Lincoln Electric POWER MIG SVM170-A Service Manual page 35

E-3
FIGURE E.3 — OUTPUT RECTIFICATION AND FEEDBACK.
AUX
115 VAC
LINE
SWITCH
RECONNECT
TRANSFORMER
OUTPUT RECTIFICATION AND
FEEDBACK CONTROL
The AC output from the main transformer secondary
weld winding is rectified and controlled through the
SCR rectifier assembly. Output voltage and current are
sensed at the shunt and output terminals and are fed
back to the control board. The control board compares
THEORY OF OPERATION
GAS
MOTOR
SOLENOID
WIRE
SPEED
CONTROL
ARC
BOARD
VOLTAGE
RECTIFIER
DIODE
G
BRIDGE
A
T
30 VAC
E
SNUBBER
S
BOARD
I
G
N
A
L
POWER
ENHANCEMENT
CHOKE
MAIN
SCR
RECTIFIER
NOTE: Unshaded areas of Block Logic
Diagram are the subject of discussion
POWER MIG 255C
FAN
SPOOL
GUN WIRE
DRIVE
MOTOR
TACH
GUN TRIGGER
FEEDBACK
C OUTPUT
POSITIVE
F
A CHOKE
TERMINAL
E
P
E
A
D
C
B
I
A
T
C
O
NEGATIVE
K
R
TERMINAL
S
SHUNT
the commands of the ARC Voltage Control poten-
tiometer with the feedback signals. The appropriate
gate firing pulses are generated by the control board
and applied to the SCR rectifier assembly. The control
board controls the firing of the SCRs, thus controlling
the output of the machine. See SCR Operation. The
control board also powers and commands the gas
solenoid, fan motor, the wire drive motor, and spool
gun receptacle.
E-3