ESAB Fabricator 181i, Service Manual

Page: 59 / 84

ESAB FABRICATOR 181i
Manual 0-5449 6-19 TROUBLESHOOTING
The mains supply voltage is connected via a double pole switch to the input rectifier Q1. 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 limiting 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 trans-
former 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 reg & - 15VDC supplies and provides a voltage signal
proportional to the output DC welding current to allow the control circuit to regulate welding current.