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dgta 3rd December 2012 12:23 AM

Blowing a power transformer winding open
This is a slightly unusual circumstance, bear with me.

I have a HP711A lab supply with a bad PT. Investigation shows one of the HV secondaries (250V) putting out only 60VAC open circuit. The PT gets slightly warm and starts smoking very lightly after a minute or so. I figure that winding is partially shorted.

Replacing the whole PT is impractical (many secondaries) but I can easily add a small PT for that winding only, if I can clear the short. I'm thinking blow it open with a short HV jolt. Has anyone done something like this? Maybe charge a good size cap to 400V and zap it? Any other ideas? Thanks.

danielphantom 3rd December 2012 01:10 AM

Somehow I think that the results you would get from that would be very unpredictable and may cause more problems than you already have. But hey, what do I know?

good luck

DF96 3rd December 2012 10:15 AM

You are just as likely to weld it together properly. Your best bet is to investigate getting it professionally rewound. There are people who specialise in this, as it is often required for maintaining old equipment.

rikkitikkitavi 3rd December 2012 12:15 PM

Any energy you put into the winding with the purpose to blow it open will spread through the insulation to the others, probably causing further damage.

Already the heat loss from the shorted winding could have damaged insulation on the others, and worse primary-secondary insulation. Any PT smoking is not good...

You have two options: scrap it or have it rewound entirely, AFIK, unfortunately.


freax 3rd December 2012 12:18 PM

What I have seen done before is the affected winding (in this case a bias winding on a Yaesu FT-101) which was smoking after being attacked by some dumb kids who had easy access to a nail and a hammer and decided to take it upon themselves to break into a fellows home back in the 1970s and attack his radio with said nail and hammer.

In this case the HAM radio operator had left the affected winding unloaded and then a substitute second transformer was simply wired in to replace that one winding.

This would be useful to you if you can somehow surgically isolate this secondary winding from the part which is causing the short and then substitute it in the previously mentioned manner. This would be possible if the short is near to the last few layers of insulation.

This would only work if it was not smoking when you take the load off the winding otherwise.

Failing this I suggest you put the transformer on some Champix to quit smoking. haha

CBS240 3rd December 2012 12:33 PM

If the secondary is smoking, then it is already shorted internally. Somehow you need to remove it from the core. Some type of transformers are easier to wind/unwind such as toroidal. If it is a EI type with lots of glue and epoxy resin it will not be an easy job. If it comes to it, there are toroidal transformer kits that are available and you can wind all the secondaries you want. (within spec of course:D)

powerbob 3rd December 2012 03:25 PM

If you really wanted to try to blow the secondary winding open you would use a lower voltage maybe10vdc to 50vdc to charge up a large value capacitor of 10‘s of thousands of uF and apply it to the secondary as fast as you can with thick wires. This will blow apart any short. But you may have a short of one secondary winding to another. The mini explosion could also make things worse, damaging more winding’s. But it is no good the way it is now anyway.

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