ESAB LHF 405 PIPEWELD, Service Manual

Page: 17 / 34

Edition 071024
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LH20/1:8 Synchronisation and thyristors firing
The synchronisation circuit consists of synchronised ramp generators,
comparators, and firing pulse forming circuit.
Signal from the synchronising winding goes via R66(53)(40) and diode limiter made
V26(23)(20) and V27(24)(21) diodes, and then through divider R67(54)(41),
R68(55)(42) to non--inverting input of the comparator A5b(A4b)(A3b). The
R69(56)(43), R70(57)(44), C34(26)(18), R71(58)(45) parts connected to the output
of this comparator make up synchronised ramp generator. The amplitude of the
ramp is adjusted by means of the R71(58)(45) potentiometer. Ramp voltage is
compared with the output voltage of the regulator in comparator A5c(A4c)(A3c).
Thus the firing phase shift is created. The signal from the comparator enables the
pulse generator A8(A7)(A6). Generator output controls the output driver made on
transistors V8(6)(4), V9(7)(5). Driver may be disabled by the positive signal on the
cathode of the V42(41)(40) ) in a case of overtemperature or the lost of
synchronisation. Shorting the R22 by setting switch ????? to SHORT makes the
thyristors driving angle more narrow thus decreases control range of output
voltage.