THERMALLY FUSED TRANSFORMER

[kevinkr]

Thermal fuses are relatively robust and I would not expect one to blow unless the transformer was seriously overheated. Usually they are rated for a temperature just below the point where the wire insulation in the hottest part of the transformer will fail. Properly designed transformers place the fuse in a location where the highest temperature is likely and not near the surface. They do occasionally fail due to other causes, but as AndrewT mentioned there is usually a reason. I'd check all associated supply components carefully and really think about replacing that transformer - they generally do not blow under electrical overload, but as a result of over temperature.

[!]

^ With all due respect, no, thermal fuses aren't robust. They will always fail after a number of years even if the target temperature was never reached.

Thermal cycling (or time?) will make thermal fuses fail. I've replaced far too many to count where there was no overheating condition, only that the thermal fuse failed itself.

Granted, caution is necessary to be sure it didn't fail for the designed reason it was there, which is quite possible. Even so, on many circuits where I found a thermal fuse the fault, replacing it with no other changes resulted in years more operation of a device with no other changes.

So... don't believe any of us, test the circuit because the fuse is a potential fault point in and of itself, or it could be an indicator of another fault point.

[simon7000]

If there is a problem in a transformer it gets hot. The issue is if the transformer failed due to a weak fuse or if it is running hot. A temperature reading will show you the core temperature.

On high power transformers you can pick a rise of 50, 80, 115 or 150 C. The lower the rise the greater the cost. If you are cheap you do pay more for actual electrical draw. If maintenance is an issue you spend more money.

For a small transformer 20 C is the most I would expect to see. If there is an internal short it will go way beyond that.

Any failure of components past the transformer should cause the AC line fuse to blow. A bad transformer will overheat even without a load.

[tauro0221]

Hi,
I would let it running like it is but making sure that you have the right size inline fuse. just in case the transformer failed. I also ran into this problem. I will called this mechanical failure due to years in services.

[kevinkr]

Originally Posted by !

^ With all due respect, no, thermal fuses aren't robust. They will always fail after a number of years even if the target temperature was never reached.

Thermal cycling (or time?) will make thermal fuses fail. I've replaced far too many to count where there was no overheating condition, only that the thermal fuse failed itself.

Granted, caution is necessary to be sure it didn't fail for the designed reason it was there, which is quite possible. Even so, on many circuits where I found a thermal fuse the fault, replacing it with no other changes resulted in years more operation of a device with no other changes.

So... don't believe any of us, test the circuit because the fuse is a potential fault point in and of itself, or it could be an indicator of another fault point.

Not my experience at all with tens of thousands of power transformers involved... (Consumer audio product) Of course the transformers in question were competently designed and used good quality Japanese thermal fuses.

Also most of the devices here at home have thermally fused transformers - the only failures I have seen in some 20 yrs involved a partially shorted rectifier, and the thermal fuse in a hairdryer I misapplied for heat shrink tubing. (And even that one ran for a couple of years before it failed.)

Otherwise I completely agree with the cautions suggested.

[tauro0221]

Hi,
Thermal fuse will also failed for high current. They are sizing by current that they can handle and the temperature.

[kevinkr]

Originally Posted by tauro0221

Hi,
Thermal fuse will also failed for high current. They are sizing by current that they can handle and the temperature.

IMLE should not be sized by current rating, but practices may differ. Usually a separate fuse or fusible resistors would (should) be fitted to deal with other component failures, or inadvertent short circuit so that the transformer itself is not compromised, but I will admit this is not always or perhaps even usually done. As a cost cutting measure I can see it being done since repair cost and long term customer satisfaction are not usually part of the design equation for cheap consumer electronics, as a design matter it is nothing other than a poor design practice.

[!]

Originally Posted by simon7000

If there is a problem in a transformer it gets hot. The issue is if the transformer failed due to a weak fuse or if it is running hot. A temperature reading will show you the core temperature.

On high power transformers you can pick a rise of 50, 80, 115 or 150 C. The lower the rise the greater the cost. If you are cheap you do pay more for actual electrical draw. If maintenance is an issue you spend more money.

For a small transformer 20 C is the most I would expect to see. If there is an internal short it will go way beyond that.

Any failure of components past the transformer should cause the AC line fuse to blow. A bad transformer will overheat even without a load.

If there is a problem causing it to run hot, that's power loss... it won't be outputting the correct voltage at current which can be detected with a multimeter easier and more accurately (since we don't actually know what the (amp) designer's target rise over ambient temp is for the transformer).

The other factor is you have to have the amp closed up to get the running state temperature reading and in that situation, internal chassis temp could easily exceed 20C rise over ambient without fan cooling.

Failure of components past the transformer might or might not cause a fuse to blow. Take for example a leaky transistor. It may not cause enough current flow to blow the AC fuse, particularly in a linear PSU powered amp as the fuse may be sized sufficient to withstand power on surge current due to the transformer and bulk capacitors after it, if it doesn't have a fairly restrictive current limiting built in.

[!]

Originally Posted by kevinkr

Not my experience at all with tens of thousands of power transformers involved... (Consumer audio product) Of course the transformers in question were competently designed and used good quality Japanese thermal fuses.

Also most of the devices here at home have thermally fused transformers - the only failures I have seen in some 20 yrs involved a partially shorted rectifier, and the thermal fuse in a hairdryer I misapplied for heat shrink tubing. (And even that one ran for a couple of years before it failed.)

Otherwise I completely agree with the cautions suggested.

The key thing might be whether it is an ideal, perfectly manufactured, quality Japanese thermal fuse and the transformer isn't a bit undersized for the load. Certain transformers or other devices may have their own particular high fault rate components.

The test is simple enough, replace a failed thermal fuse and reproduce the operating state the device was in when it failed. If the replacement thermal fuse does not trip with nothing else changed...

[tiomiguel]

This is a very interesting question with OPPOSITE opinions.

I have uploaded images from the transformer and its situation

HK990 has FOUR transformers: Two big toroids, in top is the smallest standby transformer, and under it and the central board is THE PREVIOUS section transformer, who fuse failed. this thermal fuse is set to 125 ºC as seen in picture.

The transformer is in very bad place, between the large heatsinks of the HK990 (very high biased: 170W idle power), as you can see. I think the transformer has been oerheated by its enviroment time after time... the transformer is relatively small...

After fuse changed, I put the amplifier over aluminium pieces for standing 3 cm over ground, for easier air circulating.

I think this tranformer has a bad situation, and repeated EXTERNAL overheating MAY caused thermal fuse to open... with the images, perhaps question will be clearer... ¿WHAT DO YOU THINK?

¿replace the entire transformer or keep working actual with new FUSE?????????