E-3
E-3
PRECISION TIG 275
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OUTPUT RECTIFICATION,
CONTROL BOARD AND FEEDBACK
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 senses the status and settings of the various
operator controls such as the Stick/TIG switch, the
output controls, the remote control receptacle, the
local/remote switch, the balance control and the post-
flow control. Circuitry on the control board evaluates
these commands, compares them to the feedback
information received from the shunt and sends the
appropriate gate firing signals to the output SCR
bridge. The control board regulates the firing of the
output SCRs, which control the output of the machine.
See
SCR Operation
.
The control board also monitors the thermostats and
controls the gas solenoid, the thermal light, the high
voltage transformer and the cooling fan. The optional
advanced control panel also plugs into and interfaces
with the control board.
THEORY OF OPERATION
FIGURE E.3 – OUTPUT RECTIFICATION, CONTROL BOARD AND FEEDBACK
POWER
SWITCH
RE
CO
NN
EC
T
P
AN
EL
OPTIONAL POWER
FACTOR
POLARITY
SWITCH
BYPASS BOARD
WORK
FRONT
ELECTRODE
REAR
ELECTRODE
78
V
AC
X1
X2
AC
AC
DC
DC
SHUNT
CH
O
KE
HIGH FREQUENCY
TRANSFORMER
HIGH VOLTAGE
TRANSFORMER
THERMOSTAT
115 VAC
115 VAC
11
5 V
AC
FAN
GA
TE
LE
AD
S
FE
ED
BA
CK
GAS
SOLENOID
11
5 V
AC
11
5 V
AC
LED's
THERMOSTAT
20 VAC
20 VAC
CONTROL BOARD
BACKGROUND
CIRCUIT
FROM
CONTROL
BOARD
63
.5
VA
C
CORRECTION
CAPACITORS
REAR GANG
SCR
BRIDGE
ADVANCED CONTROL
PANEL RECEPTACLE
MINIMUM
OUTPUT
CONTROL
MAXIMUM
OUTPUT
CONTROL
PROTECTION
BOARD
REMOTE
RECEPTACLE
METER
STICK
TIG
SWITCH
BALANCE
CONTROL
POST
FLOW
LOCAL/REMOTE
SWITCH
