ESAB LAW 510 Service Manual page 26

12
THYRISTOR CIRCUITS
The thyristor firing circuits consist of operational amplifiers IC7 and IC9. Their
inverting inputs are connected to the processor and their non- - inverting inputs are
connected to a fixed voltage of 1.35 V.
If the +5 V power supply is lost, potential divider R99 - - R100 pulls the input
voltage to the non- -inverting inputs down to - - 0.3 V, thus disabling the firing pulses.
Resistors R143 - - R148 limit the thyristor gate currents to about 1.3 A.
In order to make sure that the thyristors always fire, each firing pulse is duplicated.
The peak value of the firing voltage is about 22 V when the thyristors are
disconnected. Never disconnect individual thyristors, as this can unbalance the
thyristor bridge and destroy other thyristors.
If a thyristor fails, always check the gate pulses before starting up the new thyristors.
If a thyristor has short- -circuited, the other thyristors will also have been damaged,
which can reduce their lives. If one thyristor has failed, all should be replaced.
Checking the thyristors
Remove connector XS14 from the circuit board. Open the cover over the thyristors
(see item 401 on page 51). Remove the screws that connect the shunt (= the
cathode connection) to the thyristors and insert a piece of paper between the shunt
and the thyristor cathodes. Measure the resistance of the thyristors with a DMM.
The gate and cathode can be tested at connector XS14.
Make each measurement in both directions of polarity (i.e. reverse the test
connections from the DMM). The measured values must be in the range as shown
below, regardless of polarity.
S
S
S
claw0de1
Gate - - cathode: 5 - - 40 τ
Anode - - cathode: not less than 10 kτ
Anode - - gate: not less than 10 kτ
- - 26 - -
claw0e18