Patent GB-948064-A (GB)
Title: A driving arrangement for motor vehicles comprising an electromagnetic clutch and a control device therefor
IPC: H02P29/00
Filed: 1960-07-13
Abstract: 948,064. Transistor switching circuits. ELEKTRO-MECHANIK. July 13, 1960 [March 23, 1960], No. 24417/60. Addition to 921,898. Heading H3T. [Also in Division F2] In the automatic clutch control of the parent Specification wherein a transducer 61 emits an electric signal which varies continuously with the speed of the driving engine and, above engine idling speed, varies the energization of a clutch winding 63, through a transistor amplifier 62, for smooth vehicle starting, automatic circuit means reduces clutch energization to prevent engine stalling at starting or low speeds due to low engine temperatures; adjustable automatic circuit means counteracts variation in transistor amplification due to variation in amplifier ambient temperature; adjustable circuit means effects variation in transistor amplification to suit the torque/ speed characteristics of various engines; and manually or automatically controlled circuit means prevents engine stalling at starting or low speeds due to low air pressure at high altitudes. The transducer 61 comprises a resistor 64b in series with an ignition contact breaker 64a and an ignition coil primary winding 64. The voltage derived from the resistor 64b the frequency of which is dependent on engine speed, controls, through a resistor 65, a transistor 66, the collector current of which flows through a variable resistor 67 and the primary winding of a transformer 68. The transformer 68 is so saturated at low ignition frequency that a low voltage appears at the secondary winding terminals. As ignition frequency increases transformer saturation decreases and the secondary voltage increases at the same rate. The secondary voltage is rectified by a circuit 69, is smoothed by a capacitor 70 and controls the transistor amplifier 62. A thermistor 71, the resistance of which drops with temperature rise, is secured to the engine and, with resistors 72, 73, 74, forms a Wheatstone bridge circuit, the centre branch of which comprises the emitter-to-base path of a transistor 75. When the engine is cold the thermistor 71 resistance is high, the transistor 75 is conducting, and D.C. flows through the resistor 73, the transistor 75, a resistor 76 and the primary winding of the transformer 68 so that a voltage appears at the transformer secondary terminals which is lower than that without the introduction of the thermistor 71 circuit. As engine temperature rises the thermistor 71 resistance decreases and the transistor 75 conductivity and D.C. transformer saturation decrease steadily to zero. The rectified transformer secondary current flows through a resistor 77 and a thermistor 78 and with the voltage derived across the latter it controls the amplifier 62 which comprises transistors 79, 80, 81. With the same transformer secondary current and with a rising ambient temperature of the transistors 79, 80, 81, a decreasing control voltage for the amplifier 62 is derived from the thermistor 78. Thus, at low temperatures a greater voltage drop at the thermistor 78 partly compensates, to an adjustable extent, the reduced amplification of the transistors 79, 80, 81. Upon control voltage drop at the thermistor 78 and a resistor 82, the transistor 79 becomes conducting, and current flowing through the emitter-to-collector transistor 79 path and a resistor 83 shifts the potential at the base of the transistor 80 from negative, built up through resistors 84, 83, to more positive values and gradually decreases the conductivity of the transistor 80. As a result the short circuit represented by the emitter-to-collector path of the transistor 80 disappears so that current flowing through a resistor 85, a diode 86 and a resistor 87 effects a negative potential at the base of the transistor 81 which becomes conducting and allows current flow through its emitter-to-collector path and through the winding 63. The diode 86, the threshold voltage of which is that at engine idling speed, blocks winding energization at or below idling speed. A diode 90 shunts voltage peaks occurring due to winding 63 self-inductance when the clutch current is suddenly switched off and thereby protects the transistor 81. Within the control range of the amplifier 62 the transistor 79 is controlled by a pulsatory D.C. from the rectifier 69. Thus the transistors 79, 80, 81 are operated by pulses and since they act as switches in synchronism with the ignition frequency and do not run continuously throughout their characteristic their power loss and thermal loading is reduced. Above a predetermined speed the transistor 79 becomes continually conducting and the amplifier then changes over to pure D.C. operation. A variable feed-back arrangement, comprising a variable resistor 88 and a capacitor 89 in series between the transistor 80 collector and the transistor 79 base, effects variation in amplification, to suit the torque/speed characteristics of various engines, whilst the amplifier is in pulsed operation, and ceases to be effective on change-over to pure D.C. at a predetermined speed. In low air pressure conditions, due to high altitude, a dashboard switch or an air pressure gauge saturates the transformer 68 or reduces the amplification in 62.