Internal fuse of Sharp multifuctional microwave blown

Try replacing it (with an identical type and rating). A bit like people, they can get tired. I replaced the fuse in my Panasonic microwave a couple of years ago - fine since. The general rule with fuses and circuit breakers is to replace/reset once; if they drop out again then it's time to investigate (and not to reset/replace a second time shortly after the first).

Microwaves have interlocks with several microswitches which ensure that they cannot operate unless the door is firmly closed. It is a design feature that they blow the fuse if anything is wrong with the interlocks. I would check the door switches first. Be very careful if you dismantle it - there are some high voltage caps which retain their charge after the power is switched off.

There's one big bastard in particular that'll murder you. Loads of people get killed by those every year. Make certain you discharge it as soon as the case comes off!

Do loads of people really get killed by these every year? I'm not denying the lethality of a large, high voltage, capacitor, but how many people take microwaves apart?

How many is "loads"? There seems to be very little reliable information about electrocution by stored charge in microwave oven capacitors. Perhaps it doesn't happen very often. The best I could find was this:

There is no indication whether this was from live working or stored charge.

John

That advice seems to be within my range of capabilities. Thanks for the replace once tip.

That's interesting the problem seems to have coincided with the sagging of the door on its possibly adjustable hinges. If I get as far as dismantling, I don't have the test equipment to ensure non-leakage after the event. which the service manual tells me that I would need. Maybe it would be less expensive and dangerous to buy a new one. This current one is 5 years old.

"Cursitor Doom" is a Brexiteer...

So what?

Yes, large capacitors can be dangerous to the uneducated, but I doubt people are killed by those in microwave ovens. The caps that might zap people are those in large electrical installations, and these installations have shorting bars to deal with dielectric absorption recovery. Large caps in equipment are usually required to have a resistor across them to ensure discharge within a short'ish time (that I forget). Manually discharging a large cap without proper precautions may cause a f'ing loud bang (BTDTGTTS).

If I was the OP I would not be worried. Unplug the thing, leave it for a day, open it up, find and replace the fuse, close it up, try it ... and award yourself un verre de vin for the money you've saved (or to drown the sorrow that you'll have to buy a new microwave).

In the old days, you would look at this. This was written before inverter-type microwave ovens were invented.

The high voltage is on the secondary side, the fuse is on the primary side (mains voltage).

With your fuse idea in mind, you're working on the primary side, which is safe once you unplug. Your choices are, to discharge the oil-filled cap (on a legacy microwave), or to stay away from the two terminals on top. That means not dropping any tools in that area, wires or the like. Considering the number of joules potentially inside the oil filled cap, any "discharge lead" needs a resistor in the center (and insulated paddle-handles).

The oil-filled cap, if fully energized and you drop a bare copper wire across it, the sound effect is like a shotgun blast. It can cause a loss of hearing in one ear (the ear facing the oil filled cap) for a period of ten minutes (that's what happened to me). The microwave in question, was fully assembled, a unit at work, and it just decided to arc over right after the power button had been depressed on the front. Microwave ovens that cook a lot of microwave popcorn (with butter and salt in the bag), the vapour is too much for the conformal coating on some PCBs.

As a topic, this is similar to replacing the fan on an ATX PSU. It can be done safely, but mainly because for units where replacement is supported, the connector just slides off the pin header, and the operator does not need to approach the mains cap inside the unit. Your hands don't touch the bottom of the ATX PCB. That means it is mostly safe, as long as you are not a klutz. Same goes for the microwave oven. It can have its own drain resistor across it, but a good tech assumes the drain resistor is non-functional while working in there. By assuming the capacitor is fully charged and you could lose an eardrum, it makes you a safer worker.

The voltage on the two terminals of the oil-filled cap, is on the order of 5kV or so (varies). This is too much for a home multimeter, which has a max voltage of 750v or 1000V (usually printed in white on the selector area). There are devices which divide the voltage (the meter impedance being in parallel with one of the resistors), so it is possible to scale a voltage down so a multimeter can read it. The commercial probes have plastic discs near the handle, in an attempt to prevent arcing. The individual here, uses plastic tubing and there are no additional fixings for HV. You would not use a home made "40kV" design, on an actual 20kV source, as that is just asking for trouble. Commercial versions of these would be tested that they withhold some voltage, for a period of time.

My purpose in explaining that, is to mainly point out the safety issue, that the multimeter just cannot be connected directly. If you were to plan to do that or need to do that, then some sort of probe kit would be needed. And the probe kit makes an assumption about the input impedance of the instrument. The instructables article is to demonstrate the principle of operation, and it has pictures of the impressive resistors used (don't handle the HV resistors with your fingers, wear gloves, don't contaminate the body of the gigohm resistor and degrade flash over voltage). If soldering HV components, try not to leave sharp points (which arc) and you may need corona dope over exposed areas. I've built one safety device myself for a project and doped the conductive portions. That was the power supply for my laser in university (nitrogen laser, UV output).

I love high voltage... as long as someone else is handling it.

Paul

I don't think I'm exaggerating very much at all with my claim. Figures are only available for more advanced Western countries. No figures for the 3rd world where this will be far more likely. Here's what AI says on the subject:

"The high-voltage capacitor in microwaves is a proper sneaky killer—holds thousands of volts (often 2,000–4,000V) even after the thing's unplugged for hours or days, thanks to how it stores charge for the magnetron. One accidental brush across the terminals while poking around inside, and bam: potentially lethal shock, heart fibrillation, the lot. Bleeder resistors are meant to drain it slowly, but they fail, get bypassed, or just aren't fast enough in some cases.As for solid figures on annual deaths from DIY fixes? It's surprisingly low and not well-tracked in recent years, but here's what reliable sources pin down:The most cited stat comes straight from the U.S. Consumer Product Safety Commission (CPSC): From 2006 through

Globally? Even harder to pin—data's mostly US-centric via CPSC/NCHS, and places like the UK/EU report very few (if any) publicized cases tied specifically to microwave DIY. It's a niche hazard: scary as hell when it strikes, but probably under a handful worldwide per year at most, given how common microwaves are (hundreds of millions in homes) versus how few people actually crack one open to fix.Bottom line: It's rare enough that official stats don't scream alarm annually anymore, but the risk is real and non-zero—enough that pros and safety bodies still say "don't do it yourself unless you're qualified and discharge the cap properly with insulated tools." If your microwave's kaput, bin it or call a tech—better safe than zapped."

Typical made up statistics, obvs

I never quoted any statistic at all. The only thing "made up" is your criticism.

Indeed no. They have dropper resistors inside to discharge them rapidly once power goes off

Yes. as described. Most (all?) microwave caps come with an internal 10Mohm resistor. Its such a cheap addition no one thinks of pushing safety by having it external

e.g.

Disconnect the power, leave it for as long as it takes to get it on the bench, and dismantle it.

If the mains fuse has gone that's probably the transformer gone, if it's the big HV fuse its the magnetron.

Either is likely to cost the same as a new cheap microwave.

I did re-magnetron and re-fuse mine - cost about £80 for a 1KW magnetron and fuse. Absolutely get the exact same part number for the magnetron

It was a £200 machine though

F****ng stupid frogs I reckon. Amazingly crude devices. very easy to fix

That is total bollocks.

The caps have internal bleed resistors.

People like me who have serviced them know more than a f****ng AI getting its info from keyboard warriors who never saw the inside of one.

As I said earlier, typically 1µF and 10 Megohm. Time constant 10 seconds

So by a minute its down to safe voltages. And it takes longer to get the screws out than that.

I wouldn't test it live with the cover off though, but mine had microswitches to make sure you couldn't

The HV bit is 4 components. A transformer, a stack of diodes making a

-5kV and the that's the magnetron cathode, and the anode is earthed.

If there is any measurable resistance between either heater pin and earth the magnetron is toast.

You cant test the diode stack without less than 5V to drive it as there are a lot of diodes in series in it. My old analog Avometer did quite well with a 9V cell.

You can use a standard ohmmeter to test the cap isn't shorted, or the fuse has blown

The transformer should show continuity in its windings and no short to ground other than the winding feeding the anode.

ITYM europhile

Before 'turn it off and back on again' there was 'take it apart and reassemble'. I fixed (for a year so far) a microwave oven by this technique, as the door mechanism wasn't working reliably. I've no idea what changed, but after reassembly it was positive again. I didn't find anything obviously wrong.

The one I fixed had a separate inner shell inside the outer cover, and I didn't need to disturb anything containing RF. The outer cover had a couple of screws and no obvious additional screening materials (braid, fingers etc.).

If it's genuine Japanese, it will probably last for years more. I know someone who had an AEG one die irreparably after two years. We had a Panasonic one which we replaced at more than thirty years old, as the case was starting to rust.