Lincoln Electric SQUARE WAVE TIG 355 Service Manual page 49

E-5
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
DC WELDING OUTPUT
When the polarity switch is placed in either DC posi-
tion, the AC voltage from the main transformer sec-
ondary is applied to the SCR bridge. The SCR bridge
and choke circuits are connected in a conventional full
wave bridge and filter configuration, resulting in a con-
trolled DC output. Since the choke is in series with the
negative leg of the bridge and also in series with the
welding load, a filtered DC is applied to the machine
output terminals.
FIGURE E.5a – SCR BRIDGE AND CHOKE CONFIGURATION FOR DC WELDING
THEORY OF OPERATION
FIGURE E.5 - DC 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
E-5
WORK
ELECTRODE
HI-FREQ
TRANSFORMER