Comment by MisterTea
1 year ago
Notice that door switch middle is a crowbar[1] which is there to prevent the magnetron from powering on when all else fails (and there is a lot of interlocking). My friends parents had a microwave that kept dying so I opened it up and realized one of the three switches was a crowbar and it blew the fuse. I changed the fuse and it powered on again and ran for a few more days until it blew again. They decided it was unsafe and canned it.
Ironically the switches are cheap (from Ali Express anyway) and easy to replace and TMK virtually all microwaves use the same type. But yes, when you open the door, they do the following, in order:
- open to send a "stop microwaving" signal to the controller
- open to physically open the power circuit to the magnetron transformer
- close to short out the power after the above relay (the crowbar part)
The bottom two relays (in the list, not necessarily physical order) are independent of what the control board does. If due to mechanical slop, the bottom one switches before the middle one, boom goes the fuse. Even if you're just opening the door of an idle microwave. That's why, at least in mine, it says on a label inside, if the fuse is blown, replace the door switches along with the fuse.
The sequencing is very tight. You have to push the button veeerrry slooowly to actually observe the switches clicking in sequence.