Patent GB-1193091-A (GB)
Title: Zero Crossing Switching of Semiconductor Controlled Rectifiers Supplying Non-Unity Power Factor Loads.
Inventor: WATROUS DONALD LELAND
IPC: H03K17/72
Filed: 1967-10-05
Abstract: 1,193,091. Triac control circuits. GENERAL ELECTRIC CO. 5 Oct., 1967 [6 Oct., 1966], No. 45597/67. Addition to 1,147,422. Heading H3T. [Also in Divisions G3 and H2] A semi-conductor controllable rectifier is controlled to supply complete half cycles of current to a non-unity power factor load by sensing the voltage across the rectifier to detect the rise consequent upon approach to the zero current (blocking) condition, the sensing signal produced by such detection being used to permit firing of the rectifier at the start of the next current half cycle, providing such firing is compatible with the requirements of a controlled variable or a pre-set regulated condition. In Fig. 2 the A.C. supply to an inductive load 12, 21 is controlled by a triac 11 whose voltage is sensed by a bridge 31-35 which derives a signal from transistor 35 when the triac starts to block, this condition producing a voltage rise due to the phase difference between the voltage and current waveforms Fig. 1 (not shown). The signal from transistor 35 causes turn-on of transistors 26, 27 and application of a firing signal to the triac from the D.C. supply 22, 23, 24. Should a switch 41 be closed however, the signal from transistor 35 is shunted by a transistor 37 made conductive by the switch closure and the triac is not fired for this and any subsequent half cycles corresponding to this switch condition. Instead of the overriding circuit 39, 41 shown, the control signal for transistor 37 may be derived from temperature, optical or pressure sensors. In a modification, Fig. 3 (not shown), a parallel RC circuit (40, 42) is connected between the triac and the sensing network to improve firing, the sensing network comprises two alternately-turned-on transistors (43, 44) and two diodes (45, 46), and a series RC circuit (47, 48) is connected between the network and the triac gate to cause a firing pulse of predetermined time to be generated, since the inclusion of the series circuit causes transistors 26, 27 and the appropriate transistor of the sensing network to act as a one-shot multivibrator. This modification is used to control an electric motor (12) providing temperature control for a refrigerator or air conditioning installation, the arrangement being such that transistor 37 is controlled by a differential transistor amplifier (53, 54) which includes thermistor (58) responsive to the controlled temperature and which operates as a two-state trigger circuit providing a hysteresis effect in the control operation. During start up of the circuit there is a possibility that conduction of transistors 26, 27 will prevent adequate charging of capacitor 24. Various solutions are proposed to prevent this occurrence and in one arrangement, Fig. 4 (not shown), an R.C. circuit (63, 64) is added to the modification of Fig. 3 to maintain the transistor 26 non-conducting until capacitor 24 is fully charged. In this arrangement the motor is of the capacitor start type and the series R.C. circuit (47, 48) is adjusted accordingly. Also the transistor 27 is omitted. The embodiment shown in Fig. 5 is similar in certain respects to Figs. 2, 3 and 4 but uses transistors 71, 72, interconnected in positive feedback, and a capacitor 76 to form a relaxation oscillator which turns on the transistor 26 when blocking of the triac renders transistors 35, 75 conductive, recharging of capacitor 76 also being effected by conduction of the transistor 26. The time constant of the oscillator comprising members 71, 72, 76 is such as to allow charging of capacitor 24 upon starting-up of the circuit. Overriding control of the circuit is provided by the differential amplifier comprising transistors 53A, 53B, 53C, 54 of which the former are connected to temperature-sensing networks including thermistors 83A, 83B, 83C and reference resistors 58A, 58B, 58C. Other types of signal sensors such as photo-cells or strain gauges may be used. In the described embodiment the thermistors 83A, 83B, 83C are thermally coupled with the windings of the motor 12 and prevent conduction of the triac should the windings become overheated, each thermistor controlling a respective one of the transistors 53A, 53B, 53C to exert an individual overriding control. Such control may be in addition to that exercised by a further thermistor sensing air temperature, when the motor is used for temperature control of a refrigerator or air conditioning plant.