Microwave fuse burnout

Hi,
My microwave, after about 10 years of service, burned out a fuse (20A) about a month ago, and now again today. I am not knowledgeable in electronics, but can do minor repairs. What do you think is the likely problem and is it repairable?
Thanks,
...... john

i've seen a couple of common causes of this... the high voltage rectifier diode is shorted, the high voltage capacitor is shorted, or the magnetron is arcing.... it is repairable, but a word of caution: THE HIGH VOLTAGE SECTION CAN STORE ENOUGH ENERGY TO KILL YOU INSTANTLY!!!!! only work on the high voltage section after a) making sure the oven is unplugged from the wall outlet, b) completely discharging the high voltage capacitor.

A typical cause is opening the door abruptly while heating instead of turning off or letting to time out before opening.
Is the door opening while-on a habit of yours ? Can you remember if was the case when fuse blew ?

Hi unclejed,
Is the diagnoses to determine which of the likely candidates is the problem, fairly straight forward? I have a multi-meter, and the regular tools, screwdriver, pliers, etc..
Hi Externet,
We regularly open the door while heating is in progress; but have been doing so for 10 years. Only in the last month have we been blowing fuses.
Thanks,
... john

Well, microswitches get worn and act delayed... After replacing the fuse, try not to open the door when on and check if fuse blows.
A multimeter won't detect the milliseconds of time shift.

An interlock switch mounted to trigger when the door is opened is probably getting dirty and delaying operation. It is a pretty standard micro switch. There is likely more than one. Be sure no food or grease has accumulated on any of the switches where the interlock (moving bracket) touches them. The switch that blows the fuse is usually mounted alone (as oppose to in a gang of two). You can check it with an ohmmeter, (oven unplugged from wall). It will be a more clear reading if you remove the wires from the switch, but be sure to draw a sketch of where they go before you remove them. Each switch has three contact points, and normally only two are used....you must reconnect to the correct two. If the swtch has a plastic connector plug, that makes it harder to mess this up! Use the door itself to trigger the switches as you test them one at a time. Any delay or noticeable bouncing will cause the fuse to blow. It is a safety switch to be sure the oven cannot operate with any of the switches bad. If another switch is sticking, the safety switch will bloe the fuse, so check them all. It's job IS to blow the fuse. Also look at the circuit board, if there is any sign of a burned component, it is likely the MOV, and it may need to be replaced. Since the oven works sometimes, it may be blown open but internally shorted, and vibration causes it to short occasionally. (usually red or blue disc on 2 leads...youll see a hole or signs of smoke on it), finally, a shorted high voltage transformer (the BIG ONE) could blow the fuse. (This is the least likely cause.) An arcing magnetron will smell burnt, a shorted high voltage diode will test short with the ohmmeter, and so would a high voltage capacitor. The capacitor should have a 10 megaohm resistor across it to discharge it in about 10 seconds, wait about 30 seconds after you unplug the oven, then short the cap with a pair of needle nose pliers, you should see little or no spark by then.

Good luck.

Externet said:

A typical cause is opening the door abruptly while heating instead of turning off or letting to time out before opening.
Is the door opening while-on a habit of yours ? Can you remember if was the case when fuse blew ?

Click to expand...

Sorry, but that's utter nonsense - I've repaired thousands of microwave ovens - there's no issue at all with opening the door while it's running, the multiple interlock switches simply turn the oven off.

'If' there were serious problems with the interlock switches (usually more than one of them), then it 'may' blow the fuse - but that's a good thing, as you have a serious problem, and the fuse is doing it's job.

Is the OP using a correct microwave oven fuse? - it's not just a normal type, they are microwave oven specific.

Hi Nigel,
Check any microwave schematic and find the "Monitor Switch". You will find it in various positions, all of them allow it to be DIRECTLY across the line wires after the fuse. The job of the monitor switch is to blow the fuse if certain other safety devices (switches) are not properly functioning. It often is placed in the circuit where one or two thermal switches, and at minimum, the first interlock switch affect it. Should the thermals not be open, and the first interlock not be open, and the door is opened, the monitor switch will blow the fuse. I have seen several microwaves where either a monitor switch failed, or has been stuck closed by grease build up, and guess what? You open the door and the fuse blows. In one case, the grease just delayed the interlock opening, so everything was normal by the time the meter was employed to check switches. I too have repaired many microwaves, and I can assure you, trouble at the interlocks will blow the fuse. Some early models even could blow the fuse when the door was slammed and the oven was not operating, simply due to switch bounce! Back then, the switches were adjustable, and if the tolerances were too close, a good slam would take out the fuse. It really is not utter nonsense, YMMV.

I also agree with the other things you have said to try to help the OP, and I am not discounting your knowledge. Please do not discount mine.

Cheers,
-fab

Correct. No matter how many thousand microwaves fixed; a fast door opening while on can cause the fuse to blow as intended just because one microswitch interlock did not act at unison as the other by inertial effect, wear or just timing, short-circuiting mains for a millisecond.
It is not nonsense. It is exactly the behavior of microwave ovens that get repaired just by replacing the fuse and nothing else diagnoses wrong and left as unexplained.

fabelizer2 said:

Hi Nigel,
Check any microwave schematic and find the "Monitor Switch". You will find it in various positions, all of them allow it to be DIRECTLY across the line wires after the fuse. The job of the monitor switch is to blow the fuse if certain other safety devices (switches) are not properly functioning. It often is placed in the circuit where one or two thermal switches, and at minimum, the first interlock switch affect it. Should the thermals not be open, and the first interlock not be open, and the door is opened, the monitor switch will blow the fuse. I have seen several microwaves where either a monitor switch failed, or has been stuck closed by grease build up, and guess what? You open the door and the fuse blows. In one case, the grease just delayed the interlock opening, so everything was normal by the time the meter was employed to check switches. I too have repaired many microwaves, and I can assure you, trouble at the interlocks will blow the fuse. Some early models even could blow the fuse when the door was slammed and the oven was not operating, simply due to switch bounce! Back then, the switches were adjustable, and if the tolerances were too close, a good slam would take out the fuse. It really is not utter nonsense, YMMV.

I also agree with the other things you have said to try to help the OP, and I am not discounting your knowledge. Please do not discount mine.

Click to expand...

I suggest you try reading my response properly, and your own as well - I stated your original claim was nonsense, and it still is - and I qualified it by explaining that it would only happen if there was a serious fault with one or more of the door switches. You have just stated EXACTLY the same in the post I've quoted, and I've highlighted the parts where you did.

Your original claim that simply opening the door can blow the fuse was completely untrue - which you seem fully aware of.

Hi,
Well, I bought two more fuses today. They are Bussmann ABC 20A. They say 'for microwave' on them. (I have no idea why they're opaque. One can't see the filament, and has to test it with a meter.)
That being said, I'm wondering if I can simply clean these switches, perhaps with contact cleaner or ?? (I assume they're inside the door?). Is that the obvious first step?
Thanks,
....... john

Not much to clean in a sealed microswitch. They are inside the door frame and better not disturbed. Just try to not open the door by pushing its button and instead, set/let off. It will work as diagnostic to see if failures stop or not.

I have seen many were the fuse holder no longer grips well and the fuse cooks itself.
Sometimes on the cooked fuse you will see
Like Solder coming out where the cap goes over the ceramic body
Run a long cycle maybe 4 min with two coffee cups of water ,unplug and see if fuse is hot. If so replace fuse holder

Hi,
There we no signs of seepage, so I took the fuse apart. I've never seen a ceramic fuse before; and was quite surprised to see that the inside is filled with a white powder. The filament itself was burned out in two places, leaving a roughly 1/8" piece in the powder. I will try the " set/let off" method and see if that tells me anything.
Thanks,
..... john

The Ceramic fuses are filled with flame retardent powder. Ordinary glass fuses can shatter when subject to short circuits.

Hi debe,
Ah, good to know.
Thanks,
....... john

debe said:

The Ceramic fuses are filled with flame retardent powder. Ordinary glass fuses can shatter when subject to short circuits.

Click to expand...

The vast majority of microwave fuses are ceramic, and like that - but 'some' fitted in new ovens appear to be plain glass - but presumably are specially made for microwave use?. So don't panic unduly if you ever see a glass fuse in a microwave, it 'could' be how it left the factory.

BTW, I have seen the occasional ceramic microwave fuse 'blown to pieces', so even those aren't immune - but I've repaired a LOT of microwave ovens over the decades.

Incidentally, as far as fuses for microwaves go, we used to store them entirely separately from all other fuses (in a specific microwave oven spares area), so as not to fit the wrong types by accident. I don't know if it's the same elsewhere?, but we also weren't allowed to sell them to the public (as part of the agreement for been an authorised microwave oven service centre).

HI Nigel,
Thanks for the primer. I had not seen a ceramic fuse until I bought the replacement fuse for this unit; obviously I don't do much in the way of electonics repairs.
....... john

Shinnen said:

HI Nigel,
Thanks for the primer. I had not seen a ceramic fuse until I bought the replacement fuse for this unit; obviously I don't do much in the way of electonics repairs.
....... john

Click to expand...

Personally I don't like ceramic fuses - they seem to fail more often for no reason than glass ones, and in any case seem to blow apart just as much (if not more) than glass ones do.

Hi Nigel,
One would assume that the reason for their use is cost; but I would guess that they're no less expensive than glass??
......... john