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Interesting Find, inside a transformer.

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gary350

Well-Known Member
I was testing my transformers to decide which ones are worth keeping when I found one that is an open circuit. I pulled the covering off the transformer and found a tiny little part inside. From what the part says on the outside it is a thermo over load and also a fuse rated 115 degrees C 2A 250v. I have never seen one of these inside a transformer before. Who would know to look for this device hidden inside a transformer?

I'm not sure this is a good idea to have it hidden inside the transformer. It would be nice if it were located so it can be tested to see if it is bad then replace it.

This is what is says on the side of the part.

**broken link removed**

**broken link removed**
 
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Pretty common safety device on smaller transformers nowadays. Easy fix too!
 
If the transformer burns up it would overheat and perhaps start a fire.
 
They are mandatory in some countries.
 
Yes, sorry but nothing 'interesting' at all - it's a VERY, VERY common device inside mains transformers (and has been for a great many years), it's also a VERY, VERY common failure as well - often when the transformer itself isn't actually faulty. On some transformers both connections are brought out externally, and you can bypass it :D
 
Personally I would prefer the self resetting thermal limiters like what are used in the old style iron core fluorescent lighting ballasts.
 
Pretty darn common in alot of small trannies, fans, sewing machine motors and almost any or all home appliances that will produce heat.
They are called thermal cutoffs, a one time device used specifically for temperature but like everything there is a limit on their current abilities.
They come in several temp and current ranges as well as shapes, sizes and package configurations.
Some are easy to get to but I've had some where they are buried deep under many windings, so not repairable if'n you want to be as safe as it was.
There is a dishwasher out there that uses a 98 C cutoff mounted on the main board inside the control panel. Well we all know that water evaporates at 100 C and if the vents on the other side of the control panel gets plugged with crude the steam will migrate over to the board and pop the cutoff.
Great design, they go all the time.

There was a big recall around here a couple decades ago for coffee makers i think, cause they were catching fire as the wrong or defective cutoff was failing to trip. One needs to be wary of re-using an older one.

https://www.google.ca/search?q=ther...aJIW0ggSk5YH4Aw&ved=0CFYQsAQ&biw=1680&bih=890
 
The thermo fuse is not replaceable because a transformer is damaged by overload (overheating) and should not be used again.
 
The thermo fuse is not replaceable because a transformer is damaged by overload (overheating) and should not be used again.

There is nothing wrong with this transformer. I removed covering over enamel coated copper wire it looks brand new. I put 2 new terminals on the transformer and it works good as brand new. 120 vac primary and 14 vac secondary. I plan to replace the secondary coil with an 8.5 vac coil to get +12 vdc 2a to drive the gates of 2 mosfets. I will put a fast blow fuse on the primary.

**broken link removed**
 
There is nothing wrong with this transformer.

As I said in post #5, often the fuse fails when the transformer isn't faulty - either the fuse just fails (not unknown), or the transformer has overheated because of excessive loading on it. The third option (of course) is that the transformer is faulty, and has shorted turns causing it to overheat.

I had a MAJOR row with Sony (went on for two-three years :D) - about repeated failure of the mains transformer in a rather nice Sony amplifier. The amplifier was rated at 100W+100W RMS continuous, and looked a really nice amp, the customer (who was young, and presumably played it LOUD!!) brought it back dead, and the mains transformer was O/C (or at least the internal heat fuse was). I was surprised at how light the amplifier was, and once the top was off, at how small the mains transformer was - and also at how much space there was around the transformer - as if it had the wrong transformer fitted?.

I replace the transformer under warranty, and it failed again later when the unit was out of warranty - which resulted in my row with Sony.

Comparing the transformer size and weight to others, it was only rated about 75-100W, FAR too small for a 200W amplifier, and eventually Sony agreed to supply a new transformer for free - but only the exact same 'too small' one.

As I see it the unit was designed for a suitably sized toroidal transformer, the space for it was there, and the small one even had to have a special mounting bracket to make it fit in the larger space - presumably some 'bean counter' had altered the specification in production and fitted a much cheaper transformer, assuming that no one would ever play it loud.

Anyway, Sony would never admit to the manufacturing error, and just insisted that failure rates had been low.
 
Way baaaaack, manufacturers used to put replaceable fuses on user-accessible fuse holders.

Two things could happen next:
-The user would -by ignorance- put a wrong (higher rated) fuse.
-The user would -by malice- put a wrong (higher rated) fuse.
Smoke and/or fire would then ensue.

National Safety Agencies therefore regulated that protection devices should be non-user replaceable.
 
Way baaaaack, manufacturers used to put replaceable fuses on user-accessible fuse holders.

Two things could happen next:
-The user would -by ignorance- put a wrong (higher rated) fuse.
-The user would -by malice- put a wrong (higher rated) fuse.
Smoke and/or fire would then ensue.

National Safety Agencies therefore regulated that protection devices should be non-user replaceable.


You forgot
The user would -by stupidity wrap the fuse in tinfoil and re-seat in the amp to "keep the party going". :nailbiting:
 
You forgot
The user would -by stupidity wrap the fuse in tinfoil and re-seat in the amp to "keep the party going". :nailbiting:

The variety of this I've seen repairing construction and mining equipment is incredible. Piece of rebar or threaded rod, mashed foil from a cigarette package rolled into a cylinder (so your fire starting tinder is right there), a (thankfully empty) rifle bullet casing, an Allen wrench or a driver bit. All shoved into fuse holders and in use.
 
Many years ago, working late on a Friday evening at an electronics, someone from a nearby factory came in and asked if we sold fuses. We didn't but I asked what he was after. His company had blown the fuses on a welder, and had a load of work to do over the weekend. He showed me the blow fuses and I realised that our completely different welder used the same fuses, so I lent him ours.

He was worried that would leave us short, but I reassured him that we weren't going to need the welder until Monday, the electrical suppliers over the road would be open by then, and the fuseboard had a dedicated circuit for the welder, rated the same, so I could link the fuses I lent him anyhow.

He went away relieved, and came back first thing on Monday with a big bag of fuses to replace the ones he had borrowed.
 
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