GB556521A - Improvements in electric valve circuits for controlling the supply of alternating current to a load circuit - Google Patents

Abstract

556,521. Control systems for resistance welders &c. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 2, 1942, No. 1345. Convention date, Feb. 3, 1941. [Class 38 (iv)] Thermionic oontrol.-Timed control of the number and current-intensity of cycles of A.C. supplied to a load, such as a transformer supplying a welding circuit 1, is obtained through reversely connected valves 4, 5, the second valve 5 being ignited in dependence on the operation of the first by means of a transformer 45 connected across the load circuit. The grid 8 of valve 5 is connected to a phase adjustable peaky wave transformer 36, supplying also the ignition voltage for the grid 8 of the valve 4, but an A.C. bias voltage from a transformer 36 prevents ignition until the collapse of flux in the transformer 45 has caused the ignition of an auxiliary valve 41 which, according to one arrangement, produces a D.C. voltage on a resistance 43 to annul the bias voltage. The first valve 4 may be ignited by moving a switch 17 to its lower contact, whereupon a valve 15, ignited by a peaky wave from a transformer 25 and supplied with D.C. causes a condenser 13 in a timing circuit 10 to charge ; at the same time the grid 8 of the valve 4 normally held by a tapping 34 so negative to the cathode 7 as to prevent the peaky wave from transformer 30 igniting the valve, is relatively raised in potential by the drop occurring in resistance 19 and the valve ignites every alternate half cycle until the condenser charge has sufficiently raised the potential of the cathode 7. The valves 4, 5 may be arranged to supply ignition current to ignitrons controlling the main circuit. In a modified arrangement both valves 4, 5 used to control ignitrons, have their grid potentials held down by an A.C. biasing transformer and, to ignite them, auxiliary valves are arranged to load the transformer winding 5 so as to reduce the respective bias potentials and permit the peaky wave to become effective. One auxiliary valve is controlled by the timing circuit 10 and the other by the transformer 45. Specifications 286,703, [Class 38 (ii)], and 491,771 are referred to.