From bigclivedotcom.

This is a module I found inside a microwave oven I was salvaging bits out of. I could guess what the filter components were for, but the purpose of the relay and resistors wasn’t immediately obvious.

A bit of reverse engineering revealed that there are two separate circuits on the one PCB. A classic mains filter and a very simple, but also very worthy inrush limiting system.

An inrush limiter is used to limit the sudden spike of current that can occur when some types of load are initially powered. In this case the module limits peak current to around 10A when the magnetron’s transformer is initially powered and the high voltage capacitor is in a discharged state.
After a very short delay of just a few mains cycles, the limiting is removed and the transformer receives full power.

You’d think that a circuit like that would be complicated, but this is a very simple and reliable version that would find uses in other applications. Especially when the relay was powered by a separate power supply as described at the end of the video.

Just a quick reminder – the transformer in a microwave is extremely dangerous and should not be played with unless you fully understand the dangers of high voltage at high current. Likewise, the high voltage capacitor can hold a charge – especially if its discharge resistor has failed, and poses a risk even when the oven is unplugged.
The home microwave is probably the biggest electrical killer for DIYers, handymen and hobbyists attempting repairs.

The inrush limiting circuit in a high power amplifier scores high in the "loudest bangs of my career" chart. A more complex timing circuit had failed and was not enabling the bypass relay. I fitted a new aluminium cased ceramic resistor that had failed (because it wasn’t being bypassed) and after a short delay while it pressurised internally with heated moisture, it exploded, firing ceramic shrapnel out of its core.

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