Patent GB-919166-A (GB)
Title: Improvements in or relating to devices for gradually establishing an electrical current notably for controlling electromagnetic clutches
IPC: H02P29/00
Filed: 1959-06-04
Abstract: 919,166. Controlling clutches. REGIE NATIONALE DES USINES RENAULT. June 4, 1959 [June 28, 1958], No. 19211/59. Class 80 (2). [Also in Group XL (c)] An electric control circuit for a clutch in a motor vehicle &c., giving a gradual establishment of current and a quick cut-off, comprises a semi-conductor amplifier, circuit elements determining the time varying function, and a control switch. In a first embodiment (Fig. 1), the base circuit includes a variable control resistor 8 in parallel with a negative temperature coefficient resistor 9. When a switch 7 is closed, the bias on the base of the transistor increases as resistor 9 warms up, and increases the current through the clutch operating solenoid 2 in the collector circuit. The resistor 10 may also be of a non-linear type, notably one having a positive temperature coefficient. The resulting characteristic curve is of the form shown in Fig. 2, the exact slope depending on the value of resistor 8. In a second embodiment (Fig. 4), the resistor 9 is replaced by an inductor 11 having a long time constant, while the resistor 10 is omitted. A diode 12 is connected in parallel with the inductor to protect the transistor from overload when the switch 7 is opened. In the third embodiment (Fig. 7), the current rise in the coil 2 is established in response to the opening of the switch 7. With the switch closed, a capacitor 15 is charged providing a large bias limited by a Zener diode 14 on the base of transistor 24. The collector current is consequently high, thus making the bias on the transistor 3 low and limiting its collector current through the solenoid 2. When the switch 7 is opened, the capacitor discharges through the transistor 16, which maintains the discharge current constant, thus reducing the bias on a transistor 24 and consequently allowing the current through the solenoid 2 to increase. The slope of the current rise curve is controlled by a potentiometer 22. In a fourth embodiment (Fig. 9), a capacitor 15 is normally maintained discharged by contacts 7<SP>1</SP> of the switch engaging with a contact 28, and the battery supply is disconnected. When the contact 7<SP>1</SP> is switched to a contact 29, the capacitor 15 starts to charge, lowering the bias on transistor 24, which as before allows the current through the solenoid 2 to increase. The initial current through the solenoid is determined by a resistor 25, and a diode 12 is connected across the solenoid to limit overload current on switching off. The resistor 30 may have a high positive temperature coefficient such that the initial current is increased and then subsequently reduced as the resistor warms up, giving an initial pulse as shown in Fig. 11. Potentiometers 33, 34 may if desired be omitted. Further modifications of circuits in which the rise time is controlled by the charge of a capacitor are given in Fig. 12 (not shown), in which a further capacitor is used in place of the resistor 30, previously referred to, to give an initial current pulse, and in Fig. 15 (not shown), in which the amplifier comprises three transistors connected in emitter follower configuration, allowing for use of a lower valued capacitor for a given rise time. Generators of specific waveforms may, if required, be inserted in the circuit to modify the law of current variation through the solenoid 2, or voltage responsive resistors may be included for this purpose.