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THEORY OF OPERATION
E-4 E-4
IDEALARC DC-655
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PROTECTIVE DEVICES AND
SHUTDOWN CIRCUITS
THERMAL PROTECTION
Two thermostats protect the DC-655 from excessive
operating temperatures. Excessive operating temper-
atures may be caused by a lack of cooling air or oper-
ating the machine beyond the duty cycle and output
rating. If excessive operating temperatures should
occur, a thermostat will open. This condition will be
indicated by the illumination of the yellow
Thermostatic Protection Light located on the case
front. The fan should continue to run, but the welding
output will be disabled until the machine cools and the
Thermostatic Protection Light is out.
OVER CURRENT PROTECTION
If the average output current exceeds 900 amps for
five to six seconds, the welding output will be dis-
abled. The control board LED 4 shutdown light will
turn on. The higher the current, the quicker the DC-
655 will shut down. If the average current exceeds
1200 amps, the shutdown will occur in 0.3 seconds.
The over current shutdown circuit can be reset by
opening the feeder gun trigger or switching the DC-
655 output terminal switch to the OFF position.
PEAK CURRENT PROTECTION
If the peak current exceeds 2500 amps, the welding
output will be disabled and the control board LED 4
shutdown light will turn on. The peak current shut-
down circuit can be reset by turning the DC-655 input
switch off.
REMOTE CONTROL FAULT PROTECTION
The remote control leads from the 14-pin receptacles
or terminal strip are protected against voltage faults to
the electrode circuit or auxiliary voltage supplies. If a
fault should occur, the welding output will be disabled
and the control board LED 4 shutdown light will turn
on. This fault protection shutdown circuit can be reset
by opening the feeder gun trigger or switching the DC-
655 output terminal switch to the OFF position. On
later production machines, LED 3 input shutdown light
will turn on and the input contactor will open.
SHORTED OUTPUT RECTIFIER FAULT
PROTECTION
If a short should occur across one of the SCRs, a
potentially hazardous AC voltage could appear across
the welding output terminals. If such a fault should
occur, the input contactor will open and remove the
input power to the primary of the main transformer.
Also, the control board LED 3 input shutdown light will
turn on. Welding and auxiliary output will not be pre-
sent, but the pilot light and control board power will be
present until the power switch is turned off.