Usual failure modes of magnetrons?

I agree with Phil.

The only thing I could see having a problem when you open the door prior to the microwave timed out is the actual door safety switch, and only if it carried a few amps of current - perhaps so on early microwaves. A simple signal switch doesn't care much if the current is micro-amps it will last as long as it would with no power. X number of door openings - typically a million or so for a good quality switch.

The magnetron wouldn't care either, it has a certain number of ON (powered up) hours in it, and considering the cycling the magnetron does on reduced heating times, then that too is quite a long time - for better quality units.

I've not seen the schematics for a microwave, but I assume that sensible safety requirements would have at least two switches to disable the magnetron if the door was opened when operating.

When I bought out our local TV/VCR/small appliance parts shop last year there was also a bunch of microwave parts - magnetrons, big caps, etc. I really should put them up on my site in case someone wants to fix older units. Really, someone might want to!

John :-#)#

Later generations have them wired in a make/break situation so in case a switch sticks closed or is intentionally jumped out, it shorts the supply and blows the main fuse.

You can still stick a knife in the interlock if you need to fry somebody's intruding drone. ;)

Cheers

Phil Hobbs

Total immersion is simpler. Add a bit of salt or baking soda to the water, that makes the damage irreparable.

Peter Wieck Melrose Park, PA

d the operation was bad for the magnetron. He said the unexpected interrupt ion was a shock that over time led to more frequent magnetron failures. Do you have any knowledge or opinion about this?

sers for failures that were nothing to do with them.

Door switches take more than a few amps, and that still applies to a lot of current prodution units.

Microwave interlocks use 4 switches in a self monitoring configuration. If a switch should open but doesn't, another monitoring switch shorts the powe r feed (via a resistor) which blows the main fuse. 2 series switches withou t monitoring were used in the 1970s. I've seen much worse in historic machi nes, I remember one that continued cooking with the door part way open.

NT

I saw an interesting very old Moulinex one which seemed to have a fairly loose door, but when it started cooking, an electromagnet pulled the door very tightly onto the oven. Before I tried it, I expected it to be very leaky but it was one of the least leaky I tested.

You have to catch it first though.

Cheers

Phil Hobbs

I'm looking ata schematic from an old Sharp microwave. There is such a 'monitor switch' which shorts the power supply. It is located immediately across the magnetron primary. So, should al other interlocks and control relays fail, this will indeed blow the fuse.

Now, back to the original rumor. Should someone open the door on the oven rather then wait for the control to shut off power, that short may be applied to the transformer before it and/or the magnetron has bled off energy. The resulting transient might be enough to shorten the life on something.

that's the plan

The short only occurs if the primary interlock switches fail, no other situation. At which point no-one cares what it does to equipment life expectancy.

NT

The shorting monitor switch closes every time the door opens. If the timer shuts off the magnetron and you open the door a few seconds later, there is no effect. If you open the door on a running oven, even if the series interlock switch removes AC power from the HV transformer, the shorting switch may close within milliseconds of that. So what is the time constant of the decaying field in the transformer? And what hapens if you give that field a shorted winding?

The delay in the mechanical timing of those switches is clearly always enough. If there was overlap, you'd blow the fuse every time.

fun fact, the monitor swtich as it's being called here is these days (at least the past decade if not longer) is a non-snap action switch in a standard microswitch form factor. The only rating is 1 use and enough current to blow the mains fuse. They never switch any load during the life of the microwave unless there is a fault condition. Microwave ovens are sort of like cars and have every possible extra cost engineered out. Polaroid instant cameras were like that too. It was amazing how many parts they tweaked out of the design.

Link points to a website that is under construction???