Lincoln Electric PRECISION TIG SVM186-A Service Manual page 35

E-3
POWER
SWITCH
115 VAC
Receptacle
Circuit Breaker
Protected
OUTPUT RECTIFICATION
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, AC balance control, pulse control, post-
flow control, or remote control with the feedback infor-
mation. 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 ther-
mostats, and also controls the gas solenoid valve, ther-
mal light, auto balance LED, pulse frequency LED, TIG
LED, stick LED, and the digital display.
THEORY OF OPERATION
FIGURE E.2 - Output Rectification & Feedback Control
MAIN
TRANSFORMER
INPUT
BYPASS
ASBLY
H1
X1
H2
X2
H3
18VAC
18VAC
115VAC
FAN
REMOTE
RECEPTACLE
AUTO
PULSE TIG STICK
BALANCE
FREQ. LED LED
LED
LED
AND
NOTE: Unshaded areas of Block Logic
Diagram are the subject of discussion
PRECISION TIG® 225
BY-PASS
BOARD
POLARITY
SWITCH
C
H
O
AC DC+
K
SCR
E
BRIDGE
HIGH VOLTAGE
SHUNT
TRANSFORMER
DC-
AC
F
E
E
D
B
A
C
K
115VAC
CONTROL
BOARD
THERMO-
THERMAL GAS OUTPUT
STATS
LIGHT VALVE CONTROL
AC
BALANCE
CONTROL
WORK
ELECTRODE
HI-FREQUENCY
TRANSFORMER
CIRCUIT
POWER
"ON"
DIGITAL
DISPLAY
MODE
SWITCH
PULSE POSTFLOW
CONTROL CONTROL
E-3