Lincoln Electric WIRE-MATIC SVM 117-A Service Manual page 47

E-2
OUTPUT RECTIFICATION AND FEEDBACK
MAIN
LINE
TRANSFORMER
SWITCH
RECONNECT
FAN
MOTORS
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 com-
pares the commands of the ARC Voltage Control
potentiometer with the feedback signals. The appro-
priate 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 con-
trolling the output of the machine.
Operation . The control board also powers and com-
mands the gas solenoid and the wire drive motor.
THEORY OF OPERATION
WIRE GAS
SPEED SOLENOID
ARC
VOLTAGE
CONTROL BOARD
G
A
T
E
S
I
G
N
A
L
SCR
RECTIFIER
Figure E.3 - Output Rectification and Feedback.
See SCR
NOTE: Unshaded areas of Block Logic
Diagram are the subject of discussion
WIRE-MATIC 250
WIRE
DRIVE
MOTOR
TACH
GUN TRIGGER AND THUMB SWITCH
FEEDBACK
POSITIVE
TERMINAL
CHOKE
C
A F
P E
A E
C D
I B
T A
O C
R K
NEGATIVE
S
TERMINAL
SHUNT
E-2