SOLVED: Slightly OT: Bosch tumble dryer repair

This appears to be a polyfuse, though it doesn't have the castellations that they normally have each end. There is no visible branding on the top unfortunately; not sure what the likelihood is of something useful being on the other side.
Does anyone have any ideas on who makes it or is it just a chinese generic? What are the chances of being able to identify it from the three digits 823?

Background: It's from the control board in a Bosch tumble drier of about 10 years old. The drier runs until hot and then cuts completely out. After switching off for about 10 seconds, the appliance can be restarted, but switches off again after about a minute. I suspect, assuming that there is not an additional fault, that the polyfuse is simply showing its age, accelerated by the hot conditions in the tumble drier, and is now out of spec. Replacement board is about £160, so replacing the fuse, even if just to test the theory, would be preferred.


That looks very like a thick-film 82k surface mount resistor. Have you measured the resistance across it?

Probably an 82K resistor. Odd because of the size and pad arrangement. Perhaps a one watt device.

Clearly not a polyfuse with the white ceramic ends.

Worries me a little injecting current into an unknown circuit, but 81.1k measured.

Spot on, both of you, thanks!

I was kind of worried that it might not be a fuse since it would blow my theory out of the water, but hey, back to square one again...

Sounds like the temperature sensor in the dryer is defective. That's why it runs hot. After a while the thermal cut off device (a safety device, usually referred to as a TCO) will overheat and shut the dryer down.

Sounds like the temperature sensor in the dryer is defective. That's why it runs hot. After a while the thermal cut off device (a safety device, usually referred to as a TCO) will overheat and shut the dryer down.

Thanks, that was my first thought too. However, the "temperature limiter" mounted on the heating element doesn't seem to be the issue, since with it disconnected the appliance does light up as normal when switched on. That being said, I didn't try running the appliance and then disconnecting the limiter, which could possibly make a difference. I think that circuit may be at mains voltage, so not fussed on trying it.
There are other temperature sensors, but I wouldn't expect them to cause the appliance to just go totally blank/off.

If it were a recoverable fault, I would have expected a flashing LED or something to indicate a fault. I suppose it could be the case that the controller detects a fault and just goes into grump mode with no indication - which would be poor design IMO. The delay before it powers back on could be residual charge in power supply capacitors keeping the controller alive for some additional seconds.

Bosch do have parts diagrams online, which is nice, so I have X-ray vision to an extent.

About four decades ago, when I fixed driers, dust buildup was a big problem.
It got everywhere, including around the heating element.
Clean it with compressed air, and check your outside vent/grill.
Not enough air movement will shut down the drier.

Yeah, it's a condensing drier, so all the dust gets merrily circulated round the internal ducting. The lint filters that they install don't seem very effective, since even larger bits of fluff and hair make it past. I cleaned everything out already, including the heating element, which only had two wisps of fluff in it. There was a thin layer of fine dust on many of the surfaces, but nothing that could cause a blockage or even significantly restrict flow.

The problem didn't occur with the lid off the drier and it pulled out from the wall, which was more evidence for it being a temperature related issue. I'll probably reconnect the board next but leave it hanging out the front of the machine, cover the gap with something to keep the heat in, and then retest. If the fault occurs with the board out, then it can't be anything to do with the board overheating.

So, after some further contemplation and diagnostics with the help of an electric hair dryer, I found and fixed the fault. This is the guilty party:

TNY263-268 TinySwitch-II Family
Enhanced, Energy Efficient, Low Power Off-line Switcher


With the appliance turned on but not actually running, this "switcher" was hot to the touch (please note that I turned off the mains power to the machine before touching it...). The new replacement only feels lukewarm.
The replacement cost £1.50, so a good saving over a new board.

I was interested in what could cause this component to fail, so searched online for TNY266GN and fault. In typical sod's law style, only then did I get a couple of hits to YouTube videos about how to fix exactly this problem in Bosch dryers. Previous searches had been fruitless. Grrr.

For anybody looking for a solution to this problem in the future, I'll just include the dryer model number as a keyword, though other models are likely affected too: WTE84104GB

BTW, this thread is not totally misplaced on the Arduino forum: the control board also has a ATMEGA644P and a TINY25 :wink: