GB621660A - Improvements in or relating to electronic timing control apparatus - Google Patents

Abstract

621,660. Switching systems for resistance welding and the like. WESTINGHOUSE ELECTRIC INTERNATIONAL CO. Feb. 25, 1947, No. 5433. Convention date, Jan. 18, 1946. [Class 38 (iv)] Timing apparatus, particularly for allowing energization of a resistance welding transformer 7 for a predetermined and adjustable number of cycles from a supply 13 through back-toback discharge devices 1, 3, comprises a starting valve 49 in series with a resistor 53 which controls the ignition of the leading discharge device 1 and which is shunted by a " stopping " valve 109. The last is normally maintained non-conducting by grid bias 117, but is rendered conducting to terminate ignition of discharge device 1 by a voltage which opposes the bias and which builds up on a timing condenser 61 over a rectifier 57 and adjustable resistor 59 when the starting valve is conducting. Valve 49, which is energized from a transformer 45, is normally held off by a grid voltage in phase opposition to its anode voltage, but upon closure of an initiating switch 123, a relay 77 connects a condenser 73 to the grid excitation winding of the transformer, shifting the phase of the grid voltage about 30 degrees and allowing the valve to conduct. The relay also disconnects a discharge resistor 125 from condenser 61 which then changes over the valve 49. Valve 109 is eventually fixed to short-circuit resistor 53 (which has previously been carrying most of the current passed by 4a) when (a) condenser 61 reaches a voltage approaching that of the bias 117 and (b) a hold-off A.C. voltage applied to a second grid 119 from transformer 45 passes through zero. The control voltages for the ignition grid of discharge device 1 comprise(1) an A.C. hold-off component derived from tapping 56 of transformer 45, (2) an inverted full-wave rectified component tending to fire the device and provided by rectifier 95 fed from the source 13 over transformer 97 and phase-shiftng network 105, 107, and (3) a unidirectional component derived from the passage of current through resistor 53 when valve 49 is conducting. The last component opposes the first and allows the second component to fire the device 1 each cycle. Upon conduction of stopping valve 109, the third component of control voltage disappears and the second component is unable to maintain device 1 conducting. The trailing discharge device 3 is normally maintained non-conducting by grid-leak bias provided by transformer 33 and grid-leak and condenser 29, 27. A transformer 37, energized from the welding transformer, reduces the grid excitation of device 3 when device 1 passes current and so allows the former to trail the latter.