Microwave failure modes - and fuse types

[Roger Mills]

In my holiday flat there's a Neff built-in combi-microwave which I believe to be about 10 years old.

The microwave section has just failed, although the grill function still works.

On investigation, I found a blown fuse behind the control panel. It's a glass fuse - 20mm long x 5mm dia - and is rated at 250v 10A.

The actual marking says M10D. The M apparently means medium speed to blow - somewhere between an F (fast blow) and T (slow blow - time delay). I've no idea what the D signifies.

Two questions:

1. Since *something* must have caused the fuse to blow, what's the likelihood that replacing the fuse will fix it? [Fuses *can* die of old age, but what else may have caused it?]

1. Since I can't find an identical use, what's the best type to use? Maplins have T10's and also some ceramic jobbies - apparently intended for microwaves.

[NT]

Something's not right with this picture. A microwave shuold not use a glass fuse, they need the rupturing capacity of a sand filled ceramic fuse.

NT

[Roger Mills]

It's possible that the fuse which blew was not the original - I haven't owned the device from new. But the markings on the PCB next to the fuse-holder say F1[1] and M10.

What are the special characteristics of a sand filled ceramic fuse? I can certainly get some from Maplin which would fit - but would they fail to provide adequate protection in some other way?

[1] Presumably Fuse 1 - there's another 1 amp stripy fuse labelled F2, which *didn't* blow

[NT]

Sand filled ceramics have much higher rupturing capacity than glass fuses. And the consequence of failure to blow isnt just electrical, but potentially severe internal burns of the end user.

NT

[Arfa Daily]

I've seen glass fuses on the mains input of microwaves, although equally, ceramics are also quite common. Check to make sure that nothing has happened to the door interlock switch mechanism. One of the switches places a short across the output of the other one when the door is opened. It's a sort of 'belt and braces' thing. One switch opens to isolate the supply when the door opens, and the other closes to place a short across the open one, just in case it hasn't opened. The fuse-blowing situation can occur if the switch contacts on either one weld themselves up, or if a switch comes loose on its bracket, or the mech that works it breaks. You would be ok with a T rated fuse. If you want to put a ceramic in, you should be able to get a T10A without too much problem.

Arfa

[chudford]

Not the fuse the OP is describing but there is usually a glass fuse on the HV side of the microwave. My microwave has just failed and I found this fuse had blown. I replaced it with a fuse off Ebay (there are lots to choose from). Unfortunately the problem is more severe than a blown fuse as the replacement also blew. So I guess the is a fault with the magnetron circuit. When time permits I will be stripping the microwave down to see what failed out of interest. A word of warning, don't fiddle around in the microwave circuits unless you know which bits not to go near.

[Geo]

Since the fuse is/was glass, was the glass blackened or sprayed with metal internally? If so then "something" has blown it and the replacement you buy will blow as well. If not then it /may/ have been just tired of life.

[Roger Mills]

Sorry, I'm not sure I understand the term "rupturing capacity". Are you talking about the physical disintegration of the glass (or whatever) tube - or simply about the overload current which can be sustained before it blows?

And the consequence of failure to blow isnt just electrical,

I'm not sure I understand that either! What circumstances do you have in mind?

[docholliday]

Putting it simply, the current which occurs if there is a short circuit may be many times higher than the rated capacity of the fuse (which is the maximum current it can pass indefinitely), and the rupturing capacity is the current the fuse can safely break. When the fuse blows there will be a momentary arc between the ends of the fuse element, and for higher rupturing capacity fuses the sand fill helps to extinguish this safely. As an indication, the standard BS1362 fuses used in UK 13 Amp plugs have a rupturing capacity of 6000A, while glass fuses can only break currents of the order of 100A. There's lots more at

[Andy Dingley]

High rupturing capacity - the ability to interrupt a large fault current and isolate it quickly, rather than allowing an arc to be sustained across the ends of the melted wire. Mostly an issue for high voltage or inductive loads - for a 20mm fuse, inductive loads, such as a big transformer, are the likely reason.

[NT]

neither. If fault current is above the rupturing capacity of the fuse, the fuse cant break the circuit.

Interlock failure causes fuse blows. If the fuse doesnt blow, then a faulty interlock means sometimes the thing will cook with the door open. Unless you want to be food, thats not a good idea.

If the main fuse blows, always check the shorting resistor. Sometimes that also fails, and sometimes that means no working protection circuitry, but it still cooks.

NT

[Peter Parry]

Quite good, certainly worth a try. One other failure mode which can occur and will blow the fuse is that the door micro switches fail - not usually difficult to diagnose or fix.

Any slow blow or time fuse is fine. Even if the fuse ruptures the microwave case will contain the glass fragments.

[Roger Mills]

It's blackened for about 80% of the length of the visible glass envelope

- suggesting a rapid overload.

Other posts are suggesting that this could possibly be caused by a problem with the interlock switches on the door.

I will investigate next time I'm at the flat. The only problem is that it's mounted quite up in a built-in unit - above a conventional oven - and it's bl**dy heavy!

[Graham.]

As the glass fuse did not vaporise and was only partially blackened I suspect the door microswitches did not crowbar the supply (but if it tripped the MCB as well then I wouldn't be so sure)

I think it is more likely to be, in order of likleynes:

HV diode short-circuit HV capacitor short-circuit Magnetron cathode (filament) to anode (chassis earth) short circuit.

Generally speaking a microwave can run for a long time with any of the above faults without blowing the mains fuse, because the transformer is run at near saturation so there isn't as great an increase in primary current as you might imagine. Apart from not heating the food the only symptom is a slightly louder buzz from the tranny than normal.

Microwave ovens are one of the easiest of domestic appliances to fault-find and repair IIHO, (Panasonic inverter models exempted!)

*Unplug* from mains and *discharge* the big HV capacitor with a couple of well insulated screwdrivers before delving inside, I can't emphasise those enough. Use a multimeter on ohms range to test the components above.

[Arfa Daily]

Understanding, of course, that a conventional multimeter is unlikely to produce enough voltage on any of its ohms ranges, to turn on the HV diode, so this device is likely to read open circuit, even if it's ok ...

Agreed though, that a short circuit diode will be picked up ok.

Arfa

[Windmill]

But bear in mind that the diode is likely to be actually about 10 diodes in series, so if it's OK it won't conduct in either direction unless the multimeter applies a voltage of more than maybe 6 volts. That fooled me once into thinking that the diode was open-circuit. (The meter, not my own, had only a 1.5 volt battery.)

[Mr. Benn]

The HV is the reverse voltage that the diode is rated at, not forward voltage, so you can still check it with a multimeter.

[Graham.]

then I wouldn't be so sure)

and repair IIHO,

well insulated screwdrivers

ohms range to test the components

produce enough voltage on any of its ohms ranges, to turn

so you can still check it with a multimeter.

The depletion voltage required to forward bias the diode is going to be many times higher than the typical 0.7v for a single silicon junction.

[Graham.]

Vf is 12 volts on this diode

a popular multimeter with a PP3 battery won't forward bias it.

An old Avo 8 on the other hand might just do it. I think they go very leaky rather than OC

[Roger Mills]

I'm now back at the flat, and have done the following:

1. Replaced the blown fuse with a 10A ceramic jobbie.

The result was that the panel lit up again, and I could open and close the door without problem.

However when I tried to operate the microwave (on its second from highest setting) the internal light came on as normal, it made encouraging noises (albeit a bit louder than usual) for a few seconds and then the fuse blew again.

1. I then tried a T10 glass fuse.

The panel lit up again and opening and closing the door was again ok. This time, I tried operating the microwave on its lowest setting - and it ran for the time I had set (about 15 seconds) without problems. I then tried it on its second from lowest setting, and the fuse immediately blew!

Does this additional information help to narrow down the likely underlying problem?