Lincoln Electric SQUARE WAVE TIG 355 Service Manual page 47

E-3
FIGURE E.3 - OUTPUT RECTIFICATION AND FEEDBACK CONTROL
HIGH FREQUENCY SPARK
ELECTRODE SENSE
PANEL
SWITCHES
HIGH VOLTAGE
115VAC
TRANSFORMER
REMOTE
CIRCUIT
FEED - THRU
RECEPTACLE
BOARD
GAS/WATER
SOLENOIDS
115VAC
INTERLOCK
FANS 115VAC
RECEPTACLE
INPUT
R
CONTACTOR
E
C
O
N
N
E
C
T
START/
STOP
PILOT
TRANSFORMER
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 control panels,
panel switches, and/or remote control with the shunt
feedback signal and electrode sense lead. The appro-
priate SCR gate firing commands are created by the
control board and sent to the power board where the
gate firing pulses are generated and applied through
the snubber board to the SCR bridge. The control
board controls the firing of the SCRs, which controls
the output of the machine. See SCR Operation. The
control board also powers and commands the display
board and the status board.
The electrode sense, the remote control and the trigger
signals are applied through the feed – thru board to the
control board.
THEORY OF OPERATION
UPPER LOWER
DISPLAY
CONTROL
CONTROL
BOARD
PANEL PANEL
SCR GATE COMMANDS
ELECTRODE
SENSE
15VDC
CONTROL
TRIGGER
BOARD
REMOTE INTERLOCK
COMMANDS
HI-FREQ/ WATER/GAS
COMMANDS
INTERLOCK
115VAC - HI-FREQ AND GAS/WATER
115VAC
CONTROL TRANSFORMER
115VAC
AC
AC
POWER
FACTOR
CAPACITORS
POLARITY
MAIN
TRANSFORMER
NOTE: Unshaded areas of Block Logic
Diagram are the subject of discussion
SQUARE WAVE TIG 355
STATUS
BOARD
POWER
BOARD
SNUBBER
BOARD
DC-
C
H
DC+
O
K
E
BY-PASS
BOARDS
SWITCH
SHUNT
E-3
WORK
ELECTRODE
HI-FREQ
TRANSFORMER