GB1404192A - Switching apparatus - Google Patents

Abstract

1404192 Control of D.C. motors; switching systems for consuming apparatus ELECTRO MECHANICAL CONSTRUCTION Ltd 20 June 1973 [7 June 1972] 26461/72 Heading H2H and H2J [Also in Division H3] A load, for example a D.C. motor, is supplied from a D.C. source through a transistor TR1 and silicon controlled rectifier SCR1, the transistor being switched on and off by a signal including a sawtooth waveform to produce corresponding switching of the controlled rectifier. Transistor TR1 is normally switched off by a constant negative bias applied from resistor R3, a positive bias obtained from a resistor chain VR1, R4 being insufficient to override the negative bias even with resistor VR1 set to produce the maximum bias. The positive bias is augmented by the sawtooth waveform produced by the charge and discharge of a capacitor C4 obtained by use of a constant current circuit TR5, R10, R11, R12, ZD1 and a transistor TR6 periodically made conductive by pulses applied to its base from a multivibrator circuit (not shown). Thus the transistor TR1 is switched on for portions of the positive periods of the sawtooth adjustable by the resistor VR1, the conductive periods of the rectifier SCR1 being predetermined, relative but not absolutely to the conductive periods of the transistor, by appropriate selection of the impedance values of the networks C2, R1 and R2, D1 of which the former transmits a positive pulse to the gate of SCR1 via the capacitor C2, this potential being maintained when the capacitor C2 is fully charged by the resistor R1. Members R2, D1 determine the steady state potential applied to the gate of rectifier SCR1. The maximum current supplied to the load for a particular setting of the resistor VR1 is predetermined by selection of the values of resistors R1, R2, which determine the instant at which rectifier SCR1 starts to conduct after switch-on of the transistor TR1. Switch-off of the latter causes switch-off of rectifier SCR1 due to the absence of a sufficient holding current. A safety circuit comprising members TR2, TR3, R5, R6, R7, R9, R14 D2 D3 D4 prevents the rectifier SCR1 being turned on when the transistor TR1 is made conductive, the abovementioned multivibrator circuit supplying pulses to the bases of transistors TR2, TR3 when conduction of rectifier SCR1 is to be prevented. Application of the invention to switch two silicon controlled rectifiers alternatively in 180 degree phase relationship is envisaged.