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Old 28th April 2009, 07:04 AM   #1
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Default Isolation Transformer Question

I want to use an isolation transformer on my bench and think I have found a good candidate. I have a pretty good size transformer out of an old tube Hp scope. I summed the power of all the various secondary windings and it comes to about 540 watts.

It has two sets of primary taps to accomodate 120 and 240 volt services. I suppose that they are used in series for 240. For 120 they could use just one set or they could use them in parallel (I have no way of knowing which at this point)

If I use these two sets of primary windings as primary and secondary, I have an isolation transformer.

So here's the question: How much power can it handle? Must I assume that both sets of primary windings are of equal gauge and each rated for half of the 540 watt load? Do I have a 270 watt isolation transformer here or are there other considerations? Am I correct to halve the output rating?

Thanks for reading.

Dave
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Old 28th April 2009, 08:04 AM   #2
Colt45 is offline Colt45  Serbia
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You're correct, should be good for half the power.

But since the windings are likely wound together, as they're both primaries, the isolation is probably not amazing... erm... don't try tying the output to several kV above earth.
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Old 28th April 2009, 01:49 PM   #3
vuohi is offline vuohi  Finland
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I can't answer the question, but are the primary windings separated on two halves on the bobbin? If the windings are separated by nothing else than wire insulation, I don't know, it just doesn't sound safe to me. Kind of defies the point of having an isolation transformer? Please correct me if I'm wrong.
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Old 28th April 2009, 02:23 PM   #4
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I have never measured one, but I'd expect any Hewlett Packard transformer to withstand a 1000V hipot test between primaries. 1000V is the minimum I'd consider adequate for 120VAC isolation. I have a dual-primary transformer I was thinking about using for isolation and it passed 1500VAC primary-to-primary hipot. Small dual-bobbin transformers are often wound with two bifilar primaries. I wouldn't use one of these on 220V, let alone depend on it for isolation.
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Old 28th April 2009, 03:21 PM   #5
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What about toroidal isolation transformers? Their windings are only separated by wire insulation, correct? Are they wound bifilar?

Virtually every modern iso transformer that I have seen spec'd for medical devices is a toroid (Amveco, Ulveco, etc).
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Old 28th April 2009, 03:27 PM   #6
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Isolation transformers will be wound with a layer of insulation between windings or triple-insulated wire. Medical applications will require higher breakdown voltage test (and may require a grounded shield between layers).
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Old 28th April 2009, 03:30 PM   #7
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Thanks for the ideas.

Quote:
Originally posted by Colt45
You're correct, should be good for half the power.

But since the windings are likely wound together, as they're both primaries, the isolation is probably not amazing... erm... don't try tying the output to several kV above earth.
Deal. Given that I intend to use it as bench isolation to reduce electrocution risk, the two windings should never be signficantly above earth. The next poster makes the point about insulation too.


Quote:
Originally posted by vuohi
I can't answer the question, but are the primary windings separated on two halves on the bobbin? If the windings are separated by nothing else than wire insulation, I don't know, it just doesn't sound safe to me. Kind of defies the point of having an isolation transformer? Please correct me if I'm wrong.
I rather certain you and the other posters are right about the windings being wound together and that is a legitimate point. I had not given the consequence of that full consideration. However now that I have given it more thought, I'm not sure I agree about the inherent risk of depending on insulation for safety. It seems to me that if a well built transformer, such as this one appears to be, is operated within it's design, the insulation should be as dependable as any other insulation we depend on for safety. The lack of ability to inspect for condition is a problem for me though.


Quote:
Originally posted by Tom Bavis
I have never measured one, but I'd expect any Hewlett Packard transformer to withstand a 1000V hipot test between primaries. 1000V is the minimum I'd consider adequate for 120VAC isolation. I have a dual-primary transformer I was thinking about using for isolation and it passed 1500VAC primary-to-primary hipot. Small dual-bobbin transformers are often wound with two bifilar primaries. I wouldn't use one of these on 220V, let alone depend on it for isolation.
Tom makes the point concisely with the 1000V minimum comment. I too would expect this transformer to meet high standards when new, but given its age, I think a hipot test is an an excellent suggestion.

On one hand, the argument could be made that any isolation is better than none. However depending on a "safety net" can actually be less safe if the net is not capable of catching ones fall.

Thanks for posting
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