Electrostatic Spray Gun With Self-contained Miniaturized Power Pack Integral Therewith

Abstract

An electrostatic spray gun having physically integral therewith a power pack for transforming low voltage supplied to the gun to high voltage for application to the gun electrode. The power pack is contained completely within the gun, and includes a combined oscillator and transformer which converts low voltage d.c., e.g., 11 volts, supplied to the gun via a low voltage cable to an intermediate voltage at high frequency, e.g., 6,000 volts peak-to-peak at 45 KHz; and a voltage multiplier circuit which transforms the high frequency 6,000 volt peak-to-peak power to 72,000 volts d.c. for application to the gun electrode.

Description

This invention relates to electrostatic spray coating systems, and more particularly to miniaturized power packs contained wholly within the spray gun for transforming a low voltage which is input to the gun from an external source to a high voltage for application to the gun electrode.

Electrostatic spray coating systems of the general type to which this invention relates typically include as a principal component thereof an electrostatic spray gun. The gun has a handle designed to be manually grasped in use by the operator and a barrel which at its forward end terminates in a nozzle. A spray of finely divided, or atomized, particles of coating material, such as paint, lacquer, or the like flows from the gun nozzle toward the object being coated when an actuator on the handle, such as a trigger, is actuated by the operator. An electrode, electrically insulated from the gun handle, trigger, and barrel, is mounted in the nozzle and maintained at a high D.C. potential, e.g., 72 Kv, for electrostatically charging the coating particles as they leave the nozzle. Electrostatic charging of the particles enhances, for well-known reasons, the deposition of the coating on the article being coated, which is typically maintained at ground potential.

A source of coating material is connected to the barrel of the gun via a flexible hose. Activation of the trigger activates a flow valve in the gun to permit the flow of coating material to the gun whereat it is atomized and emitted as a spray. Electrostatic spray systems also typically include a power pack or booster supply for transforming commercially available low voltage power to the high d.c. voltages which are applied to the gun electrode for electrostatically charging the coating particles.

The power packs heretofore proposed, particularly those adapted to supply electrode voltage of 50--100 Kv or more have, from a physical standpoint, typically taken the form of a box or canister-like structure which because of its bulkiness, e.g., 1 cubic foot or more in volume and 35-45 pounds in weight, is usually placed on the floor near the operator. The power packs, which plug into a conventional 120 volt, 60 Hz a.c. source, provide at their output terminal the high d.c. voltage, for example, 72 Kv, required for electrostatic charging of the coating particles. A high voltage electrical cable connects the output of the powr pack to the gun for application of the high voltage to the electrode.

From an electrical standpoint the power packs heretofore proposed include a transformer and a voltage multiplier circuit of the capacitor/diode voltage doubler type. Both the transformer and the multiplier circuit are located in the power pack canister, preferably submerged in oil, which is located on the floor near the operator and connected to the gun by a high voltage cable. The power pack transformer steps-up the voltage of a conventional input at 120 volts a.c. (340 volts peak-to-peak) to an a.c. voltage of approximately 14,000 volts peak-to-peak. This stepped-up a.c. voltage is then fed to a five-stage voltage multiplier which transforms it to a high d.c. voltage, approximately 72 Kv, as required for electrostatically charging the coating particles. The 72 Kv voltage is provided at the output terminal of the power pack, and transmitted to the gun electrode via a high voltage electrical cable.

The power packs previously proposed, while capable of providing the high d.c. voltage necessary for electrostatic spraying, have a number of undesirable characteristics. For example, specially designed electrical cables capable of carrying electrode voltages of 72 Kv or the like are required for interconnecting the power pack and the gun. These cables, because of the requirement that they safely carry very high voltages, are in practice very expensive, as well as quite stiff and bulky. The high cost of the cable, which often is in the neighborhood of hundreds of dollars, renders conventional power packs undesirable for obvious reasons. In addition, the bulkiness and stiffness of teese cables makes the spray gun physically more cumbersome and difficult to handle, thereby increasing operator fatigue.

The high voltage cables of the prior art which interconnect the gun and power pack, since they do carry a high voltage and are typically used in an explosive environment, represent a potential hazard should they become damaged, short-circuit and arc to ground, and ignite combustible coating solvents which often are present in spraying applications. The prior art gun cables are hazardous for a further reason, also attributable to the high voltage character of the cable, namely, should the cable become damaged, an operator accidentally coming in contact with it risks being electrically shocked.

It has been an objective of this invention to provide a power pack for electrostatic spray guns which is contained wholly within the gun, thereby eliminating the need for a high voltage cable interconnecting the power pack and the gun, and the attendant disadvantages of such cables, such as their very considerable cost, bulk and stiffness which increase operator fatigue; high voltage which increases the hazards of electrical shock and explosion should the cable become damaged.

The foregoing objective has been accomplished in accordance with certain principles of this invention by utilizing a fundamentally different approach to the design of electrostatic spray gun power packs. More specifically, this objective has been accomplished by providing a power pack which operates at a very high a.c. frequency. High frequency operation of the power pack drastically reduces the required capacitance of the power pack multiplier circuit, providing correspondingly drastic reductions in multiplier size. With the size of the multiplier so reduced, the power pack, whose principal volume and weight constituent is the multiplier, is small enough to permit placement inside the gun. With the power pack located inside the gun, the need for a high voltage cable to interconnect the gun and power pack is dispensed with, as are the disadvantages which accompany the use of such high voltage cables, such as increased bulk and stiffness which produce operator fatigue, high cost, and hazards of operator shock and explosion should the cable become damaged.

In accordance with additional principles of this invention, the volume and weight of the power pack are further reduced by the use of a transformer having a core fabricated of ferrite and configured in the shape of a cup. A transformer of such design is extremely compact and light-weight. Additionally, because of the geometrical cup-like configuration of the core, stray flux is kept to a minimum with the result that radio frequency interference which would normally be expected at high operating frequencies is kept to a minimum. Minimization of radio frequency interference may be desirable and/or essential under certain specific conditions of use.

In accordance with a preferred embodiment of this invention the power pack, which is contained wholly within the spray gun, is supplied from an external source with a low d.c. voltage, for example, 11 volts. The low voltage d.c. input power is transformed in the gun-contained power pack to a 45 KHz, 6,000 volt peak-to-peak voltage level by a combined oscillator and transformer. The 6,000 volt, high frequency power is in turn transformed to 72 Kv d.c. by a 12-stage voltage multiplier of the capacitor/diode voltage doubler type, also contained in the gun-housed power pack. Because the multiplier circuit is operated at 45 KHz, the necessary multiplier capacitance is but a mere fraction, on the order of 1/500th, of what it would be were conventional power pack frequencies, typically 60 Hz, utilized. This reduction in multiplier capacitance permits the power pack to be miniaturized, e.g., reduced to 3-4 cubic inches in volume and 8-12 ounces in weight, to an extent sufficient to enable it to be located entirely within the gun.

As noted, with the power pack of this invention, conventional, lightweight, flexible electrical cable can be used to supply the gun-contained power pack externally from a low voltage source. Such reduces cable cost, operator fatigue, and the hazards of electrical shock and arcing-induced explosions. This is in contrast to the power packs heretofore proposed which, because of low frequency operation, were heavy and bulky and necessarily independent of and physically remote from the gun, and required use of high voltage cable to interconnect the gun and power pack. Such cables, as noted, are stiff and bulky, producing operator fatigue; expensive; and due to their high voltage, increase shock hazards and the risk of arcing-induced explosions.

A further and also important aspect of this invention is that the capacitively stored electrical energy of the spray gun system is very materially reduced. Since ignition in a combustible environment due to arcing between the gun electrode and a grounded object is related to the electric energy capacitively stored in the spray gun system, the likelihood of ignition, and/or the electrical circuitry required to counteract, dissipate or dampen the ignition-inducing effects of capacitively stored system energy is materially reduced in the system of this invention by the very significant reduction in electrical energy capacitively stored in the system.

In conventional prior art spray gun systems of the type described earlier, there are three principal sources of capacitive electrical energy storage, namely, the capacitance of thegun, the multiplier circuit, and the high voltage electrical cable which interconnects the multiplier circuit with the gun. The gun capacitance, which is generally attributable to the geometry and spacial relationship of the electrode and nozzle structure, is in a convenional prior art system on the order of 300 picofarads, while the capacitance of the prior art multiplier circuit in practice often is in the range of 800 picofarads. Since capacitive electrical energy storage is equal to V.sup.2 C, where C is capacitor capacitance and V is capacitor voltage, gun and multiplier capacitances of 300 pf and 800 pf, respectively, coupled with use in high voltage circuits, are sources of significant capacitive electrical energy storage. The capacitance of the high voltage cable which interconnects the multiplier circuit and the gun of the prior art systems varies depending on the construction of the cable. In one form of prior art cable, disclosed and claimed in Nord U.S. Pat. 3,348,186 entitled "High Resistance Cable," assigned to the assignee of this invention, the capacitance of a standard length high voltage cable is approximately 300 picofarads. This cable capacitance, since also used with high voltages, is an additional source of significant capacitive electrical energy storage in the typical prior art spraying system.

To reduce to within tolerable limits the probability of ignition in prior art spray systems due to inadvertent contacting of the gun electrode to a grounded object, it has been necessary to neutralize, dampen or otherwise counteract the ignition-inducing effects of capacitive electrical energy stored in these highly capacitive prior art systems. In the past it has been possible to reduce this probability of ignition to tolerable levels only by adding resistance to both the multiplier circuit and to the high voltage cable. In fact, the above referenced Nord U.S. Pat. No. 3,348,186 is directed to an improved high voltage cable incorporating just such neutralizing resistance means. Thus, in the prior art spray systems, by the appropriate selection and placement of resistance in the multiplier circuit and the high voltage cable, the ignition-inducing effects of cable and multiplier circuit capacitive electric storage have been neutralized to a degree sufficient to reduce the probability of ignition to within safe limits. However, this reduction of ignition probability to a tolerable level has been at the expense of adding resistance to both the multiplier circuit and to the cable.

In the electrostatic spraying system of this invention, the multiplier capacitance, as well as the voltage of the electrical cable connecting the low voltage supply to the gun, are very markedly reduced. As a consequence, the resistance required for netralizing the ignition-inducing effects of capacitive energy storing in the multiplier circuit and the cable is negligible, if not nonexistent. For example, in the preferred embodiment, the component of capacitively stored electrical energy stored in the system which is attributable to the cable which interconnects the low voltage d.c. source and the gun is negligible due to the very low voltage, e.g., 11 volts, at which the cable operates, rendering it totally unnecessary to incorporate in the cable any resistance means for neutralizing the effects of the cable capacitance. By contrast, prior art gun cables had very considerable capacitively stored energy in the cable, due principally to the high voltage, e.g., 72 Kv, at which such were typically operated.

In addition to reducing the cable component of capacitively stored system energy, this invention reduces the capacitance of the multiplier circuit from approximately 800 picofarads for prior art multipliers to approximately 33 picofarads for the multiplier of this invention. This capacitance of the preferred embodiment multiplier circuit is on the order of one-twentyfifth of its prior art multiplier circuit counterpart. As a consequence, in this invention multiplier circuit capacitive energy storage, and hence the resistance required for neutralizing its effects, is reduced, if indeed any is necessary, by a factor of 25.

Accordingly, in the electrostatic spray system of this invention, wherein cable and multiplier capacitive energy storage is either zero or at the very least quite low, the requirement for incorporation of resistance in the multiplier and cable to neutralize, to within safe limits, the ignition-inducing effects of capacitive electrical energy storage is virtually nonexistent.

These and other advantages and objectives of the invention will become more readily apparent from a detailed description of the invention taken in conjunction with the drawings in which:

FIG. 1 is an elevational view schematically illustrating the principal components of an electrostatic spray system incorporating the principles of this invention;

FIG. 2 is a longitudinal cross-section of an electrostatic spray gun showing the manner in which the booster supply of this invention is housed therein;

FIG. 2A is an enlarged cross-sectional view of the encircled area of the gun nozzle of FIG. 2;

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 2;

FIG. 4 is a plan view of a physical circuit assembly of a preferred form of voltage multiplier;

FIG. 4A is a diagrammatic exploded perspective view of the components which constitute a portion of the assembly of FIG. 4;

FIG. 5 is a circuit diagram, including waveforms, of a preferred booster supply embodiment incorporating certain of the principles of this invention;

FIG. 6 is an electrical circuit diagram of a modified form of voltage multiplier circuit;

FIG. 7 is a plan view of a preferred physical circuit assembly of the modified voltage multiplier circuit of FIG. 6;

FIG. 7A is a diagrammatic exploded perspective view of the components which constitute a portion of the assembly of FIG. 7;

FIG. 8 is a cross-sectional view of a modified spray gun barrel showing the relationship of the voltage multiplier circuit to the coating atomizing nozzle;

FIG. 9 is a view in cross-section and partially exploded, showing a preferred form of transformer; and

FIG. 10 is a view in corss-section showing the transformer of FIG. 9 assembled.

A preferred form of an electrostatic spray gun system incorporating the principles of this invention is shown in FIG. 1. For the purpose of convenience, the illustrated system is of the "airless" type described, for example, in Bede U.S. Pat. No. 2,754,228 and Nord et al. U.S. Pat. No. 2,936,959. As those skilled in the art will understand, "airless" spraying effects the atomization of the coating material by forcing the coating stream through an orifice of the gun under high pressure, e.g., 300-1,000 psi. This is in contrast to "air spray" systems in which an auxiliary high velocity air blast is directed at a stream of relatively low pressure coating leaving the gun nozzle to produce atomization of the unpressurized coating material. Of course, while the invention is described with respect to an "airless" system, it will be understood that the invention is not limited to use only with "airless" systems, but can also be used with other types of systems such as conventional "air spray" systems, systems effecting atomization electrostatically, or systems in which atomization is effected by a combination of such techniques.

Referring to FIG. 1, the preferred system is seen to include an electrostatic spray gun 20 having a handle 22 designed to be manually grasped in use by the operator, and a barrel 24 terminating at its forward end in a nozzle 26. A spray of finely divided, or atomized, particles of coating material such as paint, lacquer, or the like flows from the nozzle 26 toward an object 34 to be coated when the gun trigger 30 is activated by the operator. An electrode 32, electrically insulated from the gun handle 22, trigger 30, and barrel 24, is mounted in the nozzle 26 and maintained at a high d.c. potential, either positive or negative, for charging the coating particles in the spray 28 as they leave the nozzle 26. Charging of the coating particles enhances, for reasons well known in the art, the deposition of the coating particles on the article 34 being coated which is maintained at an electrical potential different from that of the electrode 32, such as ground potential.

A source of coating material in the form of a supply tank 36 is connected via a suitable fluid conduit 38 to the barrel 24 of the gun 20. A pump 40, connected in line 38 between the tank 36 and the gun barrel 24, pressurizes the coating material to facilitate atomization of the coating material by the nozzle 26 without need for an auxiliary source of pressurized air, as is conventional in the "airless" spray technique described in the above referenced Bede and Nord et al patents.

An electrical power pack or booster supply 42, including a voltage multiplier 42A and a combined step-up transformer and d.c.-a.c. converter 42B, is housed within the gun for supplying a high d.c. voltage, for example 72 Kv, to the electrode 32 from a low voltage d.c. source 44, for example, an 11 volt d.c. supply, which is connected to the gun handle 22 via a low voltage line 46. For convenience the low voltage d.c. source 44 connects via line 48 to a conventional 120 volt, 60 Hz a.c. source. Of course, if desired the low voltage source 44 could be a portable, conventional battery pack, dispensing with the need for a 120 volt, 60 Hz a.c. source.

The electrostatic spray gun 20, particularly the mechanical features of its construction, is shown in more detail in FIG. 2. Referring to this figure, the gun 20 is seen to include the handle 22 and the barrel 24 which, for convenience in assembly and maintenance, are detachably connected. The handle 22 preferably is a casting of electrically conductive material, such as aluminum, and is provided with an internal cavity 50 which houses certain of the operating components of the electrostatic spray gun system including the combined step-up transformer and d.c.-a.c. converter 42B. The cavity 50 is open at its lower end 50A to permit introduction of low voltage line 46 into the interior of the gun. A grommet assembly 52 threaded into the lower end 50A of the cavity 50 frictionally engages the low voltage line 46 to the gun handle 22 at the point at which it enters the cavity 50. Alternatively, the low voltage line 46 could be detachably connected to the combined transformer and converter 42B within the gun by provision of a connector instead of the grommet assembly 52. The cavity 50 is also open at its forward end section 50B, for reasons to become apparent hereafter.

The barrel 24, which is detachably mounted to the handle section 22 of the gun 20 to facilitate maintenance and assembly, preferably is fabricated of a tough, electrically insulative material. The barrel 24 is provided with a first cavity 54 adapted to accommodate the voltage multiplier 42A. Cavity 54 is preferably closed, or fluid sealed, at its forward end. This prevents leakage of coating solvent into the cavity 54 when the gun nozzle 26 is inserted in a solvent bath, as is periodically done in use, to prevent hardening of the coating and blockage of the nozzle. A second cavity 56, also provided in barrel 24, constitutes a coating flow passage interconnecting the conduit 38 and the atomizing nozzle 26. Cavity 56 additionally houses a longitudinally reciprocable actuating rod 58 which responds to the trigger 30 for opening and closing a flow valve 61 comprising seat 61A and ball 61B. Valve 61 regulates the flow of coating material from the cavity 56 to the atomizing nozzle 26.

The atomizing nozzle 26 includes an orifice assembly 60 preferably consitituted of a generally frusto-conical conductive metal member 60A having a carbide insert 60B in which the orifice 60C is actually formed. Member 60A is securd to a generally ring-shaped mounting structure 62 of insulative material. The orifice-mounting ring 62 is maintained in operative position relative to the coating flow passage 56 by an insulative retaining collar 68 which is threaded to the front of the barrel 24.

The electrode 32 is preferably configured in the form of a needle, the inner end of which is anchored in the insulative ring 72 and in electrical contact with the conductive orifice-supporting member 60A. A conductive tab 70 is formed in the forward end of the cavity 54, and is in electrical contact with a planar electrically conductive tab 73 constituting the output terminal of voltage multiplier 42A when the multiplier is properly positioned in cavity 54. An electrical conductor 72 is connected at one end to the tab 70, and at its other end to an electrically conductive ring 74. The other side of the ring 74 is in electrical contact with the orifice-supporting member 60A. An electrical path between electrode 32 and the output 73 of the multiplier 42A therefore includes conductive elements 60A, 74, 72 and 70.

The triggr 30 is suitably pivotably connected at its upper end to the gun handle 22 as shown at 31 for movement between an outer inactive position shown in solid lines in FIG. 2, and an inner active position shown in dotted lines at 30' . Secured to the trigger 30 is an angulated arm 30A. The angulated arm 30A, when the trigger 30 is moved to the active position 30', pivots an actuating arm 80 of a microswitch 82 in the direction of arrow 84 to energize the electrode 32. A horizontally reciprocable plunger 86, slidable in a sleeve bearing 88 mounted in a bore 90 of the handle 22, transmits motion between the trigger arm 30A to which it is connected at its forward end and the microswitch arm 80 which it abuts at its rear end. In addition to actuating the switch 82 when the trigger 30 is moved to its active position 30', movement of the trigger also opens the flow valve 61 to permit the flow of pressurized coating material from the line 38 through the passage 56 to the orifice 60 whereat atomization takes place. Specifically, movement of the trigger 30 to its active position 30' rearwardly reciprocates a rod 92 which is connected at its inner end to the arm 30A. The rod 92 which slides in an axial bore formed in a seal member 94 rearwardly moves the rod 58 to which it is connected at its inner end, in turn lifting the ball 61B secured to the forward end of rod 58 from seat 61A to open the flow valve 61. A compression coil spring 96 connected between the seal member 94 and a circular shoulder 98 formed on the rod 58 at its inner end normally spring-biases the rod 58, and hence the ball 61B, against the seat 61A to maintain the valve 61 in its closed position.

The combined step-up transformer and d.c.-a.c. converter 42B is connected to a ground line 46A and to a positive low voltage line 46B, lines 46A and 46B comprising the low voltage line 46 which extends from the gun handle 22 to the low voltage source 44 shown in FIG. 1. Grounded low voltage line 46A is also connected to the electrically conductive gun handle 22 via a screw terminal 47 to ground the operator. Electrically interconnecting the output of the combined step-up transformer and d.c.-a.c. converter 42B and the input of the voltage multiplier 42A are a pair of intermediate voltage a.c. lines 49A and 49B.

The voltage booster or power pack 42, which includes the voltage multiplier 42A and the combined step-up transformer and d.c.-a.c. converter 42B and which is wholly and completely contained within the gun 20, is shown in electrical circuit diagram format in FIG. 5. With reference to FIG. 5, the combined step-up transformer and d.c.-a.c. converter 42B is seen to principally include a transistorized single-ended ringing choke, converter, or oscillator, 100 and a transformer 102 which are supplied from a conventional 11 volt d.c. source 44 on lines 46A and 46B. With the output of the d.c. source 44 at a relatively low voltage, the possibility of arcing is minimized should line 46B inadvertently become damaged and short-circuit to ground. The oscillator 100 includes an NPN transistor Q2 having its emitter connected to grounded line 46A which constitutes an input to the oscillator, and its collector connected to one side of a transformer primary winding 104. The other side of transformer primary winding 104 is connected to positive d.c. line 46B' which constitutes an input to oscillator 100 via the emitter-collector path of a switching transistor Q.sub.1. A normally open movable electrical contact 103 of the microswitch 82, which is adapted to be closed by movement of the switch 82, which is adapted to be closed by movement of the switch arm 80 in the direction of arrow 84, is connected between the positive d.c. line 46B and the base of transistor Q.sub.1 for switching transistor Q.sub.1 from its OFF, or low conduction state, to its ON, or high conduction state, to energize the oscillator 100 from the d.c. source 44 when the trigger 30 is moved to its active position 30' (FIG. 2).

The oscillator 100 also includes a smoothing capacitor 99 which is connected across oscillator input lines 46B' and 46A. A bias resistor 101 connects the base of transistor Q.sub.2 to the positive oscillator input line 46B' to bias transistor Q.sub.2 ON.

A transformer secondary winding 106, which is wound oppositely to the primary winding 104 as indicated by dots 107, is connected in the base circuit of transistor Q.sub.2. One side of winding 106 is connected to grounded line 46A while the other side of the winding is connected to the base of transistor Q.sub.2 via an a.c. coupling capacitor 97 and a resistor 95 which functions to prevent parasitic oscillations at frequencies above the desired operating frequency.

In operation, the oscillator 100 is energized when trigger 30 is moved to its active position 30' (FIG. 2). This closes movable contact 103 associated with the trigger-operated microswitch 82, switching transistor Q.sub.1 from its OFF state to its ON state. With transistor Q.sub.1 conductive, 11 volts d.c. is applied from the low voltage d.c. source 44 to oscillator input line 46B'. This d.c. voltage, after smoothing by capacitor 99, is applied across the series combination of transformer primary winding 104 and normally nonconducting oscillator transistor Q.sub.2. The initial current surge through primary winding 104 which follows induces a voltage in oppositely wound transformer secondary winding 106 of a polarity such that transistor Q.sub.2 is driven toward saturation. The current through transformer primary winding 104 continues to increase in magnitude, but at an ever decreasing rate, eventually reaching a maximum level. As the current in primary winding 104 approaches its maximum, the induced voltage across oppositely wound transformer secondary 106 approaches zero, driving transistor Q.sub.2 to its high impedance state, which in turn causes the current through transformer primary winding 104 to decrease from its maximum. The decreased current flow in transformer primary winding 104 induces a voltage across oppositely wound transformer secondary winding 106 of a polarity such as to drive the transistor Q.sub.2 further toward its high impedance state. This causes the current flow through transformer primary winding 104 to drop to a minimum value, which in turn reduces the induced voltage across secondary winding 106 to zero, removing the negative base-emitter bias from transistor Q.sub.2. This permits transistor Q.sub.2 to conduct and the current through transformer primary winding 104 to increase. The increased current in transformer primary winding 104 induces a voltage across winding 106 of a sense such as to drive transistor Q.sub.2 further into saturation, and the oscillatory operation continues in the manner above described.

The primary transformer winding 104, through which the current cyclically increases and decreases, is transformer coupled to secondary winding 110. By reason of the turns ratio, to be described, between windings 110 and 104, the a.c. voltage across winding 104, which is 22 volts peak-to-peak, is stepped-up in voltage to an a.c. voltage of approximately 6,000 volts peak-to-peak. The voltage output from transformer secondary winding 110 is connected via lines 49A and 49B to the voltage multiplier 42A.

In the preferred form of oscillator circuit 100, capacitors 97 and 99 have capacitances of 0.2 microfarads and 4 microfarads, respectively, at rated voltages of 50 volts; and resistors 95 and 101 have resistances of 10 ohms and 390 ohms, respectively. With parameters of the foregoing magnitude and a transformer 102 of the type to be described, oscillator 100 has been found to operate at a frequency of 45 KHz, providing a power output of 6 watts and a peak-to-peak voltage, as noted, of 6,000 volts.

The transformer 102 in preferred form includes, as best seen in FIGS. 9 and 10, a cup-shaped core 113 comprising identical one-half sections 113A and 113A'. Core sections 113A and 113A' are in the form of cylinders closed at one end and have internal axially extending hollow stubs 113B and 113B', respectively. Longitudinal slots 113C and 113C' are provided in opposite sides of the cylindrical wall section of the cups 113A and 113A', respectively, for reasons to be described.

The transformer 102 also includes a bobbin or spool 115 having an elongated cylindrical section 115A about which the windings 104, 106 and 110 are wound. The bobbin 115 also includes an end flange 115B. The inside diameter of the bobbin cylindrical section 115A is slightly larger than the diameter of the cup core stubs 113B and 113B' to facilitate a sliding fit between the bobbin and stubs. Winding 104 in a preferred form includes 61/2 turns comprised of 63 strands of insulated No. 42 copper wire. Winding 106 is wound inside of winding 104 and includes two turns comprised of 22 strands of insulated No. 44 copper wire. Axially displaced on the spool 115 from the inner and outer windings 106 and 104 is winding 110. Winding 110 in a preferred form includes 1,800 turns of insulated No. 42 wire. In winding the winding 110 a cross-over ratio per turn of approximately 1.0 is preferred.

After the windings 104, 106 and 110 have been suitably wound on the bobbin 115, the bobbin is placed over the stubs 113B and 113B' of the cup core one-half sections 113 and 113A', and the cup core sections brought together in opposed relationship as shown best in FIG. 10. A dielectric spacer S interposed between the adjacent ends of stubs 113B and 113B' establishes a gap G between core sections 113A and 113A'. Slots 113C and 113C' facilitate making connections to the windings 104, 106 and 110 when the core sections 113A and 113A' are assembled. Dielectric material 117 fills the space between the windings and the interior surfaces of the core when assembled to prevent breakdown. The dielectric potting material preferably has a low dielectric constant, a low dissipation factor, and good electrical insulative properties. Such potting material, at the high operating frequencies used, provides due to its low dielectric constant a minimum of stray capacitance, and due to its low dissipation factor a minimum dielectric heating loss. Potting material commercially available from General Electric Company, designated type RTV 8112, having a dielectric constant of 3.6 and a dissipation factor of 0.019, has been found suitable.

Core 113 of the type utilized in the preferred embodiment preferably is fabricated of material having a very high permeability, such as ferrite, and cores of the type commercially available from Ferroxcube Corporation of America, Saugerties, N.Y., designated Model 2213-P-L00-3B7, have been found satisfactory. With a core of this type, oscillator operating frequencies of as high as 45 KHz have been achieved with an output of power of 10 watts, an input voltage of 11 volts d.c., and a peak-to-peak output voltage of 6,000 volts, as noted hereinbefore. When such a core is used, a combined transformer and oscillator 42B is provided having a volume of approximately 1.5 cubic inches.

The voltage multiplier 42A is generally of the Cockcroft-Walton type, and includes a multiplicity n of identical voltage doubler stages 42A-1 to 42A-n which are connected in cascade configuration. Each voltage doubler stage includes two capacitors C and two diodes D connected in a manner such that during positive one-half cycles, one of the capacitors C charges through one of the diodes D, and during negative one-half cycles the other capacitor C charges through the other diode D. Ideally, the charge across each capacitor is equal to the peak voltage of the input to the voltage doubling stage, providing at the output of the voltage doubling stage, since the capacitors are connected such that their voltages are additive, an output voltage equal to twice the peak of the input voltage to the doubler stage. Since the voltage doubler stages 42A- 1 to 42A-n are connected in cascade fashion, the output of the last, or n.sup.th, doubler stage is theoretically equal to the product of n and the input voltage to the first stage. In the preferred embodiment of this invention wherein it is desired to provide a d.c. electrode voltage of 72 Kv, 12 voltage doubler stages are employed to multiply the oscillator output voltage of 6,000 volts peak-to-peak present on lines 49A and 49B to the desired 72 Kv level output at terminal 73. Multiplier circuits have been constructed with as many as eighteen doubler stages, providing a d.c. output of 95 Kv when input with 6,000 volts peak-to-peak. Accordingly, the number of multiplier doubler stages used can be varied depending on the particular application. If the number of stages is increased indefinitely, a point is reached where losses in the multiplier are so large that for a given desired output from the multiplier, the input must be increased.

The theory of operation of Cockcroft-Walton multiplier circuits of the general type described herein and referenced as multiplier circuit 42A is well known and is described in standard electrical engineering textbooks such as Electronic Fundamentals and Applications, Third Edition, John D. Ryder, Prentice-Hall, Inc., Englewood Cliffs, N.J., Section 5-12; Electronics: Circuits and Devices, Ralph R. Wright and H. Richard Skutt, the Ronald Press Company, New York, Article 11-11; and Vacuum-tube and Semiconductor Electronics, Jacob Millman, McGraw-Hill Book Company, New York, Section 14-5; as well as in an article by Everhart et al., "The Cockcroft-Walton Voltage Multiplying Circuit," The Review of Scientific Instruments, Volume 24, Number 3, March 1953, pages 221-226.

A preferred physical arrangement for the circuit components of the voltage multiplier 42A of FIG. 5 is depicted in FIG. 4. As shown in FIG. 4, the voltage multiplier 42A is constructed of a first elongated stack 120 of series connected capacitors C and a second elongated stack 122 of series connected capacitors C. Each of the stacked capacitors C includes one or more ceramic capacitors C' of disc-shape. By "disc-shape" is meant wafer-like, regardless of whether circular, square, or otherwise. When stacked the capacitors C' of a given stack 120 or 122 have their peripheral edges in substantial alignment. The opposite end faces F1 and F2 of each ceramic capacitor C' is provided with electrically conductive coatings T1 and T2. The coatings T1 and T2 associated with opposite faces F1 and F2 of ceramic capacitor C' constitute the conductive plates and electrical terminals of that capacitor. Suitably sandwiched between terminals T1 and T2 of adjacent capacitors C' and C' at appropriate points in the stacks 120 and 122 are conductive planar tabs J which function as an electrical junction between the terminals T1 and T2 of the adjacent capacitor elements C' and C' between which the tab J is sandwiched. Diodes D are interconnected between the tabs J at appropriate points in the capacitor stacks 120 and 122 as is necessary to produce the electrical circuit depicted in FIG. 5.

In a preferred form, the outer surfaces of the planar terminals T1 and T2 of each capacitor are provided with a coating of solder S or the like (FIG. 4A). With the planar terminals T1 and T2 so coated, the capacitive stacks 120 and 122 can be readily electrically and mechanically assembled by first arranging the capacitors C' and conductive junctions J in stacks in the manner required to produce the desired circuit (FIG. 5). Once arranged, the stacks 120 and 122 are fired in a furnace at a temperature, e.g., 500.degree.-600.degree.F., suitable to cause the electrically conductive solder S coated on the terminals T1 and T2 to mechanically and electrically bond the capacitors and terminals T in the desired circuit configuration, such as that shown in FIG. 4. Of course, in firing stacks 120 and 122, the temperature should be increased gradually to avoid thermal shock. Instead of coating capacitor terminals T1 and T2 with solder, electrically conductive epoxy or other similar adhesive may be used and the firing dispensed with. Diodes D are connected to the stacked capacitors 120 and 122 as required, and the entire assembly potted.

In a preferred form of the invention the capacitors C' of multiplier stages 42A-1 to 42A-11 each have a diameter of 0.359 inches and a thickness of 0.080 inches to provide a capacitance of 930 picofarads with a rated voltage of 3 Kv. Capacitors C' of multiplier stage 42A-n in the preferred embodiment are of the ceramic type and each have a diameter of 0.359 inches and a thickness of 0.203 inches, providing a capacitance of 130 picofarads at a rated voltage of 6 Kv. Capacitors of the foregoing type, designated Models 2DDS61R901 and 2DDS61U101X, are marketed by Centralab Division of Globe Union Company, Milwaukee, Wisc. A suitable type of diode D, measuring 0.120 inches in diameter with a length of approximately 0.400 inches, available from Semtech Corp., Newbury Park, Calif., Model SFM 70, has been found to operate satisfactorily. A multiplier circuit 42A constructed as shown in FIG. 4 of components sized as noted has been found to measure 1 .times. 1/2 .times. 3 and to occupy a volume of 1.5 cubic inches.

In the preferred embodiment the multiplier 42A is located in the barrel 24 of the spray gun 20, with its output 73 proximate gun electrode 32. An advantage of such an arrangment is that the physical distance over which the particle-charging high voltage is transmitted, i.e., the distance between multiplier output 73 and electrode 32, is kept to a minimum, in turn keeping high voltage insulation requirements to a minimum.

The multiplier circuit 42A of FIG. 5, due to the inclusion therein of capacitors C, in use does inherently store, albeit to a limited extent, electrical energy, such stored electrical energy even though small in amount, does contribute to a minor degree, to the possibility of ignition if the electrode 32 comes in contact with a grounded object. To dissipate the energy electrically stored in the multiplier capacitors and thereby reduce to an absolute minimum the risk of ignition, resistors R may be connected in series with the capacitors C' of each voltage multiplier stage 42A-1' to 42A-n' as shown in FIGS. 6, 7 and 7A. Preferably the resistors R take the form of ferrite discs which exhibit high impedance to the flow of d.c. current while having low impedance to the flow of a.c. current. Ferrite disc resistors R which have been found suitable are commercially available from Elna Ferrite Laboratories, Inc., Woodstock, N. Y., fabricated of 3E2 ferrite, and have a diameter of 0.330 inches and a thickness of 0.08 inches, providing resistance of 100-1,000 ohms. The disc-shape of the preferred ferrite resistors R permit them to be assembled in stack-like fashion interleaved with the capacitor C' as shown by stacks 120' and 122' in FIG. 7. With reference to FIG. 7, it is seen that the resistors R are preferably provided on their opposite faces F1' and F2' with planar conductive terminals T1' and T2'. Terminals T1' and T2' preferably are coated with electrically conductive solder S' or the like prior to being assembled in stacked fashion sandwiched between planar terminals T1' and T2' of adjacent capacitors C'. When so coated and assembled, the stacked resistor and capacitor stacks 120' and 122' can be fired in an oven, conveniently producing electrical and mechanical bonding of the stacked capacitors C' and resistors R.

If desired, and to further minimize ignition hazards, a resistor (not shown) may be connected between the outputs 73 and 73' of multiplier circuits 42A and 42A'. Resistors having resistances of up to 75 megohms or more could be used.

In accordance with a modified embodiment of the electrostatic gun 20 of this invention, the stacked capacitors 120 and 122, or stacked capacitor and resistor combinations 120' and 122', are located above and below, respectively, the coating flow passage 56', respectively, in a cavity 54' formed in the gun barrel 24', as best shown in FIG. 8. This is in contast to locating both stacks 120 and 122, or 120' and 122', above the coating flow cavity 54 as shown in FIG. 3.

While the invention has been described with reference to a preferred embodiment thereof, it is apparent that a number of modifications can be made. Other oscillators can be used, e.g., symmetrical push-pull oscillators, crystal oscillator-driven power amplifiers, etc. If an oscillator is employed of the specific type shown in detail in FIG. 5, variations in its design and operation can be made. For example, the input voltage on lines 46A and 46B' to the oscillator, which preferably is 11 volts d.c. can vary anywhere between approximately 8 volts d.c. and 24 volts d.c. If the oscillator input voltage on lines 46A and 46B' is reduced below the preferred lower limit of approximately 8 volts, the input current to the primary winding 104 of transformer 102 will be excessive in the sense that for a given power output from the preferred oscillator embodiment 100 it will be necessary to increase the diameter of the wire constituting winding 104, resulting in an undesirable increase in the size of the winding. Additionally, the increased current flow through primary winding 104 may exceed safe design limits for the emitter-collector path of transistor Q.sub.2.

If the input voltage to the preferred oscillator embodiment 100 on lines 46A and 46B' exceeds the preferred upper limit of 24 volts, it is necessary to increase the number of turns of the primary winding 104 if the primary is to operate in an unsaturated mode with respect to the B-H curve of transformer 102, as is desired for maximum power transformation efficiency. However, if the number of turns of the primary winding 104 is increased, it is necessary, if volume is to be conserved, to reduce the diameter of the wire used to make winding 104. This, however, increases the inductance of the primary winding 104, which reduces the switching speed of transistor Q.sub.2. Thus, if the input voltage exceeds 24 volts and it is desired to conserve volume, switching speed and hence frequency is affected.

The output voltage of the preferred oscillator embodiment 100, as stepped up by the transformer 102 in the preferred embodiment, is approximately 6,000 volts peak-to-peak. This output could theoretically be increased in an effort to decrease the number of voltage multiplier stages required and, hence, decrease the volume of the multiplier. However, it has been found that certain factors must be considered if the output voltage of the combined transformer and d.c.-a.c. converter 42B is increased above the 6,000 volts peak-to-peak of the preferred embodiment. For example, increasing the output voltage would require, assuming the input voltage to the preferred oscillator embodiment 100 remains constant, an increased number of turns for the transformer secondary winding 110. However, if the number of turns on winding 110 is increased, the volume of the transformer is increased. If, in an effort to increase the number of turns of the transformer secondary winding 110, while maintaining the transformer volume constant, the wire diameter is reduced, the inductance of the winding increases as does the intrawinding capacitance and the winding-to-core capacitance as a consequence of the greater surface area of the wire constituting winding 110, thereby reducing frequency. Another consequence of increasing the output voltage of the transformer secondary winding 110 is that for a given diameter of cup core 113 as the number of turns of winding 110 is increased, the voltage gradient between the outer turns of winding 110 and the interior wall of the cup core increases to a point wherein the dielectric potting material 117 which insulates the outer winding turns from the cup core may break down.

In addition to the foregoing consequences of increasing the output voltage of the preferred oscillator circuit and independent of limitations imposed by the specific structure of oscillator 100 of the preferred embodiment, there are other factors to consider. Specifically, as the oscillator output voltage is increased, the input voltage to the multiplier increases, and in order to achieve any given output voltage from the multiplier, a lesser number of multiplier stages is necessary. As the multiplier stages are decreased in number, the voltage gradient per multiplier stage is increased. For example, in the preferred embodiment twelve multiplier doubler stages are used to increase the oscillator output voltage from 6,000 volts peak-to-peak to 72,000 volts d.c. Under these conditions the voltage gradient per stage is equal to (72,000 - 6,000 volts),/12 or approximately 5,500 volts per stage.

If the oscillator output voltage (multiplier input voltage) is increased to 12,000 volts, the voltage gradient per multiplier stage is 100 percent larger. Specifically, with an oscillator output voltage of 12,000 volts and a desired multiplier output voltage of 72,000 volts, it is only necessary to multiply the input voltage by a factor of six. Hence, only six multiplier stages are required. However, with six multiplier stages and a total multiplier voltage gradient of 60,000 volts, 72,000 - 12,000 volts, the voltage gradient per stage is 60,000,/6 or approximately 10,000 volts per stage. Thus, if the oscillator output voltage is increased in an effort to decrease the number of multiplier stages and, hence, to decrease the volume of the multiplier, the voltage gradient per stage increases. This in turn increases the electrical stresses to which the capacitors in the multiplier are subjected, which may cause capacitor failure if increased sufficiently.

Another consequence of increasing the output voltage of the oscillator is that extraneous transmission of energy, which undesirably interferes with surrounding electrical devices may result. Such interfering transmission is attributable principally to the increased voltage of the oscillator output and the lack of shielding of the transformer high voltage winding leads, and only secondarily to the high operating frequency. While such transmission is not excessive in the preferred embodiment due to the shielding of the high voltage winding itself by the cup-core, transmission from the unshielded leads of the high voltage winding may become excessive if the voltage of the winding is made too high. The output voltage of oscillator 100 could be reduced from the preferred 6,000 volt peak-to-peak level. However, for a given multiplier output, if the oscillator output (multiplier input) level is reduced, an increase in the number of multiplier stages is necessary. In practice an oscillator output (multiplier input) OF APPROXIMATELY 2,000 volts peak-to-peak is a preferred minimum.

It may be desired to increase the frequency of the oscillator 100 in an effort to further reduce the required capacitance of the multiplier circuit 42A, and in turn reduce multiplier circuit volume. In this regard, the frequency of the preferred embodiment of the combined transformer and oscillator circuit 42 is determined primarily by the LC value of the assembled transformer, and in particular by the LC constant of the high voltage secondary winding 110 as approximately defined by the equation F.varies.1/LC. If the frequency of the preferred oscillator embodiment 100 is to be increased in an effort to further reduce the capacitance of the multiplier capacitors, and hence to decrease the volume of the multiplier, it is necessary to decrease the LC product. If this is attempted by decreasing L, it is necessary to increase the winding diameter which increases the volume of the transformer. If, on the other hand C is decreased, it is necessary to decrease the intrawinding capacitance by either decreasing the number of turns or increasing the wire diameter. If the number of turns is decreased, the output voltage drops. If the wire diameter is increased, the volume of the transformer increases. It is also possible to decrease C by increasing the "rake-off" angle of the winding, producing more winding "cross-overs" per turn. If the number of "cross-overs" per turn increases above 1.0 "cross-overs" per turn, which is preferred, the coil becomes mechanically unstable. Thus, while the frequency of the preferred oscillator embodiment 100 can be increased by decreasing L or C of the high voltage transformer secondary winding 110 in an effort to reduce multiplier circuit capacitance and volume, such increases cannot be made without certain operating consequences.

The frequency of the oscillator 100 of the preferred embodiment can also be increased by increasing the gap G, which is nominally 0.0125 inches, between mating sections 113A and 113A' of the cup core 113 (FIG. 10), thereby decreasing the inductance L of the transformer 113. However, as the spacing G is increased, the flux density in the core 113 drops, causing the available output power to drop.

From the foregoing it is apparent that the frequency of the oscillator, regardless of whether such oscillator is of the type shown as the preferred embodiment 100 or of another type, can be increased in an effort to decrease the capacitance required in the multiplier and, hence, the multiplier volume, and/or that the output voltage of such oscillator can be increased to decrease the number of multiplier stages required and, hence, decrease the multiplier volume. However, each such modification cannot be carried beyond practical limits without producing undesirable consequences. For example, decreasing the number of stages by increasing the oscillator output voltage increases the voltage gradient per multiplier stage and if increased without limit will damage the multiplier capacitors. Additionally, if the output voltage of the oscillator is increased without limit, certain consequences follow, when the specific oscillator 100 is used, which have been discussed such as dielectric breakdown, increased transformer volume, etc. If the volume of the multiplier is attempted to be reduced by increasing the frequency of oscillator 100, other undesirable consequences follow when the preferred oscillator 100 is used; if frequency is increased without limit, such as a drop in available oscillator output power, an increase in volume of the transformer, and/or mechanical instability of the transformer windings.

As noted, it is contemplated that oscillator and transformer circuits other than the specific circuitry described, and generally referenced by numeral 42B, can be utilized for providing an input to the multiplier 42A. Subject to the considerations heretofore noted with respect to increasing the input voltage to the multiplier, such as increasing the voltage gradient per multiplier stage, the output voltage level of the other oscillator and transformer circuits, if such others are used, may be increased over the preferred level of 6,000 volts peak-to-peak for the specific oscillator and transformer 42B shown in FIG. 5.

The frequency of other oscillator and transformer circuits, if such others are used, can exceed 45 KHz which is characteristic of the oscillator 100 of the preferred embodiment depicted in FIG. 5. However, the oscillator frequency (multiplier input frequency), regardless of what type of oscillator is used, should not be so high that the period thereof exceeds the switching time of the rectifier diodes D used in the multiplier circuit. By diode switching time is meant the time duration after the diode ceases being forward biased which is required for the diode to reach its high resistance state. If the frequency of the intermediate voltage supply input to the multiplier, that is, oscillator output frequency, is such that the period of the multiplier input waveform is less than the switching time of the multiplier rectifier diodes, the diodes will remain in their low resistance state when the reverse potential of the multiplier input is impressed on the diodes with the result that the diodes will conduct for that portion of the negative one-half cycle during which time it takes the diode to switch to its high conduction state after forward bias is removed, as well as for the entire forward bias one-half cycle. Under such conditions, the diode is conducting for more than 180.degree. of the multiplier input waveform, with the result that resistance heating will exceed design limits which are predicated on diode conduction for only 180.degree. of the electrical cycle. The increased resistance heating which occurs when the frequency specifications of the diode are exceeded may cause the diode, which will normally not be destroyed when operated at a point below its voltage and frequency specifications, to be destroyed. With commercially availabe solid state diodes, the frequency at which destruction by excessive heat dissipation occurs is in the neighborhood of 250 KHz.

Certain other problems occur if the oscillator frequency is unduly increased in an effort to decrease multiplier capacitance and hence volume. For example, the capacitor dissipation factor increases, causing dielectric heating of the multiplier capacitor dielectric material to become excessive, adversely affecting capacitor life, if the frequency exceeds approximately 400 KHz.

From the standpoint of decreasing the frequency of the input to a given multiplier circuit, certain factors must be considered in addition to the increased multiplier capacitance, and hence volume required. Among such factors are increased power input needed to provide a given power output, and increased voltage gradient per multiplier stage. Specifically, as the frequency of the input to a given multiplier is decreased, the capacitive reactance per capacitor increases. Increased capacitive reactance causes multiplier output power to drop. In order to restore output power to the desired level, it is necessary to increase power input to the multiplier, which is inherently undesirable. Low frequency operation, since such can be accomplished only by an increase in input power and hence in input voltage, has another disadvantage. Namely, it increases the voltage gradient per stage, there now being less stages required due to the increase input voltage. From a practical standpoint, it has been found impractical to reduce the frequency of the multiplier input below approximately 10 KHz.

From the foregoing description of the preferred embodiment of the invention, it is clear that applicant has provided an electrostatic spray gun system which completely eliminates the need for a high voltage gun cable. By virtue of this, the substantial cable cost; cable stiffness and bulk which contribute to operator fatigue; and risk of shock and ignition occasioned by high cable voltages, are either substantially reduced or eliminated. Additionally, by virtue of the use of a low voltage calbe and the reduction of multiplier circuit capacitance, cable and multiplier capacitive energe storage is reduced to a fraction of its former value, with the result that the need and attendant cost of cable and multiplier resistance to neutralize the ignition-inducing effects of such capacitive energy storage is now negligible, if not nonexistent.

Claims

Having described my invention, I claim:

1. An electrostatic coating spray system which generates and applies high voltage charging potentials to coatings with minimal safety hazards due to capacitive electrical energy storage comprising:

a spray gun having a nozzle from which coating material is emitted,

an electrode mounted to said gun to electrostatically charge emitted coating material when said electrode is energized with high voltage electrical energy, and

a booster supply having minimal capacitive electrical energy storage at coating charging potentials mounted to said gun for converting low voltage electrical energy supplied to said gun to high voltage electrical energy for energization of said electrode, said booster supply including:

a. an oscillator circuit responsive to said low voltage energy for transforming said low voltage energy to electrical energy at high frequency, and

b. a voltage multiplier responsive to said high frequency energy and including interconnected diodes and capacitors for multiplying the voltage of said high frequency electrical energy to high voltage energy for application to said electrode, said high frequency multiplier having minimal capacitive electrical energy storage to reduce the shock and ignition hazards associated with capacitive electrical energy discharge in an explosive environment and/or to an operator.

2. The system of claim 1 further including an electrical cable interconnecting said oscillator circuit and a source of low voltage energy remote from said gun, said cable being electrically insulated sufficiently for safe operation at low voltages and insufficiently for safe operation at high voltages.

3. The system of claim 2 wherein said low voltage source is an inverter circuit for transforming 60 Hz. a.c. current to low voltage unidirectional current.

4. The system of claim 1 further including a ferrite core transformer connected to said oscillator and to said voltage multiplier for stepping up said low voltage energy input to said oscillator to intermediate voltage energy at high frequency for input to said voltage multiplier.

5. The system of claim 4 further including an electrical cable interconnecting said oscillator circuit and a source of low voltage energy remote from said gun, said cable being electrically insulated sufficiently for safe operation at low voltages and insufficiently for safe operation at high voltages.

6. The system of claim 1 wherein said gun includes an elongated barrel terminating at said nozzle and electrode, said barrel having a cavity therein with an end adjacent said nozzle and electrode which is liquid sealed with respect thereto, said cavity having an opening remote from said nozzle and electrode to facilitate insertion of said multiplier circuit into said cavity.

7. The system of claim 1 wherein said multiplier capacitors are disc-shaped with a periphery and with opposed substantially parallel surfaces each having an electrical terminal thereat, at least some of said capacitors being arranged in at least one stack in which the peripheries of said stacked capacitors are aligned and adjacent terminals of adjacent stacked capacitors are in electrical contact.

8. The system of claim 7 wherein there are at least two stacks of capacitors, wherein said two stacks are disposed in spaced, substantially parallel relation, and wherein said diodes of said multiplier have two terminals each connected to a capacitor of a different stack.

9. The system of claim 4 wherein said ferrite core transformer includes two opposed cup-shaped ferrite core sections each having a central stub, and at least two windings wound on said stubs, a first one of said windings having relatively few turns and being connected in the input circuit of said oscillator and responsive to said low voltage and a second one of said windings having relatively many turns and connected in the output circuit of said oscillator, said windings having a turns ratio to step up said low voltage to an intermediate a.c. voltage in the range of 2,000-10,000 volts peak-to-peak.

10. The system of claim 9 further including dielectric potting material between said windings and the interior of said cup-core sections for insulating said second winding and said core, said potting material having a dielectric constant equal or lower than approximately 3.6 and a dissipation factor equal or lower than approximately 0.02 for minimizing stray capacitance and providing efficient power transformation at oscillator frequencies above approximately 10 KHz.

11. The system of claim 9 wherein said oscillator includes a transistor having an emitter-collector path in which said first winding is connected and having a base, a third winding wound on said ferrite core stubs, said third winding being connected to said transistor base and of opposite polarity to said first winding for producing oscillation of said transistor.

12. The system of claim 7 further including disc-shaped resistors mechanically and electrically connected between said stacked capacitors for dissipating electrical energy stored in said multiplier circuit capacitors should said electrode become grounded.

13. The system of claim 11 wherein said intermediate voltage is approximately 6,000 volts peak-to-peak and said first, second and third windings have turns in the ratio of approximately 1:3:600, respectively.

14. A method of electrostatic coating with a spray gun having an interconnected particle-charging high voltage electrode and a voltage multiplier circuit, including interconnected rectifiers and capacitors, mounted thereon, which method involves the application of high voltage charging potentials to coatings with minimal safety hazards due to capacitive electrical energy storage, said method comprising the steps of:

supplying a low voltage to said gun from a remote source via an electrical cable,

converting in said gun said input low voltage to an intermediate voltage at a high frequency,

multiplying in said gun said high frequency intermediate voltage using said gun-mounted rectifier-capacitor multiplier circuit, said circuit having minimal capacitive electrical energy storage at coating charging potentials,

applying the multiplied voltage output from said multiplier circuit to said high voltage electrode while emitting coating particles from said gun in the vicinity of said electrode, to thereby charge said particles, and

directing said charged particles toward an article to be coated while maintaining said article at an electrical potential different from that of said electrode.

15. The method of claim 14 wherein said converting step includes driving an oscillator circuit in said gun with said low voltage to generate high frequency oscillatory voltage and by transformer action in said gun stepping up said high frequency oscillatory voltage to an intermediate voltage of the same frequency.

16. A method of electrostatic coating with a spray gun having an interconnected particle-charging high voltage electrode and voltage multiplier circuit, including interconnected rectifiers and capacitors, mounted thereon, which method involves the application of high voltage charging potentials to coatings with minimal safety hazards due to capacitive electrical energy storage, said method comprising the steps of:

supplying a low voltage to said gun from a remote source via an electrical cable insulated for use safely at low voltages and unsafely at high voltages necessary for particle charging,

driving an oscillator circuit in said gun with said low voltage to generate a high frequency oscillatory voltage,

stepping up said oscillatory voltage to an intermediate voltage of the same frequency with a ferrite cup-core transformer,

multiplying in said gun said high frequency intermediate voltage using said gun-mounted rectifier-capacitor multiplier circuit, said circuit having minimal capacitive electrical energy storage at coating charging potentials,

applying the multiplied voltage output from said multiplier circuit to said high voltage electrode while emitting coating particles from said gun in the vicinity of said electrode, to thereby charge said particles, and

directing said charged particles toward an article to be coated while maintaining said article at an electrical potential different from that of said electrode.

17. The system of claim 1 wherein said gun includes an elongated barrel terminating at said nozzle and electrode, and said multiplier circuit has a high voltage output terminal and is mounted to said barrel with said terminal proximate said electrode.

18. The system of claim 2 wherein said cable has capaci-tive electrical energy storage not substantially greater than zero.

19. The system of claim 2 wherein said cable has insufficient resistance to dissipate without ignition electrical energy in an amount equal to that stored in an identical cable carrying a high d.c. voltage.

20. The system of claim 1 further including an electrical cable interconnecting said oscillator circuit and a source of low voltage energy remote from said gun, said cable having insufficient resistance to dissipate without ignition electrical energy in an amount equal to that stored in an identical cable carrying a high d.c. voltage.

21. The system of claim 1 wherein said multiplier circuit has a capacitance substantially below 900 picofarads.

22. The system of claim 2 wherein said cable stores substantially less electrical energy in capacitive form than a similar cable carrying a high d.c. voltage.

23. The system of claim 1 wherein said multiplier circuit has a capacitance per stage which provides, at the operating frequency, a total multiplier capacitance sufficient to produce a smooth unidirectional voltage output.

24. The system of claim 1 wherein said multiplier circuit includes multiple multiplying stages, and wherein said multiplier circuit has insufficient resistance to dissipate without ignition electrical energy in an amount equal to that stored in an equivalent multiplier circuit of approximately the same per stage voltage operating at low frequency.

25. The system of claim 1 wherein said multiplier has insufficient resistance to dissipate without ignition electrical energy in an amount equal to that stored in an equivalent multiplier circuit of approximately the same total multiplier voltage operating at low frequency.

26. The system of claim 1 wherein said multiplier circuit has a total capacitance approximately equal to a 12-stage multiplier operating at 45 KHz with an average per stage voltage gradient of 6,000 v.

27. The system of claim 1 wherein said multiplier circuit has a total capacitance approximately equal to that of a 12-stage multiplier operating at 45 KHz having a total voltage thereacross of 72 Kv.

28. An electrostatic coating spray system which generates and applies high voltage charging potentials to coatings with minimal safety hazards due to capacitive electrical energy storage comprising:

a gun from which coating material is emitted,

an electrode to effect electrostatic charging of emitted coating material when said electrode is energized with substantially unidirectional high voltage electrical energy,

a source of low voltage electrical energy external to said gun,

a frequency conversion circuit responsive to said low voltage electrical energy for converting said low voltage electrical energy to high frequency electrical energy, and

a voltage multiplier circuit, including interconnected rectifiers and capacitors, mounted to said gun responsive to the output of said frequency conversion circuit for converting said high frequency electrical energy to substantially unidirectional high voltage electrical energy for energizing said electrode, said voltage multiplier having minimal capacitive electrical energy storage to reduce the shock and ignition hazards associated with capacitive electrical energy discharge in an explosive environment and/or to an operator.

29. An electrostatic coating spray system which generates and applies high voltage charging potentials to coatings with minimal safety hazards due to capacitive electrical energy storage comprising:

a gun from which coating material is emitted,

an electrode to effect electrostatic charging of emitted coating material when said electrode is energized with substantially unidirectional high voltage electrical energy, and

a voltage multiplier circuit mounted to said gun, including interconnected rectifiers and capacitors, for converting low voltage high frequency electrical energy input thereto to substantially undirectional high voltage electrical energy for energizing said electrode, said voltage multiplier having minimal capacitive electrical energy storage to reduce the shock and ignition hazards associated with capacitive electrical energy discharge in an explosive environment and/or to an operator.

30. An electrostatic coating spray system which generates and applies high voltage charging potentials to coatings with minimal safety hazards due to capacitive electrical energy storage comprising:

a gun from which coating material is emitted,

an electrode to effect electrostatic charging of emitted coating material when said electrode is energized with substantially unidirectional high voltage electrical energy, and

a voltage multiplier circuit, including interconnected rectifiers and capacitors, mounted to said gun for converting, with an accompanying change to high frequency, low voltage electrical energy input thereto at low frequency to substantially unidirectional high voltage electrical energy for energizing said electrode, said voltage multiplier having minimal capacitive electrical energy storage to reduce the shock and ignition hazards associated with capacitive electrical energy discharge in an explosive environment and/or to an operator.

31. An electrostatic coating spray system which generates and applies high voltage charging potentials to coatings with minimal safety hazards due to capacitive electrical energy storage comprising:

a gun from which coating material is emitted,

an electrode to effect electrostatic charging of emitted coating material when said electrode is energized with substantially unidirectional high voltage electrical energy,

an electrical energy source, including a source of low voltage electrical energy external to said gun, for providing low voltage electrical energy at high frequency, and

a voltage multiplier circuit, including interconnected rectifiers and capacitors, mounted to said gun responsive to said high frequency low voltage electrical energy for conversion thereof to substantially unidirectional high voltage electrical energy for energizing said electrode, said voltage multiplier having minimal capacitive electrical energy storage to reduce the shock and ignition hazards associated with capacitive electrical energy discharge in an explosive environment and/or to an operator.

32. The system of claim 31 further including an electrical cable connected between said external low voltage source and said multiplier circuit, said cable being electrically insulated sufficiently for safe operation at low voltages and insufficiently for safe operation at high voltages.

33. The system of claim 31 wherein said gun includes am elongated barrel terminating at a nozzle from which said coating is emitted and adjacent to which said electrode is mounted, said barrel having a cavity therein with an end adjacent said nozzle and electrode which is liquid sealed with respect thereto, said cavity having an opening remote from said nozzle and electrode to facilitate insertion of said multiplier circuit into said cavity.

34. The system of claim 33 wherein said multiplier includes rectifier and capacitive circuitry, said circuitry being potted and configured to fit in said cavity.

35. The system of claim 34 wherein said multiplier circuit includes capacitors at least some of which are arranged in at least two stacks, said at least two stacks being disposed in spaced, substantially parallel relation, and includes rectifiers at least some of which have two terminals each connected to a capacitor of a different stack.

36. An electrostatic coating spray system which generates and applies high voltage charging potentials to coatings with minimal safety hazards due to capacitive electrical energy storage comprising:

a gun from which coating material is emitted,

an electrode to effect electrostatic charging of emitted coating material when said electrode is energized with substantially unidirectional high voltage electrical energy,

a source of high frequency electrical energy, including a source of low voltage energy external to said gun, a transformer having at least two windings, a first one of said windings having relatively few turns and responsive to said low voltage source and a second one of said windings having relatively many turns, said windings having a turns ratio to step up said low voltage to an intermediate voltage at high frequency, and

a multiplier circuit, including interconnected rectifiers and capacitors, mounted to said gun and responsive to said high frequency intermediate voltage for conversion thereof to substantially unidirectional high voltage electrical energy for energizing said electrode, said multiplier circuit having minimal capacitive electrical energy storage to reduce the shock and ignition hazards associated with capacitive electrical energy discharge in an explosive environment and/or to an operator.

37. The system of claim 36 wherein said transformer has a ferrite core on which said windings are wound, and wherein said windings have a turns ratio to step up said low voltage to an intermediate a.c. voltage in the approximate range of 2,000- 10,000 volts peak-to-peak.

38. The system of claim 37 wherein said ferrite core includes two opposed cup-shaped core sections each having a central stub upon which said windings are wound, and wherein said transformer includes dielectric potting material between said windings and the interior of said cup-core sections for insulating said second winding and said core, said potting material having a dielectric constant equal or lower than approximately 3.6 and a dissipation factor equal or lower than approximately 0.02 for minimizing stray capacitance and providing efficient power transformation at oscillator frequencies above approximately 10 KHz.

39. The system of claim 31 wherein said multiplier circuit includes an input and an output between which are connected multiple rectifier and capacitor stages having associated therewith multiple resistors distributed between said input and output for reducing ignition hazards due to electrical energy capacitively stored in said multiplier circuit.

40. A method of electrostatic coating with a spray gun having an interconnected particle-charging high voltage electrode and a voltage multiplier circuit, including interconnected rectifiers and capacitors, mounted thereon, which method involves the application of high voltage charging potentials to coatings with minimal safety hazards due to capacitive electrical energy storage, said method comprising the steps of:

supplying low voltage electrical energy at high frequency at said gun, including transmitting to said gun via a cable low voltage electrical energy from a supply remote from said gun,

multiplying in said gun said high frequency voltage using said gun-mounted multiplier circuit, said circuit having minimal capacitive electrical energy storage at coating charging potentials,

applying the multiplied voltage output from said multiplier circuit to said high voltage electrode while emitting coating particles from said gun to thereby charge said particles, and

directing said charged particles toward an article to be coated while maintaining said article at an electrical potential different from that of said electrode.

41. A method of electrostatic coating with a spray gun having an interconnected particle-charging high voltage electrode and a voltage multiplier circuit, including interconnected rectifiers and multipliers, mounted thereon, which method involves the application of high voltage charging potentials to coatings with minimal safety hazards due to capacitive electrical energy storage, said method comprising the steps of:

supplying low voltage electrical energy at said gun, including transmitting to said gun via a cable low voltage electrical energy from a supply remote from said gun,

stepping up in said gun by transformer action said low voltage electrical energy to an intermediate voltage at high frequency,

multiplying in said gun said high frequency intermediate voltage using said gun-mounted multiplier circuit, said circuit having minimal capacitive electrical energy storage at coating charging potentials,

applying the multiplied voltage output from said multiplier circuit to said high voltage electrode while emitting coating particles from said gun to thereby charge said particles, and

directing said charged particles toward an article to be coated while maintaining said article at an electrical potential different from that of said electrode.

42. The system of claim 31 wherein said gun includes an elongated barrel terminating at a nozzle from which said coating material is emitted, wherein said electrode is mounted to said barrel, and wherein said multiplier circuit has a high voltage output terminal and is mounted to said barrel with said high voltage terminal proximate said electrode.

43. The system of claim 31 further including an electrical cable connected between said external low voltage source and said multiplier circuit, said cable when carrying electrical energy having capacitive electrical energy storage not substantially greater than zero.

44. The system of claim 31 further including an electrical cable connected between said external low voltage source and said multiplier, said cable having insufficient resistance to dissipate without ignition electrical energy in an amount equal to that stored in an identical cable carrying a high d.c. voltage.

45. The system of claim 31 wherein said multiplier circuit has a capacitance substantially below 900 picofarads.

46. The system of claim 31 further including an electrical cable connected between said external low voltage source and said multiplier circuit, said cable when carrying electrical energy storing substantially less electrical energy in capacitive form than a similar cable carrying a high d.c. voltage.

47. The system of claim 31 wherein said multiplier circuit has multiple stages and has a capacitance per stage which provides, at the operating frequency, a total multiplier capacitance sufficient to produce a smooth unidirectional voltage output.

48. The system of claim 31 wherein said multiplier circuit has multiple stages and has insufficient resistance to dissipate without ignition electrical energy in an amount equal to that stored in an equivalent multiplier circuit of approximately the same per stage voltage operating at low frequency.

49. The system of claim 31 wherein said multiplier has insufficient resistance to dissipate without ignition electrical energy in an amount equal to that stored in an equivalent multiplier circuit of approximately the same total multiplier voltage operating at low frequency.

50. The system of claim 31 wherein said multiplier is configured to provide approximately 50 KV per cubic inch of volume.

51. The system of claim 36 wherein said transformer and multiplier provide approximately 7 KV per ounce of weight.

52. The system of claim 36 wherein said transformer and multiplier provide approximately 20 KV per cubic inch of volume.