Thermal fuse reliability

Hi All,

We have a breadmaker that stopped working at the weekend - it did everything it said on the tin apart from actually baking the bread....Once I discovered this minor oversight, I bundled the bread into the main oven and proceeded to investigate.

I assumed that the element was no longer functioning (elementary my dear Watson...) and dismantled the machine to get to see it.

A test of the element connections from the power board suggested that there was indeed an open in the element.

Once I accessed the element (and believe me, this is part #1 of the entire build), I discovered that the element itself was fine but an inline component was O/C.

It looked like a 5W resistor and I assumed it was a thermistor to control the inrush current but it actually turned out to be a thermal fuse rated at 240°C.

I have never come across these devices so I have no idea if they slowly die over the years, nor how sensitive they may be.

With everything out on the bench I jumped the fuse with some wire and watched the unit start up, run the fan and the element for 2 seconds on start up and to operate perfectly.

I have some fuses on order but the questions I am looking for answers are whether this is likely a simple old-age issue (BM was bought in the late 90s) through time or 'n' uses, or whether they are highly reliable and there is another hidden problem...

BR
Simon

PS "bin it and buy a new one" responses are not sought....

Such fuses are, in some ratings, readily available Simon

Try searching ebay, RSComponents, Farnell etc.

Others appear to be scarce ... Like the small one that failed on our Silentnight electric blanket AND its replacement.

... both for no obvious reason.

MichaelG.

.

Edit: Just realised that you have already ordered replacements 

I will add though, that they generally appear to be rated for current, temperature and voltage ... so I suspect that voltage spikes might be the killer.

Edited By Michael Gilligan on 11/05/2020 15:20:51

If you're sure it's a thermal fuse and not for limiting in rush current then yes they can age with time and more importantly number of cycles. Same as an ordinary single blow fuse. Like ordinary fuses they're not particularly accurate and operation is, to some extent, dependent upon ambient temperature.

Andrew

240C sounds very close to the baking temperature of bread, so I would guess it has been taken close to its limit quite often.

I assume there is a thermostat to control the pot temperature, maybe that is on the way out and allowed it to overheat.

Adrian

@Michael,

Biggest problem with this one was that the writing had almost faded to nothingness - but you are right, its 240°C, 250Vac and 10A.

And good to catch you responding before reading fully -

@Andrew,

Thanks, you rather confirmed my suspicions. The new one will be crimped into place and following a couple of bench tests, will be returned to operations - if I can remember where all of the screws went!

Thanks for your swift and focused responses - as usual.

BR
Simon

It's 30 years since I was professionally in the Electronics repair game. Back then thermal fuses were notoriously unreliable and we were regularly changing them in a number of different devices. Whether anything has changed in the meantime I don't know.

@Adrian, yes there is a thermocouple-like device at the opposite end of the airflow (so in the hot air flow) and that appears to function - at least, R varies with applied temperature. The fuse is at the opposite end of the airflow so I think was intended as a cut out if the fan stopped or airflow ceased.

@Stuart, the machine is ~25+ years old so this fits exactly....

Thanks again

Yes, they age and fail. Often, they are hidden in the guts of the device, so elude initial (visual) diagnostic attempts, and then one often finds them unlabelelled. Probably a cynical attempt at getting the punter to buy a replacement, i.e. designed to fail.

Bad design seems to accompany thermal cut-outs and thermostats. We have fan heaters, in which the thermostat turns off the element(s) and the fan, so heat-soak from the elements progressively cooks the entire device, presumably shortening the life of the mainly plastic components. Our cooker has a thermal cut-out 'protecting' the oven. It may be defective, because we can't use the oven at high temp. long enough for some applications. It's across the mains, so the entire cooker is disabled for ten minutes, until the thing re-sets. Brilliant.

Simon0362

I think these types of thermal fuse do "wear out". I had one fail on our water boiler the other day, probably about five years old. They have a spring inside that presses on a contact held in place by some material that melts at a set temperature. After a few hundred cycles working at near maximum current and temperature they just give up and die. The thermal fuse in my case was rated at 15 amps and so far I haven't been able to find a direct replacement. Plenty of 10 amp rated ones around. When you look at the spec sheets of some manufacturers it looks like the same fuse gets different current ratings depending on whether it is CE, UL, or CSA approved.

Peter

My reply rather assumed a solidstate polyfuse rather than a mechanical one. A picture of the offending component would help identification.

Andrew

Peter,

The current capability of a thermal fuse should be of no significance, as long as it is more than sufficient to carry the load. It is the temperature spec that is the required criterion (as long as it is a device rated for the voltage of the appliance)

Two ten amp fuses in parallel might do the job just as effectively, but at twice the price - or even a standard fuse of 5A in parallel (sized that it would fail immediately if the thermal side passed all the load on it🙂.

I have 4 battery packs for my portable hand drill. I ended up opening up and shorting out the O/C thermal fuses in them in order to carry on using them. They are all about 20 years old with 3 terminals. The thermal fuse goes to the third terminal to control the charger. When open circuit it stops the battery pack from charging even though the pack is otherwise fine.

Martin C

These thermal fuses have a spring contact held closed by solder with a specific melting point. When the solder melts the spring pulls the connection apart breaking the circuit. The solder can suffer from metal fatigue and this causes failure. If the units normal temperature control works with a new fuse then it was just an old age failure of the fuse. If the new fuse trips or unit overheats you have another fault and the thermal fuse was doing it's job.

Robert G8RPI

Andrew, it is exactly as you assumed. No photos to hand but this is the exact replacement :

https://www.reichelt.com/fr/fr/protecteur-thermique-axial-240-c-mts-240-p13258.html

it is about the size of a 1W resistor with a tapered end between the body and axial lead at one end.
Simon