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INPUT VOLTAGE AND MAIN TRANS-
FORMER
Single-phase input power is brought into the rear of the
Power MIG 300 through a three-prong plug and input
cable. A reconnect panel, also located on the rear of
the machine, allows the user to configure the Power
MIG 300 for either 230V, 460V, or 575V input supply
power. An ON/OFF line switch is located on the front
panel. When the line switch is ON, AC input voltage is
applied to the primary winding of the main transformer.
The main transformer converts the high voltage, low
current input power to a low voltage, high current out-
put. The main transformer also has two isolated auxil-
iary windings. One supplies 30 VAC to a Diode Control
Rectifier, which converts the 30 VAC to 40 VDC to sup-
ply the Power Board and the Spool Gun Board. The
Power Board in turn supplies 20
VDC to the Chopper Board and +15 VDC, -15 VDC and
+5 VDC to the Weld Control Board circuitry. The other
auxiliary winding provides 115 VAC to operate a ther-
mostatically controlled fan motor through a relay, which
also receives signals from the Weld Control Board.
The fan is designed to come on automatically when the
trigger is pulled. The fan will stay on as needed for a
minimum of six minutes after the weld arc is terminat-
ed. The fan will also stay on when the machine's weld-
ing and feeding are disabled during thermostatic over-
temperature protection. (See Thermal and Overload
Protection.)
THEORY OF OPERATION
E-3 E-3
POWER MIG 300
FIGURE E.3 – INPUT RECTIFIER, CHOPPER ASSEMBLY AND FEEDBACK
NOTE: Unshaded areas of Block Logic Diagram are the subject of discussion.
