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E-3E-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
RECONNECT
PANEL
OPTIONAL POWER
FACTOR
P
OLARITY
SWITCH
BYPASS BOARD
WORK
F
RONT
E
LECTRODE
R
EAR
E
LECTRODE
78 VAC
X1
X2
A
C
AC
DC
DC
SHUNT
CHOKE
H
IGH FREQUENCY
TRANSFORMER
HIGH VOLTAGE
TRANSFORMER
THERMOSTAT
115 VAC
115 VAC
115 VAC
FAN
GATE LEADS
FEEDBACK
GAS
SOLENOID
115 VAC
115 VAC
LED's
T
HERMOSTAT
20 VAC
20 VAC
CONTROL BOARD
B
ACKGROUND
CIRCUIT
FROM
CONTROL
BOARD
63.5 VAC
CORRECTION
CAPACITORS
REAR GANG
SCR
BRIDGE
ADVANCED CONTROL
PANEL RECEPTACLE
MINIMUM
OUTPUT
C
ONTROL
MAXIMUM
OUTPUT
CONTROL
PROTECTION
BOARD
REMOTE
RECEPTACLE
M
ETER
STICK
TIG
SWITCH
BALANCE
CONTROL
POST
FLOW
LOCAL/REMOTE
SWITCH