Replacing Microwave's internal fuse

I had a microwave that I liked. When it quit working, I found that there was no output from the transformer, and no replacement available. So I dug into the transformer, (destroying it) and found a blown fuse deep inside the windings. Had I known that the suckers pulled this dirty trick, I could have installed a fuse externally.

In my experience, many blown microwave fuses are caused by opening the door before or instead of pushing the stop button when the microwave is running. For some reason, it really seems hard to not do this....I'm often guilty myself. See the below excerpt from a microwave service manual.

"Interlock switches
Various door interlock switches prevent inadvertent generation of microwaves unless the door is closed completely. At least one of these will be directly in series with the transformer primary so that a short in the relay or triac cannot accidentally turn on the microwaves with the door open. The interlocks must be activated in the correct sequence when the door is closed or opened.
Interestingly, another interlock is set up to directly short the power line if it is activated in an incorrect sequence. The interlocks are designed so that if the door is correctly aligned, they will sequence correctly. Otherwise, a short will be put across the power line causing the fuse to blow forcing the oven to be serviced. This makes it more difficult for an ignorant consumer to just bypass the door interlocks should they fail or to run the oven with an open door as a room heater - and protects the manufacturer from lawsuits. (That interlock may be known as a "dummy switch" for obvious reasons and is often not even mentioned in the schematic/parts manifest.) Of course, should that switch ever actually be used, not only will the fuse blow, but the switch contacts will likely be damaged by the high initial current! This also means it probably wouldn't be a bad idea to replace the interlock switch which might have been affected if your oven fails with a blown fuse due to a door problem.

Failed door interlocks account for the majority of microwave oven problems - perhaps as high as 75 percent. This is not surprising considering that two of the three switches carry the full oven current - any deterioration of the contacts results in increased resistance leading to their heating and further deterioration. And, opening the door to interrupt a cook cycle results in arcing at the contacts. Complete meltdowns are not unusual! If any defective door switches are found, it is probably a good idea to replace all of them as long as the oven is already apart.

The typical door switches and their function:


•Door Sensing: Input to the microcontroller to indicate the state of the door.


•Interlock Monitor: Shorts out the AC line (and blows the main fuse) should the Primary Interlock not open due to incorrect sequencing of the door switches or a failed switch.


•Primary Interlock: In series with the high voltage (magnetron) power supply so cuts power when the door is open.

Note that if the Door Sensing switch should malfunction, peculiar behavior may occur (like the fan or turntable operating at the wrong time) but should never result in microwaves being generated with the door open. "

Among other things, microwave oven fuses can blow from...old age (perhaps assisted by poor connectivity with the fuse holder causing undue heating), from accumulated crud or maladjustment causing the door safety switch to misbehave and operate its contacts out of order, or from a short in the magnetron or failure of the high voltage power supply.

If the oven works after replacing the fuse, the third option is presumably not the cause.

I had a low voltage situation in my old trailer 30 amp, wife turned on too many things. Before the breaker tripped, with the Microwave running, it took the microwave fuse out and the capacitor. Replaced them both and everything was fine. Funny thing is I had a cap lying around from another microwave (home unit) that had a bad magnatron. So all I had to buy was a fuse.

The internal fuse in the microwave in our previous coach also blew. Or at least that's what I thought. After removing it though, and finding that a replacement fully restored normal operation, I took a close look at the inside of the fuse with a handheld 100x microscope. It quickly became obvious that the fuse element had simply detached from one end cap rather than blowing from an overload. I suspect vibration was the real culprit, possibly combined with a fuse that wasn't assembled quite as well as it might have been. In normal stationary residential use, that fuse may have lasted years longer.

Now the only question is WHY did a 20A 'FAST acting' fuse blow in the first place :H