Control Circuit For Mercury Arc Lamps

Abstract

A starting and regulating circuit for mercury arc lamps comprising a voltage doubler network in which a pair of capacitors parallel connected across and a.c. power supply are alternately charged during alternate half cycles through signal controlled solid stage switching and rectifying means to provide a starting potential for the lamp, are then alternately discharged through the lamp and a series inductor and alternately act to limit current flow through the lamp; in which conduction of the switching means is delayed each half cycle until supply voltage has substantially increased; and in which conduction of the switching means is variably delayed in accordance with variations in an operating condition of the lamp to maintain a predetermined operating condition.

Description

This invention relates generally to circuits for the operation of mercury arc lamps which include means to apply the required starting voltage to the lamp and ballast means to limit the current flow therethrough. It further relates to means in circuits of this kind to manually vary the effective power supplied to the lamp thereby to effect a selected operating condition of the lamp, and to means responsive to variations in an operating condition of the lamp, as caused by variations in the supply voltage or lamp aging, to automatically vary the effective power supplied to the lamp as required to maintain the selected operating condition of the lamp.

It has been found that a voltage doubler network having the required capacitance and switching means to effect the alternate charging of a pair of capacitors and the alternate discharging thereof through a lamp throughout each full half cycle of an a.c. power supply provides a suitable starting and ballast circuit for the operation of some mercury arc and other types of gas-filled lamps. The addition, however, of suitable inductance in series with the lamp reduces current peaks and effects a more continuous flow of current through the lamp. This provision has been found to be essential to the operation of other types of lamps.

It is well known that variations in supply voltage and lamp aging effect objectionable variations in the light output of mercury arc lamps and that means operative to maintain a more constant light output under these varying conditions is highly desirable.

Accordingly, it is an object of this invention to provide a generally new and improved ballast and regulating circuit for mercury arc and other types of gaseous lamps in which the effective power supplied to the lamp is maintained substantially constant under conditions of varying supply voltage.

A further object is to provide a ballast and regulating circuit for mercury arc and other types of gaseous lamps in which the effective power supplied to a lamp is automatically varied to maintain a more constant light output under conditions of varying supply line voltage or lamp aging.

A further object is to provide a generally new and improved ballast and control circuit for mercury arc and other types of gaseous lamps in which the effective power supplied to the lamp may be varied manually.

A further object is to provide an economical ballast for mercury arc and other types of gaseous lamps comprising a voltage doubler network having oppositely charged capacitors, in which a considerable reduction in the usually required capacitance to maintain rated output of the lamp is achieved by provision of switching means operative to delay charging and discharging of the capacitors until the supply voltage has increased each half cycle to some predetermined value.

Further objects and advantages will appear from the following description when read in connection with the accompanying drawing.

In the drawing:

FIG. 1 is a schematic illustration of a regulating ballast for mercury arc lamps constructed in accordance with the present invention;

FIG. 2 is a schematic illustration of a modified form of the regulating ballast shown in FIG. 1.

FIGURE 1

Referring to FIG. 1 of the drawing, capacitors C.sub.1 and C.sub.2 with respective diodes D.sub.1 and D.sub.2 are connected in parallel across a.c. power source terminals T.sub.1 and T.sub.2 through bi-directional solid state switching means, such as a Triac Q.sub.1, the conduction angle of which is controlled. The diodes D.sub.1 and D.sub.2 are arranged in opposed polarity with respect to each of the supply terminals so that, during one half cycle of the power supply when Q.sub.1 is conducting in one direction, one of the capacitors may be charged to line voltage, and during the succeeding half cycle when Q.sub.1 is conducting oppositely, the other capacitor may be charged to line voltage while the charge on the one capacitor is maintained by a blocking diode. The described network will function, therefore, as a voltage doubler ballast with a potential of substantially twice that of line voltage appearing across junction points 10 and 12 to start a lamp 14.

A mercury arc lamp 14 and an inductor L.sub.1 are series connected across junctions 10 and 12. The system also includes a second inductor L.sub.2 connected in series with Triac Q.sub.1 during both half cycles and in series with capacitors C.sub.1 and C.sub.2 during alternate half cycles. Control of the firing angle and therefore the conduction angle of Triac Q.sub.1 is accomplished by a gate control network comprising a fixed resistor R.sub.1, a photoconductive element 16, a variable resistor 18, and a capacitor C.sub.3, all series connected across the Triac Q.sub.1. Bi-directional, breakdown switching means, such as a Diac Q.sub.2, is connected between the control electrode of Triac Q.sub.1 and a point 18 between the capacitor C.sub.3 and variable resistor R.sub.2. Diac Q.sub.2 is a solid state, bi-directional, voltage breakdown switch which blocks current flow in both directions until a predetermined threshold voltage is applied thereacross. When the threshold potential is applied, Diac Q.sub.2 conducts in either direction according to the polarity of the applied potential and thereby fires the Triac Q.sub.1.

When T.sub.1 is positive C.sub.3 is charged to the threshold of Q.sub.2 at a time during that half cycle of the power supply determined by the resistance of series connected R.sub.2, photocell 16 and R.sub.1, and upon conduction of Q.sub.2, a positive firing signal is applied to the gate of Triac Q.sub.1. When T.sub.2 is positive C.sub.3 is again charged to the threshold potential of Q.sub.2 at a time determined by series connected R.sub.2, photocell 16 and R.sub.1, and when Q.sub.2 conducts, a firing signal is again applied to the gate of Triac Q.sub.1. The variable resistor R.sub.2 provides means for manually adjusting the circuit so that it will supply a measure of effective power to the lamp 14 which will effect a predetermined operating condition thereof at a given supply voltage. The photoconductive element is arranged to respond to variations in the light output of lamp 14, its resistance to electrical current flow varying inversely with the light output of the lamp so that more rapid charging of capacitor C.sub.3 and therefore earlier firing of Q.sub.1 occurs as the light output of the lamp increases.

OPERATION OF FIGURE 1

In starting, when power supply terminal T.sub.1 is positive and Triac Q.sub.1 is conducting, capacitor C.sub.1 will be charged through Triac Q.sub.1, inductor L.sub.2, and diode D.sub.1. During the succeeding half cycle when supply terminal T.sub.2 is positive, C.sub.2 will charge through diode D.sub.2, inductor L.sub.2, and Triac Q.sub.1, and when sufficient potential occurs across junctions 10 and 12 to effect ionization and conduction through the lamp, capacitor C.sub.1 will discharge through inductor L.sub.1, the lamp, and diode D.sub.2. When conduction through the lamp occurs charging of C.sub.2 will continue and C.sub.1 will also charge in the opposite direction through L.sub.2 and Q.sub.1 during this half cycle.

During the next succeeding half cycle, when T.sub.1 is again positive and with the vapor in the lamp ionized, C.sub.1 will again discharge through inductor L.sub.1 and lamp 14, and capacitor C.sub.2 will discharge through inductor L.sub.2 and Triac Q.sub.1 in the same direction with respect to lamp 14. Also, during this half cycle, C.sub.2 will also charge in the opposite direction through lamp 14, inductor L.sub.1, and diode D.sub.1. It will be seen that either C.sub.1 or C.sub.2 limits the current through the lamp and that L.sub.1 and L.sub.2 act to reduce peak lamp current and prolong its flow.

Under conditions of operation of a voltage doubler ballast in which the switching means is rendered conductive immediately following the zero voltage point each half cycle of the power supply, so that the capacitors charge during substantially 180.degree. each half cycle, the effective power supplied to the lamp for a given power supply voltage will be a minimum and the capacitance required will be a maximum. It has been found, however, that the effective power supplied to the lamp through a voltage doubler ballast circuit increases as the conduction angle of the switching means each half cycle is reduced by delaying the firing angle thereof until the supply voltage has increased substantially.

When the firing of Triac Q.sub.1 is delayed each half cycle until supply voltage has increased substantially, the abrupt application of higher voltage to L.sub.1 and L.sub.2 increases self-inductance therein and the value of capacitors C.sub.1 and C.sub.2 as power factor corrective means increases. Also, when the firing of Triac Q.sub.1 is so delayed so as to reduce its conduction angle, the circuit can be considered to be operating at a higher frequency, therefore requiring less capacitance.

I have found that when the firing of Triac Q.sub.1 is delayed 45.degree. or more following zero voltage each half cycle, the capacitance of C.sub.1 and C.sub.2 may be reduced to one half the value required for a given supply voltage when the circuit is operated through substantitally 180.degree. each half cycle. This reduction in required capacitance permits a considerable reduction in the cost of producing a non-regulating voltage doubler type ballast. Also, this reduction in size, and therefore cost, of capacitors C.sub.1 and C.sub.2 permits the manufacture of a regulating voltage doubler type ballast at substantially the cost of one which is non-regulating, with the greater capacitance required for conduction throughout substantially the entire cycle.

Some additional inductance is required when the firing angle of Triac Q.sub.1 is delayed any considerable amount. The inductor L.sub.2 supplements inductor L.sub.1 in reducing current peaks through the lamp and in continuing current flow therethrough. By positioning that portion L.sub.2 of the total inductance so as to be alternately in series with C.sub.1 and C.sub.2, the peak capacitor charging currents are also limited.

FIGURE 2.

Referring to FIG. 2, the Triac Q.sub.1 and diodes D.sub.1 and D.sub.2 of FIG. 1 are replaced by a pair of silicon controlled rectifiers Q.sub.3 and Q.sub.4 arranged in opposite polarity with each of terminals T.sub.1 and T.sub.2. Additionally, a means 20 to indicate current flow through mercury arc lamp 14, comprising a small incandecent lamp 22 and a parallel connected resistor 24, is provided.

A firing signal is applied to each of the SCR's during each conducting half cycle thereof through respective transformer secondary windings 26 and 28 connected between their control electrodes and the cathode sides thereof. A transformer primary winding 30 common to both secondary windings 26 and 28 alternately induces a positive firing pulse in the secondary windings 26 and 28. Primary winding 30 is connected across the supply terminals T.sub.1 and T.sub.2 in series with Diac Q.sub.2, variable resistor R.sub.2, photoconductive element 16, fixed resistor R.sub.1 and inductor L.sub.2 and in parallel with capacitor C.sub.3.

OPERATION OF FIGURE 2.

When silicon controlled rectifiers Q.sub.3 and Q.sub.4 are alternately conducting during alternate half cycles of the power supply, the capacitors C.sub.1 and C.sub.2 are alternately charged and discharged through lamp 14, as in FIG. 1. The firing angle of the SCR's is determined by the time required to charge capacitor C.sub.3 to the threshold voltage of Diac Q.sub.2 through the resistors R.sub.1 and R.sub.2 and photocell 16. When Diac Q.sub.2 conducts in one direction during one half cycle, an energy pulse flows through primary winding 30 in a corresponding direction and therefore through one of the secondary windings 26 or 28 in a direction to apply a positive signal to the gate of one of the SCR's. During the alternate half cycle when Q.sub.2 is caused to conduct oppositely, a positive firing signal is applied to the other SCR.

When the current flow through lamp 14 varies due to any reason, such as supply voltage variation, from that predetermined to effect a predetermined optimum operating condition of the lamp, the light output of incandescent lamp 22 will vary directly with the current flowing through lamp 14. The relative resistance of resistor 24 and the filament of lamp 22 is preferably such that a considerable portion of the radiation from the filament is in the red and infra-red frequency band to which an inexpensive, reliable, cadmium sulphide, photoconductive element 16 is responsive. The inductors L.sub.1 and L.sub.2 function similarly to their counterparts in FIG. 1 to limit current peaks through the lamp and capacitor charging current.

Experiments were conducted with a ballast and regulating circuit constructed in accordance with FIG. 1 of the drawing. In these experiments it was found that when the supply voltage was adjusted to provide 360 watts at lamp 14 and with Triac Q.sub.1 conducting substantially a full 180.degree., the lamp power could readily be increased to 440 watts by decreasing the conduction angle of Q.sub.1, that is, by retarding the firing thereof until the supply voltage has increased substantially each half cycle. It was also found that the circuit could be readily adjusted to provide 400 watt lamp power with 20 per cent variation in input voltage.

Further, in this circuit having a 10 millihenry inductor L.sub.1, a 13.8 millihenry inductor L.sub.2, and a capacitance of 43 microfarads, it was found that rated lamp power in a 400-watt mercury arc lamp could be maintained under conditions in which the supply voltage was varied from 110 to 165 volts a.c.

Claims

I claim:

1. In a starting and ballast circuit for mercury arc lamps, a source of a.c. power, an inductor, an arc lamp, a pair of capacitors, parallel circuit branches connecting said capacitors in parallel across said power source, rectifying means in each of said circuit branches in series relationship with said capacitors and arranged in opposite polarity, and circuit connections connecting said lamp and said inductor in series relationship between said parallel circuits and to points thereon between said capacitors and said rectifying means, whereby the sum of the charges on said capacitors is impressed across said lamp to start discharge therethrough and whereby said capacitors thereafter alternately discharge through said lamp and series inductor and limit the current flow through the lamp and whereby said inductor reduces current peaks and provides a more continuous current flow through said lamp.

2. A circuit as claimed in claim 1 which further includes voltage responsive switching means connected in series relationship with both of said parallel circuit branches and operative to delay charging of said capacitors each half cycle until the supply voltage has increased each half cycle to a predetermined value, thereby to increase the effective power supplied to the lamp and to reduce the required capacitance.

3. A circuit as claimed in claim 2 which further includes a second inductor connected in series relationship with said voltage responsive switching means and with both of said circuit branches across said power supply and operative to reduce peak capacitor charging currents and increase the total inductance.

4. A circuit as claimed in claim 2 in which said voltage responsive switching means comprises a controlled bi-directional, solid state switching device including a gating circuit therefor responsive to a predetermined supply voltage to effect conduction thereof.

5. In a starting and regulating circuit for vapor arc lamps, a source of a. c. power, an inductor, an arc lamp, a pair of capacitors, parallel circuit branches connecting said capacitors in parallel across said power source, rectifying means in each of said circuit branches in series relationship with said capacitors and arranged in opposed polarity, circuit connections connecting said lamp and said inductor in series relationship across said parallel circuit branches at points thereon between said capacitors and said rectifying means, whereby the sum of the charges on said capacitors is impressed across said lamp to start discharge therethrough and whereby said capacitors thereafter alternately discharge through said lamp and inductor and limit the current flow through the lamp, and switching means connected in series with both circuit branches controlling conduction therethrough, said switching means being responsive to variations in an operative condition of said lamp to variably delay the charging and discharging of said capacitors each half cycle of the power supply thereby to vary the inductive reactance of said inductor and consequently the effective power supplied to the lamp.

6. A starting and regulating circuit as claimed in claim 5 in which said switching means is responsive to variations in the light intensity of said lamp to variably delay the charging and discharging of said capacitors each half cycle of the power supply.

7. A staring and regulating circuit as claimed in claim 5 in which said switching means is responsive to variations in the average current flow through said lamp to variably delay the charging and discharging of said capacitors each half cycle of the power supply.

8. In a starting and regulating circuit for the operation of vapor arc lamps, an a. c. power source, first and second inductors, an arc lamp, a pair of capacitors, parallel circuit branches connecting said capacitors in parallel across said power supply, a normally non-conductive, silicon controlled rectifier in each of said circuit branches, each connected in series with a capacitor and arranged in opposed polarity, circuit connections connecting said lamp and said first inductor in series between said circuit branches and to points thereon between said SCR's and said capacitors, gating means operative to apply a firing signal to each of said SCR's at a predetermined time during their conductive half cycle, whereby the sum of the charges on said capacitors is impressed across said lamp to start discharge therethrough and whereby said capacitors thereafter alternately discharge through said lamp and series inductor and limit current flow through the lamp, and circuit connections connecting said second inductor across said power source in series with both of said circuit branches.

9. A starting and control circuit as set forth in claim 8 in which said gating means comprises a resistor and a capacitor series connected across said power source, a transformer secondary winding connected between the control electrode of each of said SCR's and the cathode side thereof, a transformer primary winding common to both of said secondary windings and a bi-directional, voltage breakdown, triggering switch, said primary winding and said triggering switch being series connected in parallel with said capacitor and in series with said resistor.

10. A starting and regulating circuit as claimed in claim 9 which further includes a photoconductive element series connected with said resistor and capacitor and being responsive to variations in the light intensity of said lamp to vary the firing time of said SCR's.

11. A starting and regulating circuit as claimed in claim 10 which further includes a filament lamp connected in series relationship with said lamp across said circuit branches and adapted to be heated by current passing through said lamp, and in which said photoconductive element is responsive to variations in the radiant energy of said filament to vary the firing time of said SCR's.

12. In a starting and regulating circuit for the operation of vapor arc lamps, an a. c. power source, first and second inductors, an arc lamp, a pair of capacitors, a pair of parallel circuit branches connecting said capacitors in parallel across said power source, a solid state diode in each of said branches, each in series relationship with a capacitor and said diodes being arranged in opposed polarity, circuit connections connecting said first inductor and said lamp in series relationship between said circuit branches and to points thereon between said capacitors and said diodes, a normally non-conductive, controlled, bi-directional, solid state switching device, circuit connections connecting said solid state switching device and said second inductor in series across said power source and in series relationship with both of said circuit branches, and gating means operative to apply firing signals to said bi-directional switching device during alternate half cycles of the power supply to effect conduction thereof at a predetermined time during each half cycle.

13. A starting and control circuit as set forth in claim 12 in which said bi-directional, solid state switching device is a Triac, and in which said gating means comprises a resistor and a capacitor series connected across said Triac, and a bi-directional, voltage responsive, triggering switch connected between the control electrode of said Triac and said R-C timing circuit at a point thereon between said resistor and said capacitor.