Lincoln Electric IDEALARC SP-255 Service Manual page 48

E-2
POWER SUPPLY OPERATION (CONTINUED)
MAIN
LINE
TRANSFORMER
SWITCH
RECONNECT
FAN
MOTORS
OUTPUT RECTIFICATION AND FEED-
BACK CONTROL
The AC output from the main transformer secondary
weld winding is rectified and controlled through the SCR
rectifier assembly. Output voltage and current are
sensed at the shunt and output terminals and fed back
to the control board. The control board compares the
commands of the keypad (or gun thumb switch) with the
feedback signals. The appropriate gate firing pulses are
generated by the control board and applied to the SCR
rectifier assembly. The control board controls the firing
of the SCRs, thus controlling the output of the machine.
See SCR Operation. The control board also powers
and commands the keypad, the LCD Display, the piezo
alarm, the gas solenoid and the wire drive motor.
THEORY OF OPERATION
PIEZO GAS
ALARM SOLENOID
LCD
DISPLAY
KEYPAD
CONTROL BOARD
G
A
T
E
S
I
G
N
A
L
SCR
RECTIFIER
Figure E.3 - Output Rectification and Feedback Control
NOTE: Unshaded areas of Block Logic
Diagram are the subject of discussion
IDEALARC
WIRE
DRIVE
MOTOR
TACH
GUN TRIGGER AND THUMB SWITCH
FEEDBACK
POSITIVE
TERMINAL
CHOKE
C
A
F
P E
A E
C D
I B
T A
O C
R K
NEGATIVE
S
TERMINAL
SHUNT
® SP-255
E-2