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Simple transformer question.

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Arcstarter

Thu Jun 26 2008, 08:19PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I have a REALLY nice stereo power supply transformer that no longer works. I think i overheated it. But the weird thing is that it pulls no primary current and has no continuity. Does it maybe have a temperature operated fuse? Maybe once it gets too hot, it will blow? Or do some transformers have a built in 'regular' fuse?

Dr. Dark Current

Thu Jun 26 2008, 09:12PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Yes it is probably some internal thermal fuse. You can try to dig into the transformer for the fuse and then bypass it, as you apparently have nothing to lose...

Arcstarter

Thu Jun 26 2008, 09:24PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Dr. Kilovolt wrote ...

Yes it is probably some internal thermal fuse. You can try to dig into the transformer for the fuse and then bypass it, as you apparently have nothing to lose...

That is what i hoped for, but it may also be a blown primary, which i would have to unwind it until i get to broken part and the i would solder it and wrap it in electrical tape, or actually i would just use a butt connector.

Dr. Dark Current

Fri Jun 27 2008, 10:42AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Arcstarter wrote ...

That is what i hoped for, but it may also be a blown primary, which i would have to unwind it until i get to broken part and the i would solder it and wrap it in electrical tape, or actually i would just use a butt connector.

Well, "blown" primary will read less resistance than a good one because the insulation has broke somewhere. There is no way for the copper wire to melt before the inssulation does.

ahron11

Fri Jul 04 2008, 11:24PM

Registered Member #1572 Joined: Fri Jul 04 2008, 11:14PM
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Posts: 19

How long have you had the system. High voltage parts are very prone to breakdown. But before I would investigate that I would look for any fuses that may have blown like Dr.Kilovolt said. But what makes you think that there is a fuse inside the transformer?

Arcstarter

Fri Jul 04 2008, 11:29PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

ahron11 wrote ...

How long have you had the system. High voltage parts are very prone to breakdown. But before I would investigate that I would look for any fuses that may have blown like Dr.Kilovolt said. But what makes you think that there is a fuse inside the transformer?

Well it came from a quite old stereo. It is not, in any way, high voltage though. It has a 50 volt tap, which is the highest voltage it has. I starteed to take all the laminates off but i stopped for now.. I need to get a new transformer, so if anyone knows of a transformer with 30-50 volts ac and a few amps(2 or more), let me know.

Billybobjoe

Fri Jul 04 2008, 11:41PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Dr. Kilovolt wrote ...

Well, "blown" primary will read less resistance than a good one because the insulation has broke somewhere. There is no way for the copper wire to melt before the inssulation does.

But copper wire can melt along with the insulation - this may be the case. I've had small(ish) transformers do this after a power surge.

Sulaiman

Sat Jul 05 2008, 01:38PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Industrially we get many small transformers with open-circuit primaries,
the chances are that when you try to repair it you will find a charred mass of wire and insulation
that can't be repaired. Sometimes we get lucky - but usually not.
Now the hard part is to find a replacement:-
If you're lucky you will find just the info. you're after
when you don't then some educated guesswork is called for.

The power/VA rating is basically by weight!
Find a transformer of similar style and weight to estimate the VA rating.
If space allows replace with a higher VA rating since this one must have been marginal.

Ideally you count primary and secondary turns - very time consuming

Voltage ratings of capacitors after rectifiers indicate the maximum possible secondary voltage
but many designers (wrongly) use higher voltage ratings than the circuit will ever see,
so the actual working voltage will be a little or a lot less than the rated voltage.
Look for linear regulators (78xx for example) to get an idea of minimum working voltage.

With your estimate use a variac with a test transformer to operate the circuit.
The correct voltage is just between 'everything just about working' and 'where's that smoke coming from?"

quicksilver

Sat Jul 05 2008, 02:48PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Perhaps you could save some money and time by getting a MOT and pulling some of the secondary windings off? I suppose that's a rather long way around the replacement issue .....but depending where you live Radio Shack has a 25-30v PS part of about $10.

Arcstarter

Sat Jul 05 2008, 11:48PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

quicksilver wrote ...

Perhaps you could save some money and time by getting a MOT and pulling some of the secondary windings off? I suppose that's a rather long way around the replacement issue .....but depending where you live Radio Shack has a 25-30v PS part of about $10.

Well the way i ruined it was by running an mot with it for around 700 volts :P. I would just make a modded mot for 50 volts but my mots are so small that with relatively thin wire i could only get around 30 volts(29). I suppose i will take this transformer apart and use the secondary windings for a new secondary on my mot. Thanks for all the help though!

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