GB1213300A - Improvements relating to electrical control circuits for firing high power silicon controlled rectifiers - Google Patents

Abstract

1,213,300. Supply control systems. SOUTH LONDON ELECTRICAL EQUIPMENT CO. Ltd. 15 Aug., 1968 [15 Aug., 1967], No. 37518/67. Heading H2H. A control circuit arranged to cause the firing of a high power silicon controlled rectifier switch is connected to the gate of the rectifier and includes a further S.C.R. whose firing is arranged to be initiated by an impulse in its own gate circuit and includes a filter circuit, presenting low impedance to transients, connected across the gate circuit of the high power S.C.R. As shown identical circuits control the firing on alternate half cycles of high power silicon controlled rectifiers SCR3 and SCR4 to supply a welding transformer WT from mains L1, L2. The identical circuits are also fed from mains L1, L2 through a transformer T and each has a further silicon controlled rectifier SCR1 fired by narrow pulses across transient preventing resistor R1 and capacitor C1. Capacitor C3 and resistor R3 provide a voltage gradient limiter protecting SCR1. A Zener diode ZD1 limits the amplitude of pulses " X " from SCR1 to give waveform " Y " and resistor RS maintains potential across SCR1 if safety relay contacts CTA are open. Capacitor C5 is a transient suppressor. An indicator circuit (Fig. 2, not shown) may be connected across SCR3 and SCR4 and include a neon N1 which flashes on transients to indicate that full conduction has not been achieved.