Can two inputs make an isolation transformer?

[scuzzyTerminator]

I just acquired a toroidal transformer with two 120V input coils.

I figure that the inputs are indended to be wired in parallel for double current or series for double volts.

Is there any reason that one can not be used as an isolated output? Perhaps insulation is a problem?

Sorry the specs are scant.

[tecman]

It is okay to use as an isolation transformer.  Just be aware that you can only use it at 1/2 the rated power, since each primary coil is designed to handle 1/2 of the power. 

One issue is that there will be no shielding between these two primary windings.  Therefore there will be come capacitive coupling between the windings, which can result in some voltage present on the second winding with respect to ground (on the first winding).  While generally low current, you can get a nice tickle from the secondary winding to ground.  A proper isolation transformer will have an electrostatic shield to prevent this from occurring.


paul

[Gyro]

Firstly, as a "Medical Isolation Transformer" it's intended for the purpose - and a very good one.  The two input primary windings are intended to be wired either in parallel (100-120V supply) or in series (200-240V supply). You need to connect both primary windings to achieve the rated wattage of the transformer.

As for the question I think you're asking - can I use one of the primary windings as an isolated secondary? The answer is a definitely not. Firstly, for the reason above, and secondly (as you guessed) because of insulation. The primary and secondary windings are formed in very particular and very well insulated layer sections, with a protective ground screen (green/yellow), also insulated, between them. The two primary windings are together, in the same layer section.

You need to use the secondary windings, as intended.

[scuzzyTerminator]

OK, thanks.

But if they can be wired in series wouldn't that create the same potential difference between the input coils as if they were wired for isolation? I would think they would need enough insulation either way.

Maybe they are not intended to be wired in series?

[Gyro]

Yes, they are insulated enough for series / parallel configuration across the mains (a few hundred volts), that's not the same insulation quality as between the primary and secondary windings of a mains transformer (several kV) and a safety isolating transformer in particular.

Stick with the intended secondary windings: Red-White and Blue-White (Check!).

P.S. You've got a very nice quality 300VA safety isolating transformer there, the medical ones are the best you can get. Be nice to it! 

[scuzzyTerminator]

Ah, if I understand you are saying that the insulation could break down with a high voltage event.

I wasn't really thinking of high voltage isolation from mains for safety but just being able to probe something with an oscilloscope, only a few hundred volts isolation.

P.S. You've got a very nice quality 300VA safety isolating transformer there, the medical ones are the best you can get. Be nice to it! 

We always hurt the ones we love.

[Gyro]

Yes, correct.

No, that's when accidents happen (just probing something with a scope!), you need the safety isolation capability. The two primaries are possibly just relying on the insulation thickness on the enameled winding wire (as a single 240V winding would).

I don't understand your reluctance to use the proper secondary windings, they should be the same voltage. 

[Kilrah]

Yup don't understand, you're wanting to use it for exactly what it's made for, but in a twisted way instead of the intended one for apparently no good reason...

https://www.plitron.com/default-factory-specs/medical-grade-toroidal-transformers/

[scuzzyTerminator]

As I said, the specs are scant and I do not know yet what the secondary voltages are. Of course I would rather use them if they are the full voltage.

You think that a medical isolation transformer only isolates and does not step up/down? Great if I could be sure.

[Gyro]

Yes, they should be the same voltage as the primaries [Edit: otherwise the output voltage would probably be stated on the label] - they don't have 100/120V taps because the ratio can be adjusted with the primary ones. If you are not able to safely measure the voltages, then you should be able to check their resistance - they should be approximately the same as the primaries (different winding diameters so not exact).

Using a single secondary will get you 150VA. Again you use parallel or series for full wattage 120 or 240V. IMPORTANT: If wiring both secondaries, you will need to make sure you connect them in phase otherwise you will get 0V (series) or an effective short (parallel).

It doesn't sound as if you are that experienced in mains wiring from your previous replies, if so, please be very careful and ask for advice. Be safe.

[electrodacus]

I think you where searching for this

[Gyro]

Perfect. 

Those embedded thermal switches (they may well be non-resetting) are another reason not to take liberties with it!

[ArthurDent]

As others have said, it may already have a split 120/240 volt secondary.  The unused wires are probably unused 100 volt primary taps. Looks like it has a shield between primary and secondary and low capacitive coupling. Use an incandescent 40 watt bulb in series with the line when you first power it up to make sure of your connections. It should be good for up to 2.5 A.

https://www.plitron.com/default-factory-specs/medical-grade-toroidal-transformers/

[scuzzyTerminator]

I think you where searching for this

Thanks. I went back to the store today and found what I think is the correct schematic. Exactly the same but with a ground shield.

Those certainly look like they are supposed to depict 1:1 windings.

[scuzzyTerminator]

It doesn't sound as if you are that experienced in mains wiring from your previous replies, if so, please be very careful and ask for advice. Be safe.

I'm actually less experienced than it sounds. I'll be careful. The best thing I got going for me is that mains freaks me out.

Thanks for all your help.

[scuzzyTerminator]

To clarify my intents, this was a case of grabbing something before they are gone and then figuring out what I got. Now these look like they are nice isolation transformers so I went back and bought two more. The last two. Couldn't just leave them there.

I also picked up these two. Labels are good. CJLTT.

Thanks again, everyone.

[Kilrah]

As I said, the specs are scant

My post above had a link to the full specs and schematic?? 

[Zero999]

I think you where searching for this

Thanks. I went back to the store today and found what I think is the correct schematic. Exactly the same but with a ground shield.

Those certainly look like they are supposed to depict 1:1 windings.

There are several different types of insulation:

Functional: can only withstand enough voltage for normal operation. Failure of such insulation will not pose a shock hazard.

Basic: used between an earthed enclosure and live parts or a non-earthed enclosure and some live parts with another layer of basic insulation in between.

Reinforced or double: used between a non-earthed enclosure and live parts.

Functional insulation is the bare minimum required to allow the device to work.

Basic insulation needs to be significantly over-rated, because its failure can result in electrical shock, although not so much as reinforced insulation, as there's  enclosure is earthed or has another layer of basic insulation, offering more protect.

Reinforced insulation needs to be twice as good as basic insulation, because there's only one layer between it and the user.

[001]

why medical isolation transformer use torroidal construction? 

in 80`th all medical trs has primary and secondary coils at DIFFERENT bobbins to eliminate parasitic capasitance

[Zero999]

Why not use a toroidal construction for a medical grade isolation transformer?

There's an earthed screen between the primary and secondary, which both offers additional protection and prevents capacitive coupling between he secondary and primary windings.

[001]

Why not use a toroidal construction for a medical grade isolation transformer?

There's an earthed screen between the primary and secondary, which both offers additional protection and prevents capacitive coupling between he secondary and primary windings.

Yea

And it adds capasitance between sec and ground so still shock hazadrd

[Gyro]

It doesn't sound as if you are that experienced in mains wiring from your previous replies, if so, please be very careful and ask for advice. Be safe.

I'm actually less experienced than it sounds. I'll be careful. The best thing I got going for me is that mains freaks me out.

Thanks for all your help.

That's a useful defense mechanism in keeping you safe. Your best bet would be to properly mount it in a metal case. For the mains input it would be best to use an IEC connector to avoid the trip hazard of a trailing mains lead when carrying. The Input ground pin should be wired directly to the case, ensuring good connection with a tag, serrated washers and lock nuts. The protective ground wire from the transformer and the ground from the Output socket should go to the same point.

The Input Live connection should go straight to a panel mounted fuse holder, sleeve all connections. Try a 3.15A Slow blow fuse to cope with input surge current. You may want to add a double pole switch and neon indicator, of just go straight to the transformer primaries, wired as the diagram. You could either use soldered connections with plenty of heatshrink sleeving or a mains rated terminal block.

The secondary wiring is the critical part for a safety isolating transformer - if either of the output connections accidentally comes into contact with the case (ground), it will completely nullify the purpose of the transformer. Wire the secondaries in parallel (0-0, 115-115, assuming your mains is 110-115V), straight to the output connector. If you need to extend the wires then use multiple layers of heatshrink sleeving on the joints, or use a terminal block with an additional layer of insulating sheet, with decent overlap, underneath it.

It sounds as if you got a very good deal if you went back for the other transformers. They look useful and clearly marked. Make sure you parallel the primaries again for proper operation.

If unsure of anything, post photos as you go.

[Gyro]

Why not use a toroidal construction for a medical grade isolation transformer?

There's an earthed screen between the primary and secondary, which both offers additional protection and prevents capacitive coupling between he secondary and primary windings.

Yea

And it adds capasitance between sec and ground so still shock hazadrd

Although if it is a medical grade one, steps (insulation thickness etc.) will have been taken to minimize capacitance - there are maximum specs for these things! It is a reputable brand - there is no shock hazzard.

[001]

Why not use a toroidal construction for a medical grade isolation transformer?

There's an earthed screen between the primary and secondary, which both offers additional protection and prevents capacitive coupling between he secondary and primary windings.

Yea

And it adds capasitance between sec and ground so still shock hazadrd

Although if it is a medical grade one, steps (insulation thickness etc.) will have been taken to minimize capacitance - there are maximum specs for these things! It is a reputable brand - there is no shock hazzard.

But what capasitance value is ok?

[Gyro]

All covered (all approvals and specs) in the web page already quoted if you bother to look.. https://www.plitron.com/default-factory-specs/medical-grade-toroidal-transformers/.

For the OP's 300VA model, capacitance is less than 0.78nF, Leakage current less than 13uA. Below the threshold of human perception. As I said before NO shock hazard.