Protective Devices And Circuits - Lincoln Electric IDEALARC SVM 122-A Service Manual

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PROTECTIVE DEVICES AND
CIRCUITS
Two thermostats protect the DC-400 from excessive
operating temperatures and overload conditions.
Excessive operating temperatures may be caused by
insufficient cooling air or by operating the machine
beyond the duty cycle and output rating. The primary
thermostat, located on the nose of the bottom center
primary coil, will activate if the machine is overloaded.
The machine will not have output, the amber thermal
protection light will glow, and the fan will continue to
run.
The secondary thermostat, located on the lead con-
necting the secondary coils together, will open either
with an excessive overload or insufficient cooling. The
machine will not have output, the amber protection
light will be on, and the fan should continue to oper-
ate. When the thermostats reset, the amber protec-
tion light will be off.
THEORY OF OPERATION
Upon restart, if the fan does not turn or the air intake
louvers are obstructed, the input power must be
removed and the fan problem or air obstruction cor-
rected.
The DC-400 is also protected against overloads on the
SCR/diode bridge assembly through an electronic
protection circuit located on the control board. This
circuit senses an overload on the power source and
limits the output to 550 amps by phasing back the
SCRs.
A circuit is also provided to protect the circuitry from
accidental grounds.
leads (#75, #76 or #77) are accidentally connected to
the positive output welding lead, the DC-400 output
will be reduced to a low level, thus preventing damage
to the machine. If an erroneous connection is made
between the remote control leads and the negative
output welding lead, the control board self-restoring
fuses will blow, preventing damage to the machine.
IDEALARC DC-400
LINCOLN
ELECTRIC
If the external remote control
®
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