Lincoln Electric SQUARE WAVE TIG 355 Service Manual page 50

E-6
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
AC WELDING OUTPUT
Rotating the polarity switch to the AC position changes
the welding power circuit. One lead (X2) of the main
transformer secondary is connected to the machine
output work terminal. The other secondary lead (X1) is
connected to one of the AC connections on the SCR
bridge. The electrode terminal is connected to the
FIGURE E.6a – SCR BRIDGE AND CHOKE CONFIGURATION FOR AC WELDING
X1
X2
THEORY OF OPERATION
FIGURE E.6 - AC WELDING OUTPUT CIRCUIT
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
other AC side of the bridge. The choke is now electri-
cally across the negative and positive SCR bridge con-
nections. With the ability of the choke to store energy
and the SCRs to turn on at the appropriate times, an
AC square wave is developed and applied to the out-
put terminals.
E-6
WORK
ELECTRODE
HI-FREQ
TRANSFORMER