Heater Repairs

While I am waiting for my axle/spring wedges to turn up I turned my attention to my non-working heater. I removed the entire cabin unit and extracted the recalcitrant control board. Sure enough the IGBT was a short circuit.

Prior to removing the control board (in the litle grey case). The arrow points at my off-board snubber capacitor.

WARNING - Technobabble alert
I replaced the IGBT, changed the PWM switching frequency from 16 kHz to 110 Hz, placed a 20V Zener accross the IGBT Gate to Emitter and finally moved the 1uF 1200 VDC snubber capacitor to directly onto the PCB. I tested the whole shebang at 12 VDC with a 24W light globe as the load.

All fixed. The snubber is now a power bulge in the gray case (red arrow). I'll RTV silicone it all up when I have tested it with 600 VDC. Note the extra heatshrink where the wires emerge from the control box and generally tidier wiring. The heater wasn't that easy to get out so I don't want to do it again soon.