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THEORY OF OPERATION
E-4 E-4
DC-600
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PROTECTIVE DEVICES AND
SHUTDOWN CIRCUITS
THERMAL PROTECTION
This welder has thermostatic protection from excessive
duty cycles, overloads, loss of cooling, and high ambi-
ent temperatures. When the welder is subjected to an
overload or loss of cooling, a thermostat will open. The
input contactor will open and remain open until the
machine cools; the white POWER light stays illuminat-
ed. No welding is possible during this cool down peri-
od. The machine will reset automatically when the ther-
mostat cools.
OVER CURRENT PROTECTION
The power source is also protected against overcur-
rents in the SCR bridge assembly through an electron-
ic protection circuit. This circuit senses currents over
780 amps on the power source and opens the input
contactor should the overcurrent remain for a predeter-
mined time (the white POWER light stays illuminated).
The predetermined time varies with the amount of
overcurrent; the greater the overcurrent, the shorter
the time. The input contactor will remain open until the
power source is manually started by resetting the
POWER ON/OFF toggle switch
.
FIGURE E.2 - GENERAL DISCRIPTION
REMOTE CONTROL FAULT PROTECTION
The power source circuitry is protected from faults on
the control circuit leads (#75, 76, or 77). If any of these
leads become common with either of the output leads,
the DC-600 will either shut down completely (control
lead fault to Negative Output), or will remain at mini-
mum output (control lead fault to Positive Output) thus
preventing any damage to the DC-600. If DC-600 shuts
down, (input contactor opens and white POWER light
stays illuminated) it must be manually started by reset-
ting the POWER ON/OFF toggle switch.
CONTROL
TRANSFORMER
R
E
C
O
N
N
E
C
T
CONTACTOR
FAULTPROTECTION
115VAC
MAIN
TRANSFORMER
CONTROL
BOARD
SHUNT
OUTPUT
CONTROL
MODE
SWITCH
POSITIVE
TERMINAL
NEGATIVE
TERMINAL
LATCHING
RESISTOR
F
E
E
D
B
A
C
K
F
E
E
D
B
A
C
K
SCR BRIDGE
115VAC
42VAC
T
E
R
M
I
N
A
L
S
T
R
I
P
FIRING
BOARD
CR2
CR1
CHOKE
14-pin
Amphenol
115VAC
Receptacle
Fan
Motor
32VAC
32VAC
32VAC
SIGNAL
G
A
T
E
S
I
G
N
A
L
OUTPUT
CONTROL
Secondary
Thermostat
Primary
Theromstat
1CRCoil
w
Pilot
Light
•
•
OUTPUT
TERMINAL
SWITCH
POWER
SWITCH
•
TERMINAL
STRIP 1
OUTPUT
CONTROL
SWITCH
2
115VAC
NOTE: Unshaded areas of Block Logic
Diagram are the subject of discussion