GB736661A - Improvements in and relating to electric timers - Google Patents

Abstract

736,661. Control systems for resistance welders; condenser arrangements. GENERAL ELECTRIC CO. Sept. 24, 1953 [Sept. 25, 1952], No. 26364/53. Class 38 (4). An electrical timing circuit, which is largely ,independent of supply voltage variations and which is particularly suitable for controlling the firing valves 6, 7 associated with resistance welding ignitrons 8, 9, comprises condensers 2, 3 which are charged with different quantities of electricity and then allowed to discharge into one another through a timing resistor 86 in a local circuit in which the voltages of the condensers are initially additive, during which period the voltage of the condenser of lesser charge, say 3, first decays to zero and subsequently builds up with reversed polarity. At some point in this period the voltage across condenser 3 becomes sufficient to block a valve 4 which then ceases to supply firing pulses to the valves 6, 7 so thus terminate the flow of welding current which was initiated by conduction of valve 4 upon charging of the condensers. Upon closure of an initiating switch 99, a valve 1, supplied from a transformer 44 and previously maintained non-conducting by negative bias obtained from a resistor 76, is fired by a pulse obtained over condenser 100, causing condenser 2 to charge over diodes 102, 104 and condenser 3 to charge over diode 103, terminal 111 of condenser going positive and so opening valve 4, which through transformer 36 annuls a holdoff bias on the firing valve 6 which subsequently conducts under the control of a voltage obtained from a phase-shifting circuit 57 and initiates flow of welding current. A winding 71 on transformer 36 applies a " turn-on " pulse to a further valve 5 which, through a transformer 47, causes firing valve 7 to trail valve 6. Valve 1 is again rendered non-conducting (and thus terminate charging of condensers 2, 3) when valve 4 conducts, by a negative pulse obtained from a winding 106 on transformer 36 and applied to condenser 100 over diode 108. Condenser 2 now commences to discharge into- and eventually reverses the polarity of -condenser 3 over timing resistor 86 and a relatively low valued resistor 89. The grid of valve 4 thus moves negatively and the valve eventually cuts-off and terminates the weld. The screens of valves 1, 4 are supplied with phase-shifted voltages which permit conduction to be initiated by the control grids only in the early portion of the supply voltage cycle. During stand-by periods, a charging circuit comprising diode 87 and resistor 88 maintains terminal 111 of condenser 3 at a negative potential to prevent conduction of valve 4. Similarly, if during a welding operation the switch 99 is released, negative bias is immediately applied to valve 4 over diode 109 to terminate conduction. The excitation arrangements for the firing-valves 6, 7 are as disclosed in Specification 659,996. A potentiometer 98 controls the relative voltages to which the condensers 2, 3 are charged and allows the use of condensers of equal capacity.