GB593587A - Improvements in or relating to electrical apparatus for energising a load circuit for a timed period - Google Patents

Abstract

593,587. Supply systems for consumption apparatus. IGRANIC ELECTRIC CO., Ltd., and FIELD, J. A. April 18, 1945, No. 9690. [Class 38 (iv)] In an electrical system for energizing a load such as a resistance welding device for a period dependent upon the time taken to charge a condenser, the timed period is automatically prolonged or shortened upon a decrease or increase, respectively, in the voltage supplying the load, which is energized 'upon closure of contacts RC1 associated with a relay coil Rw. The relay is energized to initiate the timed period, upon the closure of starting switch s, during the charging period of condensers C1 and C2 connected across the output of a rectifier t energized from the A.C. supply o to which the load is also connected. Normally open relay contacts RC2 close to short-circuit switch s and maintain the charge on condensers C1 and C2, normally closed contacts RC4 open to remove the short-circuit on condenser C3 and normally open contacts RC3 close to complete a charging circuit for this condenser through the resistance network r2, r6, r5, r1 and r7, which condenser builds up a biassing voltage for the normally conducting valve v through which the circuit of the relay coil Rw is maintained subsequent to its initial energization. When the valve becomes less conducting, due to sufficient " build-up " on the grid, contacts RC1, RC2, RC3 and RC4 drop out, condensers C1 and C2 discharge through resistance r3 and relay coil Rw and condenser C3 discharges over contacts RC4. The combination of neon tube n and resistance r4 maintain voltage on the screen of valve v almost constant within a supply voltage range of Π10 per cent so that whilst the anode current of the valve is almost independent of anode voltage, this current varies inversely as the control grid voltage. The timed period is variable between a minimum and a maximum by variable rheostats r1 and r2 and resistance r3 and winding Rw are of such a value that condensers C1, C2 are discharged in about <1>/10 second.