Can a split primary (115/230) transformer be used as an isolation transformer? Applying 115V to one primary winding, and taking 115V out of the other primary winding? If so, what would the VA rating need to be to draw one amp from the isolated output?

 

Not really it is now a auto-transformer.
And the VA remains the same.
Unless they are two distinct and separate windings.
Then it would be primary and secondary. So the only point would be isolation?
But to take advantage of the total Va they are usually paralleled for 120.
Max.

 

Max, I think Shortbus is talking about two separate 115 V primary windings.

I think in most cases, it would work, at >1/2 VA rating.

 

I would be cautious if using the windings in a way not intended by the designer of the transformer. Although the split windings might provide insulation good enough to prevent destructive breakdown the leakage current between the two windings might be high for a particular application, and there might not be sufficient creepage distance to safely use it as an isolation transformer.

 

Dick
The winding would have to be able to withstand the voltage. They are designs to be wired in parallel (115Vac) and in series (230Vac). Using the primaries as isolation transformers would have the same voltage potentials as if they were wired in parallel (115Vac).

 

True, but the insulation requirements are different. Between the two primaries you need only basic insulation. For a true isolation transformer you need reinforced insulation. So if it is a true dual primary transformer (four wires instead of three), they you will have the isolation but not the protection of a purpose-built device.

ak

 

OK, both of you, AK and DC, make a good point. This would not be a classic (rugged) (Bullet-proof) (high) isolation transformer. It would be a cheap light weight isolation transformer. I still stick to my first statement, in most cases, it would work.

 

I would think it very unusual to find a transformer with two isolated primaries.

 

I would think it very unusual to find a transformer with two isolated primaries.

You mean like these?: http://www.stancor.com/pdfs/Catalog_2006/Pg_005.pdf

 

there are lots of split primary transformers. most industria control transformers are done this way do give dual input voltages (120 or 140 volts ac) as wull as some smaller ones. I have some small transformers with two 120 volt primaries and a 6 volt ac secondary that I use for low power tube power suplies. one primary is used for the 120 vac input, and the other is used for the plate supply gives about 170 volt after rectification and filtering, the secondary for the filament. just watch the power ratings and dontg pull too much.

 

I would think it very unusual to find a transformer with two isolated primaries.

Signal Transformer has zillions of them. The idea is that at 110 the two primaries are in parallel and at 220 they are in series This way the 110 V input current in each winding isn't double the 220V current for the same power level.

ak

 

The reason I asked this is I need a small isolation transformer and I have a bunch of salvaged transformers that have dual separate primaries. Used to be transformers were fairly inexpensive. But looking now, they are fairly expensive. May be due to most things having a switched power supply, don't know.

The transformers I'm talking about are separate primaries Two rated at 115V when wired in parallel and 230V when wired in series. I'd rather not have to buy a new transformer when I have these unused ones sitting there. But I do want isolation from the mains. This is just for my own use, not something that would be sold. I got the idea for this from one of the vintage radio sites, they did it for a 'B' battery eliminator.

If I do this should I remove the secondary winding or just heat shrink over the lead ends?

 

No real need to remove the secondary.
Max.

 

Signal Transformer has zillions of them. The idea is that at 110 the two primaries are in parallel and at 220 they are in series This way the 110 V input current in each winding isn't double the 220V current for the same power level.

Yes, I forgot that they put the windings in parallel for the lower voltage, which would require separate primary windings, not a split winding.

If you use a transformer with two 120V windings for isolation then the usable VA rating is the question.
When the windings are in parallel to operate for 120V use, each winding only carries 1/2 the current.
But, since the rating of a transformer is related to heat buildup, mostly from I squared R through the windings, and there is no secondary current, then you should be able to operate at about .707 of the full VA rating, assuming that the transformer was designed for 1/2 the heat build-up being caused by the secondary current.

 

If you use a transformer with two 120V windings for isolation then the usable VA rating is the question.
When the windings are in parallel to operate for 120V use, each winding only carries 1/2 the current.
But, since the rating of a transformer is related to heat buildup, mostly from I squared R through the windings, and there is no secondary current, then you should be able to operate at about .707 of the full VA rating, assuming that the transformer was designed for 1/2 the heat build-up being caused by the secondary current.

I think the available VA capability will be greater than 1/2 the full VA rating for the reason you give, but it will not be as much as .707 times the full rating. The rating of a transformer is indeed related to the heat buildup--in particular it's controlled by the hot spot temperature of the hottest winding. Normally, the designer tries to make the hot spot temperature of all the windings be the same; this gives the maximum VA capability.

Using only the two primaries means that the heat buildup is not spread throughout all the copper in the transformer but is concentrated in the primaries only (core losses should be the same). When drawing .707 times the full VA those two windings will dissipate about as much power as all the transformer's copper is rated for in normal use (with all the copper participating in the heat generation). This would be about twice the power they dissipate in normal operation and their hot spot temperatures will likely be greater than they would experience in normal operation, which would not be good.

If the secondary windings are dissipating no power, the heating they normally produce will not be adding to the temperature rise of the primaries so the primaries can dissipate somewhat more than in normal use without exceeding rated hot spot temperature, but I don't think double normal dissipation in the primaries would be allowable. The only way to be sure would be to measure the hot spot temperature using the UL method of measuring the increase in resistance of the windings due to heating.

 

So, anytime you use a part outside of its normal topology, it is good to de-rate it.

 

yes.

 

Thank you all for your answers. But to be safe I'll make sure my fire insurance is up to date.

 

Shortbus, don't worry. It will smell really bad before it catches fire.

 

I've smelled that smell many times in my projects. Sometimes I think I was the inspiration for Lynyrd Skynyrd.