Patent GB-712217-A (GB)
Title: Improvements relating to electric control circuits
IPC: G01R27/18
Filed: 1952-08-26
Abstract: 712,217. Thermionic relay circuits. GENERAL ELECTRIC CO. Aug. 26, 1952 [Aug. 31, 1951], No. 21468/52. Class 40 (6). [Also in Groups XXXV and XXXVI] In an electrical control apparatus comprising a pair of inversely connected discharge devices such as ignitrons in series with the circuit to be controlled; each such device is arranged to be triggered by its associated thermionic valve circuit normally biased to cut off, but driven to conductivity by alternate half cycles of alternating potential superimposed on the bias and so triggering its discharge device, and the respective alternating potentials are substantially in phase but adjustable in magnitude to ensure that the thermionic trigger circuit of one selected discharge device operates after that of the other discharge device, the time difference being determined by the difference between the magnitudes of the alternating potentials, and being always less than “ the period thereof. Fig. 1 shows apparatus for current impulse testing the insulation of the armature windings of D.C. machines wherein condenser 11 is negatively charged by halfwave rectifier 12 energized from the A.C. mains through adjustable auto-transformer 16 and mains transformer 17. The condenser is connected through two parallelled and inversely opposed ignitrons 18, 23 and through inductance 21 between adjacent armature commutator bars 2 and 3 separated by insulation 4 across which winding 1, and the deflection plates of a cathode ray oscillograph are connected. On ionizing ignitron 18 by pulsing its igniter electrode 19, condenser 11 discharges through the winding 1 and inductance 21 to set up a transient oscillation and for the C.R.O. to record a complete waveform determining the state of the winding and its insulation, a path must exist for the inverse or negative excursions of the transient wave, which is prepared by ionizing the inverse ignitron 23 by pulsing its control electrode 24 in advance of the control electrode of the discharge ignitron 18. Both ignitrons are controlled by similar firing circuits 25, 25a in which condensers 26, 26a are positively charged through limiting resistors 29, 29a and half wave rectifiers 27, 27a from mains transformers 28, 28a and these condensers are dischargeable through discharge polarizing reactors 40, 40a and the control valves 30, 30a to impulse the ignitron control electrodes 19, 24. The control valves are equally negatively biased by voltages derived across capacitors 33. 33a from halfwave rectifiers 34, 34a energized by section 32, 32a of the mains transformer secondaries and applied to the control electrodes 31, 31a through grid stabilizing resistances 36, 36a and the secondaries of transformer 38, 38a superimposing in-phase alternating voltages on the respective unidirectional bias voltages. Since transformer 38 is energized from an adjustable potentiometer across winding 32 and transformer 38a is energized direct from winding 32a the alternating voltage on the grid of valve 31a is always instantaneously greater than that on the grid of valve 31 at corresponding instants of a cycle, so that when the triggering circuits are set in operation for an insulation test, the grid of control valve 30a is always driven positive before the grid of control valve 30, so that valve 30a conducts and triggers ignitron 23 before valve 30 conducts to trigger ignitron 18, the time difference being always less than “ period of the alternating voltage. The ionization of the ignitrons persists for a sufficient time to provide a forward and reverse path for the transient oscillatory current in the test circuit until the latter has died away. The device is applicable to regulating the order of firing of triggered discharge tubes in control circuits in general.