Thermal Arc 181i FABRICATOR Service Manual page 59

TROUBLESHOOTING FABRICATOR 181i
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 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 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 0-5152 6-19 Troubleshooting