Theory Of Operation - Lincoln Electric POWER MIG SVM170-A Service Manual

E-2
FIGURE E.2 — INPUT LINE VOLTAGE AND MAIN TRANSFORMER.
AUX
115 VAC
LINE
SWITCH
RECONNECT
TRANSFORMER
INPUT LINE VOLTAGE
AND MAIN TRANSFORMER
The desired single phase input power is connected to
the POWER MIG 255C through a line switch located
on the front panel.
A reconnect panel allows the user to configure the
machine for the desired input voltage. This AC input
voltage is applied to the primary of the main trans-
former. The main transformer converts the high
voltage, low current input power to a low voltage, high
current output. In addition, the main transformer also
has an isolated center tapped 30 VAC auxiliary winding
that supplies power to the Control Board for the SCR

THEORY OF OPERATION

GAS
SOLENOID MOTOR
WIRE
SPEED
CONTROL
ARC
BOARD
VOLTAGE
RECTIFIER
DIODE
G
BRIDGE
A
T
30 VAC
E
SNUBBER
S
BOARD
I
G
N
A
L
POWER
ENHANCEMENT
CHOKE
MAIN
SCR
RECTIFIER
NOTE: Unshaded areas of Block Logic
Diagram are the subject of discussion
POWER MIG 255C
FAN
SPOOL
GUN
WIRE
DRIVE
MOTOR
TACH
GUN TRIGGER
FEEDBACK
C OUTPUT
POSITIVE
F
A CHOKE
TERMINAL
E
P
E
A
D
C
B
I
A
T
C
O
NEGATIVE
K
R
TERMINAL
S
SHUNT
gate drive circuitry. The weld power windings connect
to the main SCR Rectifier and via the Snubber Board
to the Rectifier Diode Bridge. This AC voltage is recti-
fied and then regulated by the control board. The
resultant +15 VDC and +5 VDC voltages supply power
to the control board circuitry. The 115 VAC winding
supplies power, through the control board, to the ther-
mostatically controlled fan motor and also provides
voltage for the auxiliary receptacle
The fan is designed to come on automatically when
weld arc is established. The fan will stay on for a mini-
mum of six minutes after the weld arc is terminated.
The fan will also stay on when the machine's welding
and feeding are disabled during thermostatic over-tem-
perature protection. (See Thermal and Overload
Protection.)
E-2