What's wrong with my transformer?

[madan1]

Happy new year everybody

Here I have an odd case (at least for me) of a toroidal power transformer that blows fuses even without any load on the secondary side.
When powered through a current limiter, I measure no significant AC on the secondary side ( like 7-9V on the 35 output ).
I've done the measures with a component tester..
The switch on the primary side is for 110-220V and both inputs act as short when connected to mains.

[stj]

Re: What's wrong with my transformer?

stick a lamp in series with the input, it's probably just inrush current

[madan1]

Re: What's wrong with my transformer?

stick a lamp in series with the input, it's probably just inrush current

Did that, the lamp lights up and on the "kill a watt" I see the full lamp wattage. Meanwhile on the secondary side of the transformer the voltages are way too low ( 9v-ish instead of 35 ).
What is confusing me is that between any red and black input I measure at least 2k Ohms, which seems to be normal(?) transformer input value.

If it was dead short or open I would have already scrapped or rewinding it (depends on how hard is to find), but now I'm just baffled.


p.s. The only bad readings are between the red cables ( the extension coil for the 110/220V), but still does not explain why it shorts with the black inputs.


p.s.2 Alright, just got an idea - it is sparking (and can't be seen on the tungsten lamp because it is not that sensitive to such interruptions)! So now have to figure out how to find the spark (if any) before trying to rewind it.

[R_J]

Re: What's wrong with my transformer?

Is that a switch between 1&2 and 3&4? you can't short a winding with a switch. I would think the primary should only have 2 separate windings.
What is this transformer from?

[madan1]

Re: What's wrong with my transformer?

The diagram with the switches does not look correct, you can't short a winding with a switch. What is the transformer from?

The transformed is from an audio amplifier (carlsbro orion active speaker) and this is the way it has been designed. I guess when the switch shorts the extended coils it works like bypass (just a guess, never really have been interested in transformer designs and construction).
I will post a photo of the switch.. just have to find it.


p.s. Here is a photo. The blue cables come from the mains, the most right switch connectors are not used... but, now I noticed that indeed they are shorted under the heatshrink.
This means that when 220V is selected, the middle red and blue cables are shorted.


p.s.2 And just to confirm - when 120V is selected, both red wires and both black wires are shorted to each other and to the mains(blue wires). The switch solderjob and heatshrinks seem original.

[R_J]

Re: What's wrong with my transformer?

The transformer only has two primary windings, they should be separate, I suspect you have a short between the windings.

In the 110v position, black and red of the two windings are in parallel, in 220v position the two windings are in series

[madan1]

Re: What's wrong with my transformer?

The transformer only has two primary windings, they should be separate, I suspect you have a short between the windings.

Huh, 2 coils totally makes sense.
Here is what I get if each red-black is a separate coil.


p.s.
Still, the lowest Ohm readings that I get between the possible input wires is over 2k.


Well, I guess a transformer surgery will have to tell the full story.. and I hope it will not be an autopsy .

[R_J]

Re: What's wrong with my transformer?

I would guess a primary dc resistance of between 5Ω ~ 30Ω per winding

[momaka]

Re: What's wrong with my transformer?

Sounds almost like shorted windings somewhere.
Typically, I find the primaries of large transformers to be in the Henry range or at least many 10's of mH.

Do you have a different AC transformer that outputs anywhere between 12 and 24V AC? If yes, feed that in the secondary windings and see what the primary windings read when they are all disconnected from each other. Ideally, use something like a 12V transformer out of a UPS, which has decently high current. Then, using one or more 12V halogen bulbs (50 Watts minimum, 2x 50W should suffice) wire them in series between the UPS transformer secondary and the secondary of the audio transformer. If there are shorted windings, those halogen bulbs will glow and stay lit. And you may be able to determine which (if any) 110V winding is causing the issue, as that would be the one with the lower voltage output.

I would forgo resistance testing of the windings at this point, because it would not tell you if there is a shorted turn or not. If you have a component tester, disconnect both 110V windings and measure their inductance separately. If one of them has shorted turns, it will show a much lower inductance. Also check inductance of the secondary windings too.

[CapLeaker]

Re: What's wrong with my transformer?

i guess you need a insulation to check insulation breakdown under HV or ring tester, but i am sure the input of the transformer is shorted. Only because you can't measure a direct short with you low voltage DMM, doesn't mean nothing for higher voltage if the insulation is compromised and able to arc over.

[harp]

Re: What's wrong with my transformer?

When you connect together two or more windings of transformer, the orientation of windings is crucial to operate properly.

For detect spark, maybe to use AM radio, and lissen hum...?
Btw, you can build equipment aka ring tester for testing transformers windings.

[madan1]

Re: What's wrong with my transformer?

So after several hours of unwinding, finally reached the primary side... and voila - a short right on the black wires input. Unfortunately it had affected the windings beneath and of course - the affected spot is right at the other end of the coil and to reach it have to unwind the whole coil. After some cleaning found two significantly affected wires and after insolating them with some heatshrink, the short disappeared. The following days gonna test it with 220V and if it doesn't fail, I might try rewinding it.

When you connect together two or more windings of transformer, the orientation of windings is crucial to operate properly.

Yep, totally makes sense considering the magnetic polarity depends on the current direction.. still, trap for those who do not pay enough attention. By the way, what gonna happen if the orientation is reversed?

i guess you need a insulation to check insulation breakdown under HV or ring tester, but i am sure the input of the transformer is shorted. Only because you can't measure a direct short with you low voltage DMM, doesn't mean nothing for higher voltage if the insulation is compromised and able to arc over.

Yep, lot's of carbon on the burnt place and no direct copper to copper connection. No way a 9v multimeter could get short there.

I would guess a primary dc resistance of between 5Ω ~ 30Ω per winding

7-ish Ohms indeed.

Do you have a different AC transformer that outputs anywhere between 12 and 24V AC?....

I like the idea and next time I have a semi-dead transformer on my bench gonna try it. I do not have a powerful 12V transformer, but have enough powerful audio amps to use them instead of a transformer .

[harp]

Re: What's wrong with my transformer?

By the way, what gonna happen if the orientation is reversed?*

I think that voltage is lowering for windings value, but produce much more heat in core, like constant inter-semi-short.

BTW, I use a carton tube of toilet wc paper for holding bunch of wire, it is show handy to me, and very available

[madan1]

Re: What's wrong with my transformer?

Well, a total of 10 hours of unwinding/winding later I have sore fingers and a working transformer.
Not the prettiest one, but still - working with well balanced voltages. Indeed I was really surprised how balanced ended the voltages on the secondary side - both 35 and 14.5 have less than 0.05VAC difference even though I had to cut some wire on both.
I also found out what happens when the secondary side is shorted.. good thing all tests were done through a lamp.

BTW, I use a carton tube of toilet wc paper for holding bunch of wire, it is show handy to me, and very available

Not applicable here - the carton tube is too thin and also too large to pass through the toroid hole.


By the way, the toroidal transformer winding machines are really clever.

[CapLeaker]

Re: What's wrong with my transformer?

Good job!

[momaka]

Re: What's wrong with my transformer?

Well, a total of 10 hours of unwinding/winding later I have sore fingers and a working transformer.

Nice!

Great to hear you got it working.

I also found out what happens when the secondary side is shorted.. good thing all tests were done through a lamp.

I like the idea and next time I have a semi-dead transformer on my bench gonna try it. I do not have a powerful 12V transformer, but have enough powerful audio amps to use them instead of a transformer .

Perhaps go to a shop that sells household lightning and ask for models that use 12V bulbs. The key is to find lights that use an "oldschool" line transformer like yours shown here. Doesn't matter if it's toroidal type or regular EE/EI core.
However, I suspect you will find these older transformers will be expensive to buy by themselves, due to materials used in them (lots of Copper wire.)

A microwave oven transformer (MOT) can also be modified to do that if you can find one for cheap. Just open the core, remove the HV windings, and add your own low-voltage ones.