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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Sep 2021, 3:33 pm   #1
McMurdo
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Default Odd use of thermal fuse.

A caravanning chap I know has asked me for a thermal fuse, and shown me a picture of it. It's a standard-looking microtemp-style axial fuse, 91degC. It's on the electric water heater. Sounds right to me, 91 would hopefully stop the water boiling in the event of a control fault on the electronic thermostat. The one on his unit is open.

However, he's told me it's on the outside of the heater, mounted on the logic pcb...the photo confirms it. There's no contact with anything hot.

His motor-rewinding aquaintance has said it's probably being used as a normal fuse and relies on an overcurrent event to heat it up internally and blow it.

I remain sceptical about that, though I'm happy to be corrected.

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Old 13th Sep 2021, 4:05 pm   #2
Richard
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Default Re: odd use of thermal fuse

They are normally rated at 10A.

Using it as an over current device sounds unlikely, it would be slow to heat up, its a horrible idea.

Is the PCB near enough to get heated by the tank?
I have seen them used attached or very near boiler vent pipes.

We need to see the pic(s)

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Old 14th Sep 2021, 7:30 am   #3
duncanlowe
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Default Re: Odd use of thermal fuse.

Does the photo show the PCB as mounted. Or could it be that when mounted the device is close to the tank?
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Old 14th Sep 2021, 7:34 am   #4
McMurdo
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Default Re: Odd use of thermal fuse.

He assures me the pcb is nowhere near the tank and in any case is outside the lagging of the cylinder.
He's going to bring it over today for me to fit a new one (he shorted it out to get it working while he was on holiday).
I think I deleted the photo.
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Old 19th Sep 2021, 6:43 pm   #5
Richard
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Default Re: Odd use of thermal fuse.

What was the outcome?
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Old 19th Sep 2021, 6:48 pm   #6
McMurdo
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Default Re: Odd use of thermal fuse.

He brought me the board on friday and I soldered a new fuse in. The fuse is butted right up to a molex multiway connector on the pcb with various other bits and bobs on it, mainly a wiring harness break-out board to various faston tabs, so I can't image what the purpose is. Changing like for like is all I can do.
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Old 19th Sep 2021, 7:01 pm   #7
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Default Re: Odd use of thermal fuse.

How odd.
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Old 21st Sep 2021, 7:13 pm   #8
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Default Re: odd use of thermal fuse

Quote:
Originally Posted by Richard View Post
They are normally rated at 10A.
Someone brought an oil filled radiator round today for me to have a look at as he couldn't get his head around the circuit. It had one of these thermal fuses fitted that was rated at 15A, 117C. It was open circuit, so I temporarily removed it and bypassed it to prove the heater worked - he'd found it in a skip!
A quick look on-line seemed to only produce the 10A versions available in the UK, the heater being rated at 2.5 KW. I re-connected it as original with the faulty fuse back in place and he took the heater away saying that he would try to obtain one, but suspect he'll probably go for a 10A one, which he might get away with for a while. He asked if it would be OK to run it without one, but I suggested that in the unlikely, but possible event that his house burnt down (or worse), the insurance might not be too happy if the heater was found to be the cause. The fact that the 15A thermal fuse had obviously failed tended to make you think that a 10A one would be pushing it a bit from the calculations - it showed obvious signs of running hot and was clamped to the rim of the element.
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