Main Circuit Description - Thermal Arc Fabricator 211i Service Manual

TROUBLESHOOTING FABRICATOR 211i

5.14 Main Circuit Description

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Turn off power and disconnect mains supply plug from receptacle before working on the unit. Allow two minutes for
capacitors to discharge after disconnection from mains supply voltage.
The mains supply voltage is connected via a double pole switch to the input rectifier Q1 through an EMC filter. Overvoltage
protection is provided by varistor CY1.
The rectifier output charges the main capacitor bank (C16, C17, C18, C19, C22 and C23) to high voltage. Inrush current limit-
ing is provided by a PTC which is then bypassed by relay J1 after a few seconds.
The primary igbt transistors (T1, T2, T4, and T5) switch the transformer primary at high frequency and varying duty cycle. The
transformer return wire is taken from the junction of the capacitors C20 and C21 (the voltage at this point is approximately
half the DC bus voltage).
Secondary output voltage from the transformer is rectified by the output diodes (T13, T14, T15, T16, T17, T18, T19, and T20)
to DC. This DC is controlled by the PWM of the primary side igbt transistors, and is filtered by an inductor before connecting
to the welding output terminals.
A thermal overload device (thermistor) is fixed to the rectifier heatsink. When an over temperature occurs, the control circuit
inhibits the trigger, gas solenoid, wire drive system and the welding output. The thermal overload indicator LED on the front
panel is illuminated.
The current transformer TR8 provides a signal to the control circuit to indicate both transformer primary current, and also
detect transformer saturation. The Hall effect current sensor is powered from regulated + & - 15VDC supplies and provides a
voltage signal proportional to the output DC welding current to allow the control circuit to regulate welding current.
Manual 0-5158 5-21 TROUBLESHOOTING