MXPA99001246A - Circuit to control the application of electricity to a coil of an electrical current interruption apparatus - Google Patents

Abstract

The present invention relates to a control circuit (30) that drives the coil (20) of a contact device (10) in response to a control voltage being applied. The control circuit (30) includes a power source (31) that produces a regulated output voltage from the control voltage. The appearance of the regulated output voltage initiates a timer (34), which controls the width of the pulses produced by a PWM controller (33). The pulses control a transistor (Q2) that applies the control voltage to the coil (20). Initially, the pulses cause a high current to flow through the coil (20) to close the contact device (10) after a time interval, the timer (34) signals the PWM controller (33) to shorten the pulses which applies less current to the coil to keep the contact device closed. A return time circuit (36) provides a relatively low reverse voltage drop path in parallel with the coil (20) to maintain the electromagnetic field during the periods between the pulses. When the control voltage is removed to open the contact device (10), the return time circuit (36) provides a high reverse voltage drop path to quickly buffer the current of the coil.

Description

CIRCUIT TO CONTROL THE APPLICATION OF ELECTRICITY TO A COIL OF AN APPLIANCE OF INTERRUPTION OF ELECTRICAL CURRENT BACKGROUND OF THE INVENTION This invention relates to apparatuses, such as contact devices, for switching electric current; and more particularly with a control circuit for applying electricity to a coil in the apparatus, for opening and closing a set of switch contacts. The application of electricity to motors and other large loads is often controlled by the type of relay known as a contact device. The contact device has one or more sets of electrical switch contacts that are typically biased by spring to an open state. When a solenoid coil of the contact device is energized, an electromagnetic field is produced which forces the switch contacts to close. In this way, the contact device allows a relatively small current and voltage, applied to the coil, to switch a much larger current and / or voltage to the load. With some types of contact devices, a larger current is required to initially close the contacts, than is required after the same to maintain the contacts in the closed state. As a consequence, in some applications, such as a battery-powered unit, it is desirable to reduce the current of the coil after the contacts close, in order to conserve energy. One technique for controlling the current is to modulate the amplitude of the pulses of the electricity applied to the coil of the contact device, and to vary the duration of the pulses to alter the magnitude of the current applied to the coil. With pulse amplitude modulation, the energy stored in the coil can be used to produce "return time" current during the off period of each pulse cycle, in order to maintain the electromagnetic field that keeps the contacts closed . In this way a low impedance feedback path is established around the coil for this return time stream. However, this low impedance feedback path has the disadvantage of delaying the decrease of the electromagnetic field when the contacts are to be opened. This delays the physical separation of the contacts, and increases the arcing conditions between the separation contacts. In addition, external devices, such as transient suppressors, connected through the coil terminals of conventional contact devices, can adversely affect the speed at which the contacts open to shut off the load.

SUMMARY OF THE INVENTION A general objective of the present invention is to provide a control circuit for an electromagnetically operated current switching apparatus, a circuit which provides for the rapid reduction of the magnetic field of the coil during the shutdown of the load. Another object of the present invention is to provide an amplitude pulse modulation control circuit for the coil of the switching device, which provides a relatively low energy dissipation path through the coil, while energizing the control circuit , and a high dissipation path when the load is going to be turned off. Another objective is to provide a control circuit that minimizes the effects on the operation of the coil due to external devices connected to the terminals of the coil. These and other objects are satisfied by a control circuit for an electrical switching device having a set of contacts that are operated by an electromagnetic coil. The control circuit includes first and second input terminals for receiving a control signal for operating the electrical switching device. A first transistor has a conduction path connected in series with the electromagnetic coil, between the first and second control terminals. A controller applies a series of electric pulses to a control terminal of the first transistor to switch that transistor to a conductive state, and apply current pulses to the coil. The series of pulses has a first duty cycle for a previously defined period of time, after the application of the control signal to the first and second input terminals, and after that the series of pulses has a second cycle of work that results in less current flowing through the electromagnetic coil than the one that flowed during the previously defined time period. A return time circuit has a first diode and a second transistor connected in series, to provide a conductive path in parallel with the electromagnetic coil for the current produced in the electromagnetic coil, when the first transistor is non-conductive. The second transistor is biased to a first conductive state by the control signal. After the removal of the control signal from the first and second input terminals, the second transistor is biased to a second conductive state by the current produced in the electromagnetic coil, with the second conductive state being less conductive than the first conductive state . In this way, the first conductive state acts to maintain an electromagnetic field produced by the coil between the occurrences of the electrical impulses. The second conductive state produces a voltage drop in the path for the current produced in the electromagnetic coil, when it is desired to deactivate the switching device. This action dissipates significant energy to rapidly reduce the magnetic field stored in the coil, which results in the rapid opening of the switch contacts. Brief Description of the Drawings Figure 1 is a partial cut-away view of a contact device with which the present invention can be used. Figure 2 is a schematic circuit diagram of one embodiment of a control circuit, in accordance with the present invention. Detailed Description of the Invention With reference to Figure 1, a contact device 10 of a single electromagnetic pole has a plastic housing 12 with first and second power terminals 14 and 16. The first power terminal 14 is connected to a first stationary contact 15 attached to the housing, and the second power terminal 16 is connected to a second stationary contact 17. An electromagnetic solenoid 18 is embedded in cavities in the inner surfaces of the housing 12. The solenoid 18 has an annular coil 20 with a core 21 and an armature 22, located inside the central opening 24 of the coil. The armature 22 includes an arrow 26 which freely passes through the core 21, and is connected to a movable contact arm 28. When the coil 20 is energized with electric current, the armature 22 moves upwards, in the orientation shown in Figure 1, action forcing the movable contact arm 28 against the two stationary contacts 15 and 17, to complete a electrical path between the first and second power terminals 14 and 16. When the current is removed from the coil 20, a spring 29 forces the movable contact arm 28 away from the two stationary contacts 15 and 17, opening the electrical path. A contact device of this type is described in U.S. Patent No. 5,004,874, the disclosure of which is incorporated herein by reference. Inside the housing 12 of the contact device is an electrical circuit 30, which is shown in Figure 2, which controls the application of electricity to the coil 20. The user activates and deactivates the contact device 10 by applying and removing Direct current voltage through terminals 38 and 3L9 of the coil control. When activated, the control circuit 30 applies a series of direct current pulses to the coil, in order to close the contacts of the switching device 10. The amount of current that has to be applied to the coil 20 to move the movable contact arm 28 against the stationary contacts 15 and 17 is greater than the magnitude of the current that is required after the same to maintain the electrical path through of the contacts. As a consequence, the control circuit 30 applies pulses with relatively long duty cycles, in order to apply sufficient current through the coil 20, to close the contacts. After a previously defined period of time, which is long enough to ensure contact closure, the control circuit reduces the duty cycle and, consequently, the current of the coil to a lower level which is precisely sufficient to maintain the movable contact arm 28 against the stationary contacts 15 and 17. The control circuit 30 comprises a power supply section 31, an output booster section 32, a pulse width modulation (PWM) current controller 33, a timer 34, and a return / decrease time circuit 36. . The power supply 31 provides stable, regulated voltage to the timer 34 and the pulse width modulation current controller 33 over a wide range of input voltages (eg 10 vdc to 50 vdc). The positive control terminal 38 is coupled to the power supply input node 40 by the DI diode, the node 35 and a current limiting resistor Rl. The D2 Zener diode extends between the input node 40 and the earth, to provide protection against overvoltages of the power supply. The resistor R2 and the diode D3 Zener are connected in series between the input node 40 and the ground. The Zener D3 diode is the reference element of primary voltage that produces at its cathode some 8.4 volts nominal with respect to the earth, which is fed to the base of a Ql Darlington NPN transistor. The capacitor Cl couples the base of the transistor Ql to ground, as a noise filter, and also to delay the rate of rise of the voltage in the base during energization. This reduces the instantaneous driving current of ignition in the capacitors C2 and C3, reducing the voltage in those capacitors, as well as in the transistor Ql. The Ql Darlington transistor has an outlet connected to the input node 40, and an emitter coupled to a first node 42 of the power supply output. The transistor Ql acts as a current amplifier follower of the emitter, to provide regulated output voltage of nominally 7.2 vdc over a range of current loads, and over a wide input voltage range. The first output node 42 of the power supply 31 is connected by a decoupling diode D4 to a second output node 44 of the power supply 31. The second output node 44 is coupled to ground by capacitors C2 and C3 connected in parallel. The decoupling diode D4 feeds the regulated voltage to the voltage comparators in the timer 34 and the pulse width modulation current controller 33. Capacitor C2 acts as a filter element to maintain the voltage during brief interruptions of input energy and negative transient currents. A much smaller C3 capacitor is in parallel with capacitor C2 to provide more effective high frequency noise suppression. During the switching off of the contact device 10, the diode D4 prevents the reverse current of the capacitor C2 from flowing back to the first output node 42, and in other circuit stages that must be rapidly delayed to zero. This current cycle can adversely affect the operation of the return / decrease time circuit 36. The timer 34 controls the duration of time that the control circuit 30 sends the high level attraction current to the coil 20., for initially activating the contact device 10. A timer input node 52 is directly connected to the second node 44 for outputting the power supply 31. The diode D6 and the resistor R8 are connected in parallel between the input node 52 of the timer and an intermediate node 54 which is coupled by the capacitor C5 to ground. The intermediate node 54 is connected by the resistor R9 to the reversing input of a first voltage comparator 56. The non-inverting input of the first voltage comparator 56 is connected to the intermediate node of a voltage divider formed by the resistors. RIO and Rll connected in series, between the timer input node 52 and the ground, to form a reference voltage source.The output of the first voltage comparator 56 is connected to the input terminal 58 of the current controller 33. Pulse amplitude modulation The input terminal 58 is connected, by an acceleration resistor R12, to the second output 44 of the power supply 31. Since the output stage of the comparator is an open collector resistance R12, it becomes a current source inside the cathode of the diode D7 when the collector is off.The blocking diode D7 couples the input terminal 58 to the non-inverting input of a second voltage comparator 60. The non-inverting input is also connected, by the polarizing resistor R13, to the second output 44 of the power supply and to ground, via the resistor R14, thereby forming a reference voltage source. The resistor R17 is connected between the output of the second voltage comparator 60 and the non-inverting input, to provide hysteresis for the ON-OFF threshold of the comparator. The reversing input of the second voltage comparator 60 is connected, via the resistor R15, to the second end 61 of the coil 20 of the contact device, whose end is connected to ground by a low resistance current detecting resistor R16. The reversing input of the second voltage comparator 60 is also coupled to ground via the capacitor C6. The output of the second voltage comparator 60 is connected to the base of a transistor Q3 NPN in the output driver 32.

The base of the transistor Q3 is connected to the intermediate node 50 of another voltage divider formed by the resistors R6 and R7 which are connected in series between the second node 44 of the power supply outlet and the ground. The output driver 32 has a second Darlington transistor Q2, here a PNP type with an emitter connected to the input node 35, and a collector connected to a first end 47 of the coil 0 of the contact device. A D5 Zener diode is connected through the emitter-collector junction of transistor Q2 Darlington to provide overvoltage and transient protection, and a C4 capacitor couples the emitter to ground for noise suppression. A voltage splitter formed by the resistors R3 and R4 has one end connected to the input node 35 and an intermediate node 48 connected to the base of the transistor Q2. The other end of the voltage divider of R3 / R4 is connected to earth by the series connection of the collector-emitter path of transistor Q3 and resistor R5. Currently when transistor Q3 is in an ON state it operates in a current limiting mode. When its emitting current reaches a level such that the voltage drops through the resistor R5 approaches the level established in the base terminal by the resistor splitter R6 and R7 (minus the Vbe drop), the negative polarization of the The base self-limits itself, and the voltage drop from the collector to the emitter is adjusted to maintain the current at its level. This effect is desirable since the current that is drawn through the resistor R4 to drive the base of the transistor Q2 is constant no matter what the supply voltage is at the input 38. The return / decrease time circuit 36 has a input node 62 connected to the first output node 42 of the power supply 31. The input node 62 is connected via the emitter-collector conduction path of the transistor Q4 PNP, the diode D8 and the resistor R18, to an intermediate node 64. A voltage splitter formed by the resistors R19 and R20 is connected between the input node 62 and the earth, with an intermediate node 66 connected to the base of the transistor Q4. The intermediate node 64 of the return / decrease time circuit 36 is connected to the base of the Q5 Darlington transistor with its emitter connected to the first end of the coil 20 of the contact device and coupled by the resistor R21 to its base. The collector of transistor Q5 is connected by the DIO diode polarized inverse to the ground and to its base by the D9 Zener diode. When the control circuit 30 is energized by the application of voltage to control the terminals 38 and 39, the voltage across the capacitor C5 in the timer 34 initially is at a level of zero, which is coupled through R9 in the inversion input of the first voltage comparator 56. This results in the output of the first voltage comparator 56 being open, allowing by the same that the resistor R12 brings the node 58 to the regulated supply voltage. Under these conditions, the lower side of R12 is routed at node 58 through diode D7 in the voltage splitter of R13 / R14 of the second voltage comparator 60 in the pulse amplitude modulation current controller 33. This biases the reference input of that comparator 60 to a high level. With the comparators 60 in a high output state, the resistor R17 tends to attract the reference level up slightly, and the high output state also turns on the transistors Q3 and Q2. These transistors remain conductive until the voltage across the current sensing resistor R16 exceeds the reference voltage applied to the non-inverting input of the second voltage comparator 60, at which time the comparator output decreases. This action attracts one end of R17 down, which reduces the reference level in the non-inverting input of the comparator 60. This positive feedback around the comparator ensures the positive and rapid switching of the comparator. With the output of comparator 60 low, transistors Q3 and Q2 turn off. The transistors Q3 and Q2 are turned on again once the voltage across the current sensing resistor R16 drops below the reference voltage for the second voltage comparator 60. The resistor R17 is selected to provide a small amount of voltage hysteresis to the tip reference of the tipper at the comparator input. This differential establishes a slight difference in the current detection levels where the comparator is turned on and off. The level differences, in conjunction with the L / R ratio of the coil, and the time constant of the Resistor R15 and the capacitor C6 determine the actual operating frequency of the oscillation behavior of the pulse amplitude modulation, and the amount of fluctuation in the regulated current. Because the reference voltage produced by the timer 34 at the non-inverting input to the second voltage comparator 60 during this initial phase of the operation of the circuit is relatively high, the amplitude of the current pulses applied to the coil 20 are relatively long, resulting in a large initial coil current. As time passes, capacitor C5 is charged through resistor R9. When the voltage of the capacitor achieves the level at the non-inverting input of the first voltage comparator 56, the output of that latter device changes downward, diverting the current of R12 to the ground, nullifying the influence of the resistance R12 on the R13 / R14 voltage. This action removes the contact closure bias level at the non-inverting input of the second voltage comparator 60, causing the current applied to the coil 20 to be reduced to the lowest level, now determined only by the voltage divider of R13 / R14, level that is required to keep contacts 15, 17 and 28 closed. Specifically, a lower reference voltage is now applied to the non-inverting input of the second voltage comparator, which shortens the current pulses applied to the coil 20 by the switching action of transistors Q3 and Q2. This steady state condition will be maintained until the control circuit 30 is turned off by removing the positive voltage from the positive control terminal 38. During the shutdown of the control circuit 30, the voltage across the main power supply capacitors C2 and C3 rapidly decreases due to the load of the circuit. To ensure that the timing capacitor C5 is quickly discharged, the reverse diode D6 is included through the resistor R8, and becomes forward polarized when the supply voltage decreases below the charged level of C5. In this way the circuit "resets" itself quickly during shutdown, allowing the ignition timing to reoccur if it turns to apply power soon after shutdown. This situation would occur when an engine is controlled by the contact device 10 which is rapidly "folding" on and off. The current through the circuit 30 of the coil is regulated by means of rapidly switching (pressing) the output transistor Q2 on and off, and by changing the ratio of the on-off time, modulating the pulse amplitude ( PWM) by means of the same the current of the coil. The coil current is accurately detected and controlled during both the contact and closure attraction phases, with the ratio continuously adjusted to compensate for changes in supply voltage and variations in coil resistance. During the short "OFF" intervals of the pulse amplitude modulation between the current pulses, a smooth coil current flow is achieved, by providing a cycle of return time around the coil 20, through the diode DIO. During normal operation, the DIO diode is maintained in a low impedance cycle around the coil, by having the polarized transistor Q5 fully conductive through the power supply 31. This is achieved by the regulated voltage supply from the power supply node 42, through the transistor Q4, the diode D8, and the resistor R18 at the base of the transistor Q5. During the interval of the pulse width modulation between pulses, the polarity of the coil is reversed in an attempt to keep the current flowing in the same direction it was flowing during the ignition interval, in accordance with the Law of Lenz. In this way the diode DIO and the transistor Q5 are forward biased, and they conduct the return time current around the coil 20.

When the control circuit 30 is turned off, however, it is essential that this return time cycle quickly dissipates the energy of the stored coil, so that the opening movement of the contact device is not damped or delayed. When the voltage is removed from the positive control terminal 38, the negative bias of the base is removed in transistor Q5 of transistor Q4, diode D8 and resistor R18, which tends to turn off transistor Q5 and open the cycle of transistor Q5. return time. However, the reverse polarity generated by the coil now biases the base of transistor Q5, through the D9 Zener diode, forcing the collector to emitter voltage of the transistor to be set at this level. This voltage drop, in conjunction with the return time current, produces significant energy, thereby rapidly dissipating the field of the coil, which allows the contact device to open quickly. Therefore, the return time circuit 36 provides a double function, a low impedance return time cycle during the normal pulse amplitude modulation operation, and a power dissipator during shutdown. During this shutdown interval, transistor Q4 ensures that there is no leakage path to unintentionally provide a negative forward bias, within the base of transistor Q5. Since the polarity of the coil is reversed during this time, the emitter of the transistor Q5 is forced negative with respect to the ground, and any path from the ground to the positive control terminal 38 can provide a forward negative polarization, through of the transistors Ql and Q4, the resistor R18 and the diode D8 to the base of the transistor Q5, keeping it on, and passing over the negative polarization of the clamping voltage of the diode D9. During that situation, however, the base of transistor Q4 would be biased to off, opening its collector circuit, and ensuring that the cycle to the base of transistor Q5 is open. Diode D4 also prevents power supply capacitors C2 and C3 from being discharged again through transistor Q4 at the base of transistor Q5 during shutdown, since, otherwise, the negative polarization of these components could keep power on the transistor Q4. An important benefit of the present return / decrease time circuit 36 is that the coil 20 is provided with a controlled return time decrease cycle. Ordinarily, a coil of the contact device is connected directly through the control terminals 38 and 39, whereby the control power is turned on and off to energize the coil. When the control line is turned off, the energy of the coil is typically dissipated rapidly in the switch's arc. If some other load is also connected through this input, that is, in parallel with the coil, the energy of the coil can be dissipated more slowly during shutdown, in the form of a time-of-return current, through that other load. A common practice, although undesirable, when installing contact devices in applications, is to add a diode across the terminals of the coil to suppress any reverse voltage transients that the coil could print back on the control line. The prolonged decrease can dampen mechanical movement, delaying the contact separation, increasing the arc duration, resulting in increased contact damage and prolonging the time of maximum arc voltage accumulation for power interruption. In this way the higher voltage direct current levels can become particularly crucial. With this circuit, the return time energy of the coil is dissipated in an internal controlled cycle, and therefore it is not fed directly back through the input terminals, where those external loads could affect it. On the other hand, the present circuit prevents the transient voltages of the coil from being applied again on the user's control lines connected to the terminals 38 and 39, thereby eliminating the need for the suppressors described above.

Claims (15)

1. CLAIMS 1. A control circuit for an electrical interruption device having a set of contacts that are operated by an electromagnetic coil, said control circuit comprising: first and second input terminals to receive a control signal to operate the electrical interruption; a first transistor having a conduction path connected in series with the electromagnetic coil between the first and second control terminals, the first transistor having a control terminal; a controller that generates a series of pulses that are applied to the control terminal of the first transistor, where the series of pulses biases the first transistor to apply a first level of current to the electromagnetic coil for a defined period of time and then the first transistor is polarized to apply a second level of current to the electromagnetic coil, where the first level is greater than the second level; and a return / decay time circuit having a first diode and a second transistor connected in series and the electromagnetic coil, where the second transistor is biased by the control signal to provide a first voltage drop for the current produced by the electromagnetic coil during intervals between each pulse of the series of pulses, when the control signal is removed the second transistor is polarized to provide a second voltage drop for the current produced in the electromagnetic coil, in which the second voltage drop is greater than the first voltage drop.
2. 2. The control circuit as defined in claim 1, wherein the defined period of time begins upon application of the control signal to the first and second input terminals.
3. 3. The control circuit as defined in claim 1, wherein the series of pulses has a first duty cycle during the defined period of time and has a second duty cycle after the defined time period.
4. The control circuit as defined in claim 3, wherein the controller varies the first and second duty cycles to provide pre-defined first and second current levels, respectively, independently of the voltage changes of the control signal and resistance changes of the electromagnetic coil.
5. The control circuit as defined in claim 3, wherein the controller comprises: a timer that responds to the control signal by producing a timing signal after the defined period of time; and a pulse width modulation controller that produces the series of pulses, the duration of each pulse responding to the timing signal.
6. The control circuit as defined in claim 5, wherein the timer comprises: a source of a reference voltage; a resistor-capacitor network that produces a voltage that varies with time in response to the control signal; and a comparator that produces the timing signal in response to the voltage that varies with time having a pre-defined relationship with the reference voltage.
7. The control circuit as defined in claim 5, wherein the pulse width modulation controller comprises: a reference voltage source that varies in response to the timing signal; a current sensor that produces a sensor voltage that indicates the magnitude of the current flowing through the electromagnetic coil; and a comparator that produces the series of pulses in response to the sensor voltage having a pre-defined relationship with the reference voltage.
8. 8. The control circuit as defined in claim 1, wherein the return time circuit comprises a third transistor that is driven tornado by the application of the control signal and when it becomes conductive, biases the second transistor to the first state driver.
9. 9. The control circuit as defined in claim 1, where the return time circuit excites the second transistor to saturation to produce the first voltage drop. .
10. 10. The control circuit as defined in claim 1, wherein the second transistor produces a second fixed voltage drop.
11. The control circuit as defined in claim 1, wherein the return time circuit comprises: an input node coupled to the first input terminal; a pair of resistors connected in series between the input node and the second input terminal, and forming a first node between the pair of resistors; a third transistor having a conduction path and a control terminal coupled to the first node; a second diode; a first resistor, wherein the conduction path of the third transistor, the second diode and the first resistor are connected in series between the input node and a control electrode of the second transistor; a second resistor connected between the control electrode and one end of a conduction path of the second transistor, which one end is coupled to one side of the electromagnetic coil; and a Zener diode connected between the control electrode and another end of a conduction path of the second transistor, which one end is coupled to another side of the electromagnetic coil.
12. The control circuit as defined in claim 1, further comprising a power source connected to the first and second input terminals, and producing a regulated output voltage that is applied to the controller and the return time circuit for polarize the second transistor to the conductive state.
13. 13. A control circuit for an electrical interruption device, having a set of contacts that are operated by an electromagnetic coil, said control circuit comprising: first and second input terminals for receiving a control signal to operate the control device. electrical interruption; an energy source connected to the first and second input terminals and producing a regulated output voltage; a first transistor having a conduction path connected in series with the electromagnetic coil between the first and second control terminals, the first transistor having a control terminal; a timer having a first state for a defined period of time beginning upon the application of the control signal to the first and second input terminals and having a second state after the defined period of time; a pulse width modulator connected to the timer and comprising a source of a first reference voltage derived from the regulated output voltage where the first reference voltage is greater during the first state than during the second state of the timer, the width modulator of pulse having a current sensor that produces a sensor voltage in response to the magnitude of the current flowing through the electromagnetic coil, and a comparator that produces a polarization pulse that is applied to the control terminal to make the first driver transistor whenever the first reference voltage exceeds the sensor voltage; and a return / decay time circuit having a conduction path connected in parallel with the electromagnetic coil, the conduction path formed by a first diode and a second transistor connected in series, where the second transistor is biased by the voltage of regulated output to provide a first voltage drop for the current produced in the electromagnetic coil, and when the control signal is removed from the first and second input terminals, the second transistor is biased, to provide a second voltage drop where the second Voltage drop is greater than the first voltage drop.
14. The control circuit as defined in claim 13, wherein the return time circuit further comprises: an input node in which the regulated output voltage is received; a pair of resistors connected in series between the input node and the second input terminal, and forming a first node between the pair of resistors; a third transistor having a control terminal coupled to the first node and having a conduction path; a second diode; a first resistor, where the conduction path of the third transistor, the second diode and the first resistor are connected in series between the input node and a control electrode of the second transistor; a second resistor connected between the control electrode and one end of a conduction path of the second transistor, which one end is coupled to one side of the electromagnetic coil; and a Zener diode connected between the control electrode and another end of a conduction path of the second transistor, which other end is coupled to another side of the electromagnetic coil.
15. 15. The control circuit as defined in claim 13, wherein the timer comprises: a voltage divider that produces a second reference voltage from the regulated output voltage; a capacitor that is charged from the regulated output voltage to produce a voltage that varies with time; and a comparator that produces a timing signal that changes states in response to voltage that varies in time exceeding the second reference voltage.