Output Rectification And Feedback Control - Lincoln Electric SQUARE WAVE TIG 175 Service Manual

E-3
FIGURE E.3 Control Board and Output Feedback Control.
INPUT
POWER
SWITCH
H1
H2
H3
FAN
RECEPTACLE
OUTPUT RECTIFICATION AND
FEEDBACK CONTROL
The AC output from the main transformer secondary
is rectified and controlled through the SCR bridge.
Output current is sensed at the shunt as a low voltage
signal and fed back to the control board. The control
board compares the commands of the mode switch,
output control or remote control with the feedback
information. The appropriate SCR gate firing signals
are created by the control board and sent to the SCR
bridge. The control board controls the firing of the
SCRs, which control the output of the machine. See
SCR Operation . The control board monitors the
thermostats, and also controls the gas solenoid valve,
thermal light and the power "ON" light.
NOTE: Unshaded areas of block logic diagrams are the subject of discussion.
THEORY OF OPERATION
MAIN
TRANSFORMER
X1
X2
18VAC
18VAC
115VAC
REMOTE
THERMAL
LIGHT
SQUARE WAVE TIG 175
BY-PASS
BOARD
POLARITY
SWITCH
C
H
O
AC DC+
K
SCR
E
BRIDGE
SHUNT
DC-
AC
F
E
E
D
B
A
C
K
CONTROL
BOARD
THERMO-
GAS OUTPUT
STATS
VALVE CONTROL
WORK
ELECTRODE
HI-FREQUENCY
TRANSFORMER
HIGH VOLTAGE
TRANSFORMER
CIRCUIT
115VAC
POWER
"ON"
LIGHT
MODE
SWITCH
E-3