transformer blowing fuses

[barsukas]

My first post here, might be a stupid questions, sorry if it is.

So, my amp keeps blowing fuses, i took the transformer out, tried to turn it on and it blew a fuse with nothing connected to the secondary windings. Resistance of the primary winding is about 105 ohms, and the fuse is 0.25A 250V, resistance of it is 16ohms. I am really new to electronics, but i think that the resistance of the primary winding is much smaller, than it should be.

Is there a problem with the transformer or could there be something entirely different?

 

[chuckey]

The problem with transformers is that if a turn shorts out to an adjacent turn, it totally destroys its transformer action but its resistance changes by only .01% or so.
What should its resistance be? Looking at your picture, at a guess the transformer should handle 25 Watts, as they are about 90% efficient, it will dissipate 10% X 25W = 2.5 W. One half of this will be in the iron loss and a half in the copper loss due to the resistance of the windings. So loss in resistance of copper is 1/2 X 2.5 = 1.25W. Again a half should be lost in the primary winding, so the primary winding losses .625 Watts. The current it takes is 25W/230 ~ .1 A. P = I^2 X R, .625 = .1 X.1 X R , R = 62.5 ohms. So give or take your 105 Ohms is correct, its just that the transformer has a different power rating with different losses.
Inspect the transformer carefully to see if there is any damage - knocks to the windings or blackened areas. It might just be possible to carefully pull the turns apart at an area of damage to fix the problem.
Frank

 

[Pjdd]

It sounds like your transformer has some shorted turns and has to be replaced. However, some more info is needed to make the diagnosis certain.

1) Have you been using the amp for some time without any problem?
2) What is your AC voltage - 110V or 230V? That bit of info will help judge if the primary resistance is reasonable or not. That's because 110V transformers have fewer primary turns and bigger wires than 230V types. All other design parameters being equal, a 110V transformer has roughly one quarter the primary desistance of a 230V model.

You haven't given the transformer ratings, but judging from its size relative to the multimeter, I'd say that it's somewhere in the region of 10-15VA. For a 230V transformer of that capacity, a primary resistance of 105 ohms is very low and indicates a seriously shorted primary. Not clearly so for a 110V type.

 

[FvM]

A 0.25 A fuse with 16 ohm resistance is very unusual. You shouldn't see more than 1 or 2 ohm. Thus I doubt about the fuse type you're using.

If you use a medium time lag fuse (M type), it won't be blown even with full transformer saturation for a halfwave. Otherwise the transformer has a windings short.

But a fast acting (F) fuse most likely can't stand the inrush current.

 

[arbabar]

Use some High Ampere fuse beacuse i think your using 220v and 2A Transformer.
and also you must note the sound of transformer which is usually produced when transformer is working and check its voltage at the secondary coil.

 

[Pjdd]

A 0.25 A fuse with 16 ohm resistance is very unusual. You shouldn't see more than 1 or 2 ohm. Thus I doubt about the fuse type you're using.

Ah yes, I meant to bring that up too but forgot about it. Good thing you didn't. One possible explanation is that the type of DMM the OP is using sometimes has very high residual resistance in the ohms range if it has been idle for a long time.

But a fast acting (F) fuse most likely can't stand the inrush current.

I've used fast-blow fuses of 0.2-0.25A at the input of 230V transformers in many low-power projects. I even used them in one production batch of about a hundred units a long time ago. But perhaps the OP's transformer was made with very different design parameters from mine.

 

[barsukas]

To answer some commetns above - voltage is 230 and the resistance of the fuse is not 16, but closer to 15 ohms, still no 1 or 2 and visualy can't spot any damage on the transformer. But there's another thing. In the vast internets i dug out a manual for the amp and found out that there should be a T type fuse. The one that blew was F type, and for some reason it worked for a long time with no problems. Guess i'll just get a T fuse and see if that works, im beginning to think that i disassembled my amp for no reason at all...

 

[RetroTechie]

In the vast internets i dug out a manual for the amp and found out that there should be a T type fuse.

That's more like you would expect, especially in an amplifier that usually has some big capacitors in its power supply (and thus large inrush current).

Remember a fuse is (usually) not there to make sure the current stays below an arbitrary limit at all times, it's there to make sure things don't catch fire / damage is limited when something breaks.

 

[FvM]

Yes, that's plausible. As said, a F fuse will be easily blown by the inrush current, a M or T fuse won't. You don't need to raise the fuse current rating.

A 2A fuse in contrast won't disconnect even a massive secondary short and not prevent the transformer from fatal damage. Very dangerous.

 

[Pjdd]

The OP says that the fuse keeps blowing even when the transformer is out of circuit, so it's not the filter cap inrush current. He's been using the amp for quite some time, apparently with the same fuse type, so it's not that either. That leaves only the transformer itself.

 

[FvM]

It's in fact "the transformer itself", but regular operation as well.

As previously mentioned, inrush current is also caused by transformer core saturation. At worst case, the first halfwave has double the regular peak core flux, surely exceeding core saturation flux. The maximum saturation current is even higher than capacitor load current, because the secondary winding resistance isn't in effect.