GB495700A - Improvements in and relating to electric timing circuits - Google Patents

Abstract

495,700. Valve circuits. BRITISH THOMSON-HOUSTON CO., Ltd. March 16, 1937, No. 7712. Convention date, March 16, 1936. [Class 40 (v)] An electrical impulse timing circuit suitable for electric welding comprises a circuit of large time constant including a condenser, charged from a supply source through a resistance, and a second circuit of smaller time constant energized from the first to provide the timing impulse. The first circuit of large time constant comprises condenser 13 charged from the D.C. source 8 through resistance 18. Across resistance 18 is shunted the second circuit 21, 22 of small time constant and which controls the grid bias and operation of the impulse valve 1, which is normally rendered inoperative by negative bias from tapping 12 on potentiometer 10, 11. At the beginning of an operating cycle, condenser 13 charges up from source 8, the valve 14 being biassed to cut-off by the potential drop across resistance 18. When the charging current falls valve 14 becomes conductive and discharges condenser 13 ; inductance 20 limits the discharge current and also imposes a transient negative potential on the anode of 14 to paralyse the valve at the end of the discharge period. Curve A of Fig. 2 shows the potential variations at terminal 32. Curve B is the voltage impressed on the control electrode of valve 1 when switch 18 is on contact 29, while C represents the steady negative bias applied to valve 1 from the tapping 12. Condenser 13 discharges during interval o-x, and condenser 22 charges during the subsequent interval x-z. During x-n, 13 is again charging, while 22 discharges in circuit 18, 21 during z-n. The period x-y of curve A is that during which the potential of terminal 32, and therefore the control grid of valve 1, is positive. Curve E is an alternating current of peaky waveform injected into the control grid circuit of valve 14 by transformer 24, which may be fed from the supply mains 5 ; curve F is the waveform of the supply 5 and G represents the impulses generated across load resistance 6 during every interval o-x. During the interval x-n the valve 14 is not conductive, until the positive cathode potential B has fallen to such an extent that the peaks E are effective to trigger the valve into operation. Curve H represents the current in the D.C. circuit 6 when the grid of valve 1 is connected to 31 by switch 28. The period between successive energizations of the welding circuit may be varied by varying resistance 18 while the length of the energization time is adjusted by resistance 21. Specification 416,374 is referred to. Reference has been directed by the Comptroller to British Specifications 341,801, [Group XXXVII], and 426,201, and U.S.A. Specification 1,936,789.