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How to replace thermal transformer fuse

Ruger

Aug 22, 2020
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New guy to the board....thanks for taking my question.
I tend to be long winded soooo....I'll try to be brief. I have an older Hotpoint self cleaning electric stove in a rental property. The neutral wire from the utility pole broke and hit one of the hot wires on the tri-plex. The power surge blew out all the GFI's in the 2 apartments. It blew out the transformers in the 2 furnaces....and something on the electric stove circuit board. The burners work....but that is about it. I can't find a circuit board as I think they no longer make it. I can't find a transformer for the board either. I found a post from 2017 here....with a similiar situation. "Steve" was quite helpful and knowledgeable with his help. I ohm tested the transformer and am almost positive it is the fuse in the transformer. I don't how I did it....but I was able to expose the fuse and get the numbers off of it with no damage to any of the wiring. The transformer is still attached to the board
All that said....I can get the fuse....but am not sure even my brain surgery schooling will help me in getting that thing out of there. Is it possible to solder the new fuse outside the windings....or does it have to be close the windings?
Didn't really go for brain surgery....it was heart surgery. :)
Thanks in advance,
Rob R.
 

kpatz

Feb 24, 2014
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If it's a regular fuse that blows on overcurrent it can be outside the windings. If it's thermal... that is, it opens if the transformer windings get too hot, it will need to be in contact with the windings to detect this.

Have you tested the fuse with a multimeter to confirm it's open?
 

Ruger

Aug 22, 2020
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Hi Kpatz....
Thanks for your response.
It is definitely a thermal fuse. One end attaches to a pin that comes up through the control panel. It has a tube around it.....and goes up to the thermal fuse. I was able to put a very small pin on the wire between the pin and the wire side of the fuse. I got continuity. When I go to the pin....and the other side of the fuse....it is dead. The fuse was wrapped on top of the windings and attaches to the very thin wires that go back down into the windings. I don't know how I didn't break them....but I didn't.
 

Ruger

Aug 22, 2020
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If it's a regular fuse that blows on overcurrent it can be outside the windings. If it's thermal... that is, it opens if the transformer windings get too hot, it will need to be in contact with the windings to detect this.

Have you tested the fuse with a multimeter to confirm it's open?
If it's a regular fuse that blows on overcurrent it can be outside the windings. If it's thermal... that is, it opens if the transformer windings get too hot, it will need to be in contact with the windings to detect this.

Have you tested the fuse with a multimeter to confirm it's open?

If the fuse needs to read the temperature of the windings....can I wrap it on the windings but leave the fuse wire long enough to solder where I need to so I don't have to take the transformer apart?
 

kpatz

Feb 24, 2014
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If you can get the fuse body in more or less the same spot as the original, you can leave longer leads on it to solder it and it should be fine.
 

Ruger

Aug 22, 2020
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Thanks....that is the only thing I can think of without having to de-solder the transformer from the board....and then taking the transformer apart. I guess I should put a tube or sheath of some kind on the exposed fuse wire?
 

Alec_t

Jul 7, 2015
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Take care that heat from the soldering doesn't trip the fuse.
 

Ruger

Aug 22, 2020
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Thanks Alec_t....
That is the reason I want to leave as much wire away from the fuse as I can. I definitely will use heat sinks. The fuse is rated at 150 degrees Celsius. That helps. If you zoom in on the pic....you will see a kind of tube that encases the fuse wire. Is there a tubing I can get to encase the new fuse? I thought of shrink wrap....but don't want to apply any more heat than I have to.
 

Ruger

Aug 22, 2020
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Take care that heat from the soldering doesn't trip the fuse.


Since I now have access....and can bypass the fuse for testing....what is the correct way to test the transformer without power?
 

kpatz

Feb 24, 2014
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Test continuity between the two ends of the primary coil, and the two ends of the secondary coil, and make sure neither are open. There should be no continuity if you test between primary and secondary.

Is the fuse wired to the primary or secondary?
 

Ruger

Aug 22, 2020
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Trying to take another pic.
Anyway....I believe the fuse is on the primary side of the transformer. Looking at the board there is a male blade marked line. On the underside of the board....that circuit goes to the pin that the fuse connects to.
There are only 2 wires connected to pins on that side of the transformer.
 

Ruger

Aug 22, 2020
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2020-08-23 11.41.39.jpg
Not a good pic.
The top blade to the left is marked " N".
The next one down is marked line. This one marked line is the one that goes though the circuit board to the fuse wire.
From the left to right on the transformer as pictured:
1st pin....nothing
2nd pin....wire....not sure where it goes
3rd slot....no pin at all
4th pin....nothing
5th pin....wire that goes to the fuse
6th pin....nothing
7th pin....nothing
 

Ruger

Aug 22, 2020
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I forgot to ask....the testing you recommended....is that with me bypassing the fuse? In other words....jumping across the fuse to complete the circuit

Also....just again want to say thanks for all you help....and patience.
 

Ruger

Aug 22, 2020
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Some update....
I successfully bypassed the fuse....set my meter to 200 ohms (lowest on my meter) and touched the only 2 posts on the primary side of the transformer. I got "ALL" kinds of readings from 135 down to 30 as I touched and retouched the 2 pins again and again.
The number would show momentarily....then the meter would return to 1.
Guess it is pretty obvious by now I am not an electronics expert. Actually I was a tool and die maker. That is what drives me to not give up on things....but to fix them.
 

davenn

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Sep 5, 2009
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I successfully bypassed the fuse....set my meter to 200 ohms (lowest on my meter) and touched the only 2 posts on the primary side of the transformer. I got "ALL" kinds of readings from 135 down to 30 as I touched and retouched the 2 pins again and again.
The number would show momentarily....then the meter would return to 1


that isnt a good test, indicates that there is either a fault with the primary winding, or your testing wasnt correct


the wires will be enamelled insulated, you need to scrape off a bit of the insulation before going across the end(s) of the wire :)
 

Ruger

Aug 22, 2020
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that isnt a good test, indicates that there is either a fault with the primary winding, or your testing wasnt correct


the wires will be enamelled insulated, you need to scrape off a bit of the insulation before going across the end(s) of the wire.

With the meter on 200 ohms:
Before I bypassed the fuse....I didn't get any reading. After the bypass I tried re-testing keeping one probe of the meter constantly on one of the pins. It reads lower....but is still only momentary....then it goes to 1 on the meter.

With the meter on continuity:
Touching the 2 wires on the primary side of the transformer....it doesn't beep....but consistently reads 295.
 
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davenn

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Sep 5, 2009
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With the meter on 200 ohms:
Before I bypassed the fuse....I didn't get any reading. After the bypass I tried re-testing keeping one probe of the meter constantly on one of the pins. It reads lower....but is still only momentary....then it goes to 1 on the meter.

With the meter on continuity:
Touching the 2 wires on the primary side of the transformer....it doesn't beep....but consistently reads 295.

really difficult to know for sure what you are doing
please post some photos showing where you are doing your tests and the meter readings :)
 
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