Combination microwave/oven repair

[tabbypurr]

battery & LED would do.

NT

[Harry Bloomfield]

[e-mail] expressed precisely :

Might that be one of those thermal trips, which does not self reset?

[D.M. Procida]

I bought a multimeter today, so now I just need to try finding a replacement thermosicherung.

Daniele

[D.M. Procida]

I'm admitting defeat on this one, especially since I've already bought a replacement second-hand microwave combination oven to replace it.

For reference, it's a Siemens HF83950NL: .

I bought a replacement part 15792 for it. This is what I think is a thermal fuse. When I've looked up the part numbers marked on it:

Z430KK6E E065FF1 UK T180

and pressed into the ceramic:

Z42/43

6/250 T250

that returns results that describe it as a thermal fuse.

Siemens calls it a "temperature limiter", which I guess might be close enough. The old one has no continuity. Neither does the new oe I bought and fitted - but rather thoughtlessly, I only checked that after fitting it and turning on the machine, so for all I know it has immediately blown, or maybe it's not actually a fuse and it's normal for it to be open.

The odd thing is that when the machine is plugged in, one side of this fuse is at 230V, and the other - following the cabling - seems to be neutral (or at least, it the cabling disappears into what seems to be the neutral/0V terminal of the "Capacitor-interference suppre." (part no. 00065353) which is right next to the mains terminals.

So I am not entirely sure what sort of thing this fuse/temperature limiter actually is, after all, and my oven is not fixed

I've spent several hours and ?25 on this, which is disappointing, but I haven't had an electric shock or microwaved myself, which makes up for it, a bit.

Daniele

[Tim+]

How did you fit it? Just soldered it in? Use any heat-sinking? Pre-chill it?

If you didn?t take suitable precautions you may have inadvertently ?blown? the new thermal fuse by soldering.

Tim

[D.M. Procida]

No, I just bolted it into place and attched the clamps onto the terminals.

Here it is in between the two hefty resistors:

Daniele

[Thomas Prufer]

I looks as it might be a fixed-temp thermostat.

It just *might* be a "crowbar": normally open, but if the resistors get above

250° C, it goes closed-circuit, so shorts live to neutral. If wired in behind some local fuse, this guarantees that the fuse blows immediately, certainly, and Right Now.

Not a bad plan. Not the 25? part, obv.

Thomas Prufer

[D.M. Procida]

How does one find out? The part numbers on the fuse itself don't seem to lead to any information about it. Is there some sort of standard numbering system?

It's not the worst thing to happen.

Daniele

[tabbypurr]

Did the resisors check out ok? What does the circuit diagram say the cutout-like thing is?

NT

[Thomas Prufer]

Nope, can't find out from the parts used, just from the circuit.

And if the crowbar does its thing, the fuse will fail in short-circuit mode, i.e. in a flash, with smoke, rather than overload mode, i.e. just melt through unspectacularly. So have a look, any such fuse would be connected with a wire of a hefty PCB trace.

And the thermal cutout may be OK if open-circuit, and it may not be wise to short it out, even for a quick test.

Thomas Prufer

[FAQ]

I had the same 'thermal limiter' (Siemens part no. 00152792, Z430KK6E E065FF1 UK T180). This fuse could very well been produced in or for The Netherlands (I'm also Dutch), as in the metal part of it is puched "KEMA KEUR" which is the Dutch authority on certificates for electrical safety. Under normal operation it's open. When overheated, it's shorted and will switch on the fans. So it is indeed a "crow bar". Never seen one like this before, I was always used to have continuity if the fase is ok, but this one is the other way around apparently.

[Brian Gaff (Sofa]

Well this thread has been going on for a year now and its not clear to me if this post is anything at all to do with the original problem.

Many devices over the years have used crowbar protection ensuring the fuse blows, but I've not come across one that turns fans on before! Brian

[Fredxx]

No it is a simple thermal switch, very common.

[Bob Bear]

N.B. - Standard disclaimer - Microwaves contain not only mains voltages, but extra high tension voltages too - extreme caution advised - you need to know what you are doing!!! (Such as making sure that the high voltage capacitor is discharged before starting work - they usually contain their own discharge resistors, but it is wise to check). I'm a qualified electronic engineer & I'm 100% comfortable with microwave ovens and the safety precautions that have to be taken with EHV. Do not repair if the door seal is damaged in any way. I've just fixed my own Daewoo microwave that was starting OK, but not heating. The fault turned out to be a blown 800mA 'in-line high' voltage supply fuse caused by a short circuit magnetron filament filter capacitor - this is apparently fairly common. The microwave filament, (and the associated filament transformer winding), are at the full high tension negative potential of several kilovolts and the anode is grounded. The white looking filament connections to the magnetron that pass through the magnetron case are actually 5kV DC working voltage filter capacitors of a few hundred picofarads that filter out unwanted microwave emissions from the magnetron filament by grounding them to the case. Spare capacitors are available on Ebay etc. and there are Utube videos on how to change them. I had no problem at all doing it and fixed my microwave OK by replacing the HV diode, HV fuse and both filament filter capacitors, (all tested before use).

[Tim+]

Tim