GB2095065A

GB2095065A - Solenoid control circuit

Info

Publication number: GB2095065A
Application number: GB8205996A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1981-03-12
Filing date: 1982-03-02
Publication date: 1982-09-22

Abstract

A circuit for controlling the operation of an electromagnetic device which includes a solenoid and an armature, includes a transistor switch 13 connected in series with the solenoid 12 and the d.c. supply terminals, a Zener diode 16 connected in parallel with the switch 13 and a solid state switch means 14, 15 connected in parallel with the solenoid. The operation of the switch means and the transistor switch are controlled by a control circuit 17. Switch 13 is opened and switch means 14, 15 is closed to maintain current flow in the solenoid, the switch 13 being pulsed to maintain an average current flow. When it is required to de-energise the solenoid the switch and the switch means are both opened and the Zener diode conducts to protect the switch 13 from high voltage induced in the solenoid as well as dissipating the stored energy of the magnetic circuit of the solenoid. <IMAGE>

Description

SPECIFICATION Control circuits This invention relates to a circuit for controlling the operation of an electro-magnetic device which includes a solenoid and an armature.

Circuits are known in which a transistor operating as a switch, is connected in series with the solenoid across a source of dc supply. In order to protect the transistor it is known to connect a flywheel diode in parallel with the solenoid or to connect a Zener diode in parallel with the transistor. In the first case when the transistor ceases to conduct, the magnetic flux in the solenoid collapses and the diode conducts. This limits the voltage which is developed across the solenoid and hence protects the transistor. In addition, the action of the diode is to prolong the time during which current flow takes place in the solenoid. This is advantageous in some applications for example where in order to maintain the device in an energised state, current pulses are supplied to the solenoid.In such an application the power loss is low but when it is eventually required to de-energise the device, the slow rate at which the current decays means that an appreciable time lapses before the armature is released. In the second case when the transistor ceases to conduct the collapse of flux in the solenoid causes conduction of the Zener diode and most of the energy is dissipated in the Zener diode. The circuit is therefore less efficient because of the high power loss but it does have the advantage that when the transistor ceases to conduct the armature is released very quickly.

The object of the present invention is to provide a circuit for the purpose specified in a form in which the power loss is low and the armature can be released quickly.

According to the invention a circuit for controlling the operation of an electro-magnetic device of the kind specified comprises a transistor switch for connection in series with the solenoid and a source of dc supply, a solid state switch means connected in a series circuit including said solenoid, a Zener diode connected in parallel with said transistor switch and means for controlling the operation of the transistor switch and said switch means whereby when it is required to deenergise the device said transistor switch and said switch means are rendered non-conductive so that the Zener diode conducts and when it is required to maintain the device in an energised state, by switching the transistor switch at a high frequency, said switch means is rendered conductive so as to conduct in the period when the transistor switch is non-conductive.

The accompanying drawings show in Figures 1 and 3 examples of circuits in accordance with the invention and it will be seen that terminals 10, 11 are provided for connection to the positive and negative terminals of a dc supply. The solenoid 12 of the electromagnetic device has one end connected to terminal 10 and its other end connected to terminal 11 by way of the collector emitter path of a transistor switch indicated as a p-n-p transistor 1 3. In parallel with the solenoid 12 in Figure 1 is the series combination of a diode 14 having its cathode connected to the terminal 10, and a switch means shown as an n-p-n transistor 1 5. The transistor switch 13 has a Zener diode 1 6 connected in parallel therewith.A control circuit 1 7 is indicated for controlling the supply of current to the base of the transistor switch 13 and the transistor 1 5.

The three graphs in Figure 2 indicate the voltages or currents at various positions in the circuit.

The upper graph represents the voltage signal which is applied to the control circuit 1 7 and the middle graph indicates the signal applied to the transistor switch 1 3. As will be seen from the middle graph the control voltage applied to the transistor switch 13 is of a pulsed nature, the first pulse being of appreciable length in order to ensure a rapid build-up in the current flowing in the solenoid. Once the current has built-up then the transistor switch is turned off to allow the current to decay to a value which is above the minimum value necessary to maintain the electromagnetic device of which the solenoid 12 forms part, in an energised state. The transistor switch is then switched on and off at a high frequency to maintain this level of current.

The lower graph indicates the current flowing in the solenoid and it will be seen that when the transistor switch is turned off, the current decays in the solenoid at a relatively low rate. This is because the transistor 1 5 is in a conducting state so that the diode 14 is able to act in a well known manner, as a free wheel diode. When the input signal to the control circuit 1 7 ceases then the supply of base current to the transistor 1 5 is interrupted so that the diode 14 can no longer conduct. At the same time the transistor switch is rendered non-conductive and the collapse of flux in the solenoid causes conduction of Zener diode and the rate of current decay is appreciably higher so that the electro-magnetic device is deenergised very quickly.

The circuit as described therefore has the advantage that the power loss is relatively low during the period when the electro-magnetic device is held in its energised state but at the same time the device can be de-energised very quickly when required.

In the arrangement of Figure 3 the transistor 1 5 and the diode 14 are replaced by a thyristor 18 to which the circuit 17 supplies a gate signal when conduction of the thyristor in the same manner as the diode 14 is required. With this arrangement the gate signal must be removed from the thyristor in the off period before the last pulse or during the last pulse applied to the transistor switch 13. This is to ensure that the thyristor does not become conductive at the end of the last pulse applied to the transistor switch.

This is difficult to arrange and it in effect means that the transistor switch 1 3 must be turned on again in order to turn off the thyristor thus prolonging the time during which the device is energised. In order to overcome the difficulty the collector-emitter path of a further transistor 1 9 indicated in dotted outline in Figure 3, which need not be a power transistor, can be connected in parallel with the thyristor 1 8, the transistor 1 9 being supplied with the base current by the control circuit 1 7. The transistor is switched on for a short period when the input signal to the control circuit is removed.The conduction of the transistor 19 ensures that the anode-cathode current of the thyristor 1 8 is reduced to substantially zero so that in the absence of a gate signal the thyristor becomes non-conductive.

In the example described the current flowing in the solenoid winding is allowed to rise to a high value to achieve rapid operation of the device.

This method of operation will usually take the flux density in the magnetic core of the device beyond the knee in the magnetization curve of the material forming the core. The optimum flux density considered in terms of efficiency of power consumption is at the knee of the magnetization curve and hence in order to achieve increased efficiency, it is preferred to allow the current to build up to the value corresponding to the knee of the flux density curve, and then to pulse the current so that the flux density is maintained substantially at this value. During this time the armature of the device moves to its operated position ahd then the current can be reduced to the hold on value. This method of operation is seen in the graphs of Figure 4 which correspond to those of Figure 2. It should be mentioned that the pulse widths and slopes of current flow in Figure 2 and 4 are not strictly accurate in their representation.

As an alternative to the method of operation shown in Figure 4, the current can be progressively reduced from the initial high value to the hold on value preferably as a mirror of armature movement. It should also be noted that the transistors utilized in the circuits can be field effect transistors.

Claims

1. A circuit for controlling the operation of an electromagnetic device which includes a solenoid and an armature comprising a transistor switch for connection in series with the solenoid and a d.c. supply, a solid state switch means connected in a series circuit including said solenoid, a Zener diode connected in parallel with said transistor switch and means for controlling the operation of the transistor switch and said switch means whereby when it is required to de-energise the device said transistor switch and said switch means are rendered non-conductive so that the Zener diode conducts and when it is required to maintain the device in an energised state by switching the transistor switch at a high frequency, said switch means is rendered conductive so as to conduct in the period when the transistor switch is non-conductive.

2. A circuit according to Claim 1 in which saidsolid state switch means comprises a diode and a transistor switch connected in series.

3. A circuit according to Claim 1 in which said solid state switch means comprises a thyristor.

4. A circuit according to Claim 3 including a further transistor having its collector emitter path connected in parallel with the thyristor.

5. A circuit for controlling the operation of an electromagnetic device which includes a solenoid and an armature comprising the combination and arrangement of parts substantially as hereinbefore described with reference to the accompanying drawings.