Method Of Actuating Magnetic Valves And Circuit For Carrying Out Said Method

Abstract

The specification discloses a control circuit for electromagnetic devices and a method of operating such devices in which a higher voltage is supplied for actuating the devices to pull the armatures thereof to attracted position, while a lower voltage is supplied to hold the armatures in attracted position. In the circuit the coils of the electromagnetic devices are connected across a line in series with a voltage-dropping resistor and a bypass around the voltage-dropping resistor is closed automatically when a switch pertaining to a coil to be energized is moved to coil-engaging position.

Description

The present invention relates to a method of actuating a magnetic working valve, solenoid, or the like, which is to be hold in working position for a long period of time and which is provided with a magnetic coil to be connected to a source of voltage for attracting the movable magnetic core, and which is furthermore provided with an iron jacket, iron yoke, or the like, which is closed only in attracted condition of the movable magnetic core and is adapted to be short circuit the excited magnetic flux.

Magnets of this type have, with regard to their copper windings, to be designed for a high output in view of the high current attraction in order to prevent the current which flows also in attracted condition from unduly heating up the coil packet. The overheating is particularly disturbing when a plurality of magnetic valves are provided in an electrohydraulic control system and when the magnetic valves have to remain in their working positions for a long period of time. Valves which are in effective positions over a long period of time are very large and heavy which fact represents a particular drawback when a plurality of such valves are employed and, for instance, with vehicle transmission controls because with vehicles the weight and space question is particularly important.

It is therefore, an object of the present invention to provide a control method for magnetic valves which will permit the design of small magnetic valves, even if such valves have to remain in their effective position over a long period of time, while simultaneously avoiding the danger of overheating the windings.

This object and other objects and advantages of the invention will appear more clearly from the following specification in connection with the accompanying drawing, in which:

FIG. 1 illustrates a control circuit for the actuating current of two magnetic valves which are adapted selectively to be moved into and out of their effective position while the current control is effected by making a magnetic coil effective.

FIG. 2 is a control circuit for the actuating current of a plurality of magnetic valves which are adapted selectively to be made effective and ineffective while the circuit control with four magnetic valves is carried put by making a preceding coil ineffective.

The above-outlined object has been realized according to the present invention by connecting in a manner known as per se during the period of attraction of the movable magnetic coil the coil first to a source of high-attraction voltage (100 percent) for producing a high-attracting magnetic flux in the still open yoke, and after the magnetic core has been attracted, during the entire working period of the magnetic valve to connect the coil to a low-holding voltage for producing a holding magnetic flux in the closed yoke, said holding voltage preferably amounting to about from 5to 20 percent of the attracting voltage.

For carrying out the method according to the present invention, a direct current circuit is suggested in which the current-feeding line to the exciter coil of the magnetic valve there is provided an Ohm resistor which is adapted to be bridged by a bypass line and a switch within the bypass line which switch is electrically or electronically controlled and when in rest position is open. In the current -feeding line to the exciter coil of the magnetic valve there is furthermore provided a preferably likewise electrically or electronically controlled switch which is in rest position is open and which is adapted to turn on and turn off the magnetic valve. The arrangement further more comprises a delay circuit for the delayed opening of the switch which is arranged in parallel to the resistor and brings about a delay of said last-mentioned switch when the main switch has been thrown in.

The delay circuit is expediently designed as a so-called R-C-member, and the switch is designed as a transistorized three-stage current amplifier circuit in which the base emitter section of the entrance transistor is in series with the R-C-member and in which the collecting emitter section of the exit transistor is parallel to the barrier resistor.

The circuit described so far is suitable for the current control when actuating only one magnetic valve. If the actuating current is to be controlled for a plurality of magnetic valves which are independent of each other, starting from the last-mentioned circuit, there exist two possibilities of distinguishing the control which are predetermined by the respective installation which is to be hydraulically controlled by the magnetic valves.

One possibility: When a new magnetic valve is to be made effective, another magnetic valve has to be made ineffective (switching over from one magnetic coil to another magnetic coil). Second possibility: When a new magnetic valve is made effective, no other magnetic valve is made ineffective (mere throwing in of a magnetic coil). In case of an electrohydraulic control of a planetary gear transmission for vehicles, for instance when changing the velocity, ordinarily a transmission brake or clutch is disengaged and instead thereof another brake or clutch is engaged, which means that in hydraulic control one valve has to open and another valve has to close. When the transmission is shifted to rearward drive, for instance a transmission clutch or brake which normally in rest position is closed has to be opened hydraulically to which end a magnetic valve has to be made effective.

When a multivalve control is involved with a corresponding shift-over operation, the coils of the magnetic valves and a respective pertaining actuating switch are all arranged in parallel to each other. The barrier switch, the electronic switch (transistor) bridging the barrier resistor, and the delay control (R-C-member) are common to all parallely arranged coils. For switching over from one magnetic valve to another, also when a timewise overlapping of the velocity ranges is required, according to a further development of the invention first the current supply to one coil is interrupted before the current supply to the other coil is established. Due to the preceding elimination of one current consumer, a small short time potential increase within the circuit is produced which is taken advantage of for starting the current control. If no timewise overlapping is required, it is, in conformity with a further development of the invention, suggested to throw-in a smaller resistor parallel to the switch contacts of that magnetic valve which is to remain effective for a longer period of time while the time constant of the R-C-member is designed for the required overlapping time.

If merely a valve is added to the circuit, a short time potential drop occurs within the circuit where during the switching off a small voltage increase occurred. This signal is not suitable for putting through the current control. For magnetic coils which are merely to be added to the circuit, it is therefore according to a still further development of the invention suggested that the increase in the potential at the coil which has been added itself taken advantage of for putting through the current control. To this end, between the contact at the coil side and the conductor between the condenser and the resistor of the R-C-member, there is inserted a small condenser. This condenser places the increase in potential when the throwing in the coil to the entrance of input side of the current amplifier circuit which in its turn closes the circuit.

Referring now to the drawing in detail, the general character of the circuits of both figures is now being described since basically they are the same. In these figures, the coils of the magnetic valves to be actuated are represented by the black rectangular symbol of an inductivity. Thus they symbolize the magnetic valves and the latter if they are thrown in represent a certain condition of the installation which is being controlled by the electrohydraulic control system of which only its electric part and the latter only partly is illustrated. In order to distinguish the individual magnetic valves in the drawings, the magnetic valves have been designated with the reference numerals 1 to 7. The circuit is shown for direct current, and all coils have one terminal connected to the minus pole of the direct current voltage supply furnished by the battery B.sub.1 and B.sub.2 respectively, which means to the mass M. Through the plus pole P the coils are supplied with current via the bridgeable resistor R.sub.v1 and R.sub.v2 respectively. As is customary, spark-extinguishing diodes 8 to 14 are arranged in parallel to the coils.

In conformity with the present invention, the barrier resistor R.sub.v1, R.sub.v2 which is bridged only during the attraction period of the magnetic valve is so designed that the voltage drop along said barrier resistor amounts preferably to about from 78 to 93 percent of the full voltage of the direct current network. The voltage which is available in thrown-in condition at both ends will then amount only to from 7 to 22 percent of the full network voltage. This means that in the thrown-in condition only approximately from 0.5 to 5 percent of that electric output is consumed in the magnetic valve which is consumed during the attraction (full voltage) because in the calculation of the power the voltage enters with the power two (N=U.sup.2 /R). A safe operation is also obtained when the arrangement is subjected to considerable shocks--danger of tearing off of the electromagnetically lifted magnetic core--when the magnetic core is held fast by 2 percent of the attraction force, in other words the barrier resistor is designed for a voltage drop of approximately 86 percent. Only one such barrier resistor is required in the control circuit even if a plurality of magnetic valves are to be controlled with regard to the attraction current, because even when additional magnetic valves are thrown in, a short current surge will not do any damage in view of the already thrown-in magnetic valves.

That terminal of the barrier resistor R.sub.v1 or R.sub.v2 which faces away from the plus pole is connected to a point S.sub.1, S.sub.2 respectively where during normal operation prevails the reduced voltage, or the full network voltage when a magnet is just attracting. By means of this conductor, through the intervention of switches 15-18, the coils of the respective required magnetic valves are connected to the current supply or disconnected therefrom. The entire circuit can be connected and disconnected through the main switch 19, 20 respectively.

The further structure of the circuit is no longer the same for both illustrated circuits, and for this reason the circuit according to FIG. 1 and its operation will now be explained.

The current control circuit according to FIG. 1 comprises primarily the bridging circuit including the transistor T.sub.1 and the relay R.sub.1, and the delaying R-C-member composed of the condenser C.sub.1 ; C.sub.2 and the resistor R.sub.C1 . Each coil 1 and 2 has respectively associated therewith a condenser C.sub.1, C.sub.2.

The bridging switch 21 of the relay is open when in rest position. If by means of the switch 15 the positive connection of one of the coils 1 and 2 receives the potential of the switch point S.sub.1, first a relatively high charging current passes through the pertaining condenser and also through the base emitter section of the transistor which immediately turns on, i.e. becomes conductive in its collector-emitter section. In this way a delay of microseconds only occurs after the actuation of the switch 15 before the full network voltage prevails at the coil of the relay R.sub.1, and after a delay of a few milliseconds until the relay has exerted its attracting force, the full network voltage also prevails at the magnetic valve coil which has been turned on together with the switch 15. The magnetic core of the turned on valve is attracted. Simultaneously, the pertaining condenser is charged. The charging voltage equals the network voltage minus the voltage reduced by the resistor R.sub.C1. The magnitude of the resistance and of the capacity determine the charging period. With increasing saturation of the condenser, its charging current decreases. If this charging current drops below the threshold value of the transistor necessary for switching through, the base emitter section of the transistor does not again become conductive, and the relay R.sub.1 drops off. The time constant of the R-C-member is designed for the attraction period of the magnetic valves (in the magnitude of 50 ms.), which means that the charging time of the condenser is somewhat larger than its attraction period. Only when the magnetic valve has definitely attracted, is the relay allowed to drop and the energization output is permitted to drop to the fraction still necessary for holding when the iron yoke is closed.

When switching over, the current supply to the magnetic coil energized up to this point is interrupted and its magnetic core drops off. The operation of the coil energized instead of the coil energized before or for an additionally energized coil and the control of the current therefor will take place precisely in the manner described above.

The current control according to the circuit of FIG. 2 is initiated in a manner which is fundamentally different from that described in connection with FIG. 1. When designing the circuit of FIG. 2, the designer started with the knowledge that with installations to be controlled electrohydraulically, an oil flow is made effective or ineffective while another oil flow is made ineffective or effective respectively. Such an arrangement is normally the rule for control installations for planetary gear transmissions of passenger cars. Such transmissions are controlled by hydraulically operable friction clutches or brakes and, more specifically, in such a way that when effecting a velocity change, one clutch or brake is opened while the other is closed, or one is closed while the other one is opened. It may be assumed, for instance, that when the magnetic valves pertaining to the coils 3 and 6 have attracted (illustrated position), the first velocity range of a transmission is made effective. When shifting the switch 17 from the right toward the left, the oil flow controlled by the magnetic valve 6 is turned off and instead the oil flow controlled by the valve 5 is released. Due to this change, a shift-over from the first to the second velocity range is effected. When the switch 16 is fully shifted from the left toward the right, a switchover is effected from the second to the third velocity range. Starting from the switch position shown in FIG. 1 for the assumed first velocity range, for instance, by an additional energization of coil 7 the rearward velocity range of the transmission may be made effective.

The special feature in connection with the circuit of FIG. 2 is seen in the way in which the current control is initiated when shifting the valves. Even when a timewise overlapping of the working periods of the magnetic valves is necessary for a switch-over operation, first the coil of one magnetic valve is made currentless before the other will be connected. This operation is effected by switch-over means in a normal manner.

By switching off a current consumer, the potential at the switching point S.sub.2 will increase. This minor increase in the voltage will cause a charging current for the condenser C.sub.3, the resistor R.sub.2, the base emitter section of the transistor T.sub.2 and the resistor R.sub.3. In view of this flow of electrons via the base emitter section of the transistor T.sub.2, the transistor is switched through which means that its collector-emitter section becomes conductive. In this way the base emitter section of the transistor T.sub.3 will through resistor R.sub.4 be connected to the network. A relatively strong current will flow whereby the collector-emitter section of this transistor will become conductive. Thus, through the resistor R.sub.4 the base emitter section of the transistor T.sub.4 is connected to the network whereby its collector-emitter section will become conductive and short circuit the resistor R.sub.v2. This three-step transistor circuit is necessary in order to be able by means of such low currents (fractions of a milliampere) as represented for instance by the charging current due to a potential increase in the switching point S.sub.2, to switch the high-attraction currents (about 20 amperes) which are required for attracting the magnetic cores. The right-hand portion of FIG. 2 designated with the reference numeral 25 illustrates so to speak a current-amplifying circuit with a high amplifying factor. The successive switching through of the individual transistors takes place with an unbelievable speed. As soon as one of the contacts of switches 16 and 17 opens, also the resistor R.sub.v2 is bridged by the transistor T.sub.4 designed as output transistor. At this moment, the full network voltage prevails at switching point 2 and at one side of the condenser C.sub.3. Due to this renewed increase in the potential, all of the transistors will become fully effective in case they should not have switched through by the first small increase in the potential. Thus, a circuit arrangement is involved in which a minute small increase in the potential on the strip S.sub.2 initiates an avalanchelike and very fast increase in the switching through action on the transistor T.sub.4. This bridging lasts as long as in view of the initiated increase in the potential a sufficiently high charging current flows via the condenser C.sub.3. In the meantime, the oppositely located contact of the reversing switch 16 or 17 must have been reached. The charging time for the condenser C.sub.3 is by correspondingly dimensioning its capacity and the magnitude of the resistors R.sub.2 and R.sub.3 so designed that a charging current flows at least as long as the newly energized magnetic valve has attracted. The charging current decreases with increasing charge in an exponential way. Somewhere the charging current drops below the minimum valve which is necessary for switching through the transistor T.sub.2 which means that the charging current near the end of the charging period drops below the threshold value of the transistor responsive current. This moment or the time limited thereby is of interest for the invention, and this certain time is in this connection to indicate the charging time for the condenser. When the charging current drops below this threshold value, each of the transistors will become nonconductive in a manner analogous to the manner described above in connection with the successive switching through of the three transistors T.sub.2, T.sub.3 and T.sub.4. The bridging of the resistor R.sub.v2 is thus eliminated at the end of the charging time for the condenser C.sub.3. A drop in the potential occurs at the strip S.sub.2 which drop similar to the response of the transistor row initiates a self-amplifying switching off of the electronic switch. A slight drop of the responsive threshold of the first transistor T.sub.2 initiates a spontaneous switching off of the resistor bridging. In order to be sure that the condenser C.sub.3 and the condenser C.sub.u (referred to further below) will in the intervals quickly discharge, there is provided a rectifier diode 23.

In the discussions so far the resistor R.sub.u and the condenser C.sub.u have not been mentioned. These circuit elements do not affect the function of the bridging circuit 25 of the three transistors but merely serve for a timewise overlapping of the turning-on periods when shifting from coil 3 to coil 4. For purposes of explaining such an intended overlapping of the velocity ranges, it may be mentioned that the response of the bridging circuit with the three transistors is considerably faster than the dropping off of the magnetic core in the magnetic valve.

Before, following the opening of the left contact of the switch 16, the movable core of the magnetic valve 3 moves away from the remaining fixed iron mantle to any material extent so that a larger airgap forms, the resistor R.sub.v2 has already been bridged, and at the strip 2 there prevails the full network voltage. This means that when the resistor R.sub.u has been so dimensioned as to its magnitude as the resistor R.sub.v2, the coil 3 will be passed through by a current corresponding to the holding energization in spite of the turning off by means of switch 16, and the magnetic valve will remain in its previous working position as long as the resistor R.sub.v2 is bridged. When this bridging is interrupted, two resistors will be in the current-feeding line to the coil 3. In this way the voltage drop to the coil 3 becomes so high that the remaining voltage will permit only such small energizing current to pass through the coil 3 that the magnetic filed created thereby will not be able any longer to hold the magnetic valve so that the latter will return to its rest position. The time up to the point when the bridging of the resistor R.sub.v2 is again interrupted when the switch 16 is switched from the left to the right, i.e. until the magnetic valve of coil 3 returns to its rest position, is controlled by the magnitude of the condenser C.sub.u. This time corresponds to the switching on overlapping time of the magnetic valves when changing from coil 3 to coil 4. At the time at which the right-hand contact of the switch closes, the condenser C.sub.u will be electrically parallel to the condenser C.sub.3. As a result thereof, the time constant of the R-C-member is increased. By correspondingly dimensioning the condenser C.sub.u, it is possible so to extend the common charging time of the two condensers of the new R-C-member that the overlapping time is obtained. Principally, the condenser C.sub.u could be eliminated and the condenser C.sub.3 could from the very start be made so large that its charging time corresponds to the overlapping time. This, however, has the slight drawback that with each change from one magnetic valve to another magnetic valve, the transistor T.sub.4 and naturally also the transistors T.sub.2 and T.sub.3 are under load for an unduly long time. If the switching operations are carried out particularly frequently, the relative switching on period of the transistors may become so high that in the switching intervals of the transistors, the latter will not be able any longer to cool off to the ambient temperature and might heat up to a nonpermissible extent. The intermediate switching on period is in view of the condenser C.sub.u reduced to the extent which is necessary under all circumstances because only during the changeover when an overlapping is required, namely when changing over from coil 3 to coil 4, does the switching non period of the transistor become relatively long, for instance 0.3 seconds, and otherwise is short, for instance 20 milliseconds.

The circuit according to FIG. 2 also shows a coil 7 pertaining to a magnetic valve which coil is energized without a previous turning off of another current consumer. In order nevertheless to be able to cause a slight current to flow through the bases emitter section of the entrance transistor T.sub.2, which slight current will initiate the bridging circuit, there is provided a condenser C.sub.4 which, when the switch 18 is closed, is electrically parallel to the condenser C.sub.3. The condenser C.sub.4 has a capacity which is by a plurality of orders of magnitude smaller than the capacity of the condenser C.sub.3. In order to make sure that the charging current will actually flow through the base emitter section of the entrance transistor and not through the condenser C.sub.3, a rectifier diode 26 is provided which blocks toward the condenser C.sub.3 and is arranged between the terminal 24 of the condenser C.sub.4 and the condenser C.sub.3. Following the closure of switch 18, a low-charging current flows which in a manner known per se initiates the action of the bridging circuit 25. When the coil 7 is by means of the switch 18 again separated from the current supply, an increase in the potential occurs which will initiate the bridging circuit 25 and thus will briefly overenergize the remaining energized coils. This, however, will do no harm.

As will be evident from the above, the present invention solves an energy problem which is closely related to space and weight. It is well known that magnetic valves are to be dimensioned more or less strong depending on the required relative duration during which they have to be effective so as to meet the heating up which increases with the length of time during which such magnetic valves are in action. A lifting magnet for a 15 percent relative duration of action is considerably smaller, lighter and less expensive than a lifting magnet of the same strength for a 100 percent relative duration of action. These differences in magnitude are purely thermally required. Decisive is not the duration of its active period but the electric energy introduced into the lifting magnet per time unit. The present invention has drawn the conclusion of this finding and accordingly the coil is actuated only during the brief moment during which a strong excitation of the magnetic flux is necessary in a shocklike manner with a high power. In the remaining time, the energizing current or the power is reduced to the extent which, with the now closed iron yoke of the lifting magnet, is necessary for holding the core. If magnetic valves are employed, 2 percent of the attraction force will suffice. This means that a magnetic valve controlled as to the exciter current in conformity with the present invention with a 100-percent active duration as seen from a time standpoint has to be only as large as a magnetic valve actuated according to standard methods and need be designed only for a 2-percent effective duration because the power input and the heating up will in both instances be the same. In this way, with the higher necessary active periods, reductions in the overall volume and in the price and weight are possible to from one-tenth to one-twentieth with respect to magnetic valves of heretofore known control circuits.

The advantages realized by the present invention are seen in that the magnetic coils provided for high-medium time periods of active duty can be designed as small, light and inexpensive as magnetic coils for extremely low-medium active duty while the number of elements is relatively low and while such elements can be inexpensive and mass produced. The expenses for the circuits are less than the price difference of a magnetic valve for a 100-percent relative active duty of standard operation (100-percent relative active duty means when in one cycle the inertia temperature is reached) in contrast to an operation for only 5-percent active duty which means that the expenses for the circuit pay for themselves even if only one single magnetic valve is provided, aside from space and weight advantages. The saving is considerable in connection with electrohydraulic control installations or with other installations employing lifting magnets in which a plurality of lifting magnets are provided. A saving can also be realized when the relative duration during which the magnetic valves are effective is only short, for instance when it amounts only to 10 percent, because the input of power with the operation according to the invention is still less, and the coils may when taking advantage of the present invention also thermally be designed smaller than with the customary way of operation even if the valves are made effective only for an effective duration of 10 percent. It is, of course, to be understood that the present invention is, by no means, limited to the particular showing in the drawing but also comprises many modifications within the scope of the appended claims.

Claims

I claim:

1. In a direct current system for controlling the supply of energy to the coil of electromagnetic means having moveable armature means, a supply of direct current voltage, a voltage-dropping resistor in series with said coil means across said supply, a normally open line bypassing said voltage-dropping resistor, a voltage sensitive control element in said line responsive to a supply of voltage thereto to bring about closing of said line, switch means between said voltage dropping resistor and said coil means adapted to be actuated between opened and closed positions, circuit means including delay elements in the form of a first resistor and a first capacitor in series therewith operatively connecting said switch means to said control element and operable in response to actuation of said switch means to supply voltage to said control element for a predetermined period of time commencing with the actuation of said switch means, said period of time being determined by the value of said delay elements, said control element having a connection to the said first resistor near the end thereof opposite said first capacitor.

2. A direct current system according to claim 1, in which said control element includes transistor means including at least one transistor having the base thereof connected to the said resistor near the end thereof opposite to the capacitor.

3. A direct current system according to claim 2, in which said transistor means further includes a power transistor having its collector-emitter connected in parallel with said voltage-dropping resistor, and an amplifying transistor having its collector-emitter circuit connected to the base of said power transistor and having its base connected to the collector-emitter circuit of said one transistor.

4. A direct current system according to claim 3, in which said coil means comprise the coils of a plurality of magnetic valves for a multispeed transmission, each coil having one end connected to the negative side of said source and the other end connected to a respective switch contact, a bus bar, switchblades connected to the bus bar and adapted to close on said contacts, said voltage-dropping resistor connecting said bus bar to the positive side of said source, said first capacitor and first resistor being connected in series in the order named between said bus bar and the negative side of said source, a first PNP transistor having the base thereof connected to the said resistor near the end thereof opposite to the capacitor, a power transistor having its collector-emitter path connected between said bus bar and the positive side of said source, and an amplifying transistor connected between the collector-emitter path of said first transistor and the base of said power transistor and operable to make the latter conductive when said first transistor goes conductive.

5. A direct current system according to claim 4, in which, during a transmission shifting operation, at least one coil has the current supply thereto interrupted and thereafter at least one other coil has the current supply thereto established.

6. A direct current system according to claim 4, in which the time constant of the serially arranged first capacitor and first resistor is selected to provide a delay corresponding to the time required to attract an armature to attracted position.

7. A direct current system according to claim 4, which includes a further resistor between said bus bar and the said contact pertaining to one of said coils, said further resistor having about the same ohmic resistance as said voltage-dropping resistor, and the time constant of said serially arranged first capacitor and first resistor being selected to provide a delay greater than the time required to attract an armature to attracted position.

8. A direct current system according to claim 4, which includes a further resistor between said bus bar and the said contact pertaining to one of said coils, said further resistor having about the same ohmic resistance as said voltage-dropping resistor, a second contact on which the switchblade pertaining to the last-mentioned contact is adapted to close, and a further capacitor connected between said second contact and the resistor side of said first capacitor, the time constant of said first capacitor and first resistor alone providing for a delay no greater than the time for an armature to move to attracted position and the time constant thereof when said switchblade is closed on said second contact so as to connect said further capacitor in circuit with said first capacitor providing for a delay greater than the time for an armature to move to attracted position.

9. A direct current system according to claim 8, which includes a further coil having a respectively said contact and a further coil having a respective said contact and a respective switchblade connected to said bus bar and closeable on the contact, a third capacitor connected between the last-mentioned contact and said first resistor near the capacitor end thereof, and a diode poled toward said first resistor and interposed between said first capacitor and the connecting point of said third resistor to the capacitor end of said first resistor.

10. A direct current system according to claim 4, which includes a diode connected from the negative side of said source to the resistor side of said first capacitor and poled toward said first capacitor.

11. A circuit for controlling the supply of energy to the coil of electromagnetic means having a moveable armature; a direct current voltage source, a first resistor and a first switch in series with said coil across said source, a second switch actuatable to bypass said first resistor, said circuit also comprising a delay circuit for controlling said second switch, said delay circuit comprising a condenser and a second resistor in series therewith, a transistor having the collector-emitter path connected in controlling relation to said second switch, said capacitor and said second resistor and the base emitter path of said transistor forming a series circuit connected in parallel with said coil, said transistor being operable upon conduction of said collector-emitter path to actuate said second switch.

12. A circuit according to claim 11 in which said transistor is the input stage of a current amplifier circuit, said current amplifier circuit also including an output transistor having its collector-emitter path connected in parallel with said first resistor, an amplifier transistor having the collector-emitter path connected to the base of said output transistor and having a base connected to the collector-emitter path of said first mentioned transistor.