Lincoln Electric IDEALARC SP-250 Service Manual page 42

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 con-
trols the firing of the SCRs, thus controlling the output
of the machine. See SCR Operation.
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
The control
NOTE: Unshaded areas of Block Logic
Diagram are the subject of discussion
IDEALARC SP-250
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
E-2