GB917896A - Improvements in or relating to devices for safeguarding against pulsating direct current a load to be fed with alternating current - Google Patents

Abstract

917,896. Control systems for consumption apparatus. PHILIPS ELECTRICAL INDUSTRIES Ltd. Sept. 23, 1960 [Sept. 28, 1959], No. 32775/60. Class 38 (4). In order to prevent a load supplied from an alternating current source over reversely connected discharge tubes from being subjected to currents having a direct current component, arrangements are made so that the tube which was conductive last in one operation does not begin a subsequent operation. The Figure shows an embodiment in which a welding transformer L is supplied during an even number of half-cycles from an A.C. supply over reversely connected discharge tubes 1, 2. A negative grid bias Vg prevents the ignition of a tube B 1 with the result that a tube B 3 is ignited and pulses from a transformer TP 1 cause tube B 2 to ignite whilst tube B 4 remains extinguished. Closure of a switch S short-circuits the grid bias Vg so that upon the occurrence of a positive voltage peak from the transformer TP 1 tube B 1 ignites whereupon main tube 1 ignites by the agency of transformer T 1 and tube B 3 extinguishes. Current then passes to the load L for a half-cycle of the supply producing by means of a transformer TO a positive voltage at the anode of tube Be which, upon the receipt of a grid voltage peak from a transformer TP 2 fires to produce a peak at the grid of tube B 4 , by way of transformer T 4 . Tube B 4 fires, extinguishing tube B 2 . At 180 degrees after the grid peak at B 1 a grid peak occurs at tube B 2 which re-ignites to fire the main tube 2 by way of a transformer T 2 and extinguish tube B 4 . Current then passes to the load for the next half-cycle of the supply, thereby producing by way of the transformer TO a positive voltage at the anode of tube B 5 which fires, similarly to the operation of tube B 6 . The firing of tube B 5 ignites tube B 3 extinguishing tube B 1 and hence the main tube 1. The operation will be repeated whilst the switch S is maintained closed and will be terminated by the opening of the switch S. At whatever moments the switch S is made or broken the main tube 1 will always ignite first and the main tube 2 last. Thus the transformer in the load L will never be magnetized by successive pulses of the same polarity. In another embodiment either tube B 1 or tube B 2 may be started first but in this case steps must be taken to ensure that the main tube which ended an operative period is not that which starts the next operative period. The system can be reset by a push-button Dr.