During the COVID19 isolation we tend to be using the humble microwave a lot more. You can imagine our dismay when our Panasonic Microwave decided to fail. Fail in the sense whenever the door was open, the microwave would run.
Any sane person at this point would have thrown it away and purchased a new one, so I did not take this option.
Now microwaves are pretty bulletproof. A microprocessor for timing control, an inverter for the magnetron and some switches for interlocks. However before opening one must remember:
!!!MICROWAVES CONTAIN LETHAL VOLTAGES EVEN WHEN DISCONNECTED!!!
DO NOT OPEN UNLESS QUALIFIED
If you are going to open a microwave the inverter for the magnetron will be storing energy that will take a long time to dissipate. If you come into contact with this circuit it will be fatal.
Needless to say, never operate an exposed microwave.
With these warnings in mind time to start diagnosing. Given the problem only occurs when the door is open, I decided to look around at the door switches. Turns out there are three. One on top (SPST NO) and two on bottom (1x SPST NO, 1x SPST NC).
With the switches in place, I took a multimeter out and measured all the switches to see their current positions. I did this with the door open, and then then door closed to compare the results. Turns out the top switch was stuck closed.
Pulling this switch out it was clear the plunger mechanism had completely failed.
Typing in the model on the side of the switch didn't yield much luck in sourcing a replacement other than it is 16A @ 250V. I proceeded to measure the switch with a set of digital calipers. To source suitable replacements.
I ended up ordering the following switch from RS Components. It can be used in both a NO or NC configuration and has the right ratings and measurements. I ordered three in case the other switches also failed (all three are the same dimension & rating).
Installing the switch is easy, just a matter of pulling the old one out (a pair of needle nose pliers works well) and slipping the new one in position.
Replacing that one switch was all that was required. For $3.95 the microwave has been rescued, happy microwaving :)