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Old 25th July 2006, 09:59 PM   #1
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Default Un-Winding Toroids

I have some 39-0-39 (625VA) toroids and I'm looking for 29-0-29. I have never unwound a toroidal and I don't want to wreck them. I was wondering if yee all have some insight or procedure to do it right the first time. Also, the procedure to re-wrap them back up nice?

I'm fishing for all input as I will give it a shot in the next day or so.

Cheers,

Shawn.
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Old 25th July 2006, 10:02 PM   #2
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Default VA

Also, will the VA change?

Shawn.
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Old 25th July 2006, 10:11 PM   #3
poobah is offline poobah  United States
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As far as unwrapping them is concerned... you'll have to use the Armstrong method... the trick will be in spotting how the center tap is created. This may not be possible if the secondaries weren't wound bifiler (two wires pulled at the same time side by side).

The VA rating of the primary will not change... strictly speaking. But the VA rating of the secondary will... keep in mind, this is applying the definition of VA somewhat wildy. The botton line is you must (should) keep you secondary current within it's original rated value. The wire size chosen for the secondary was based on amperage.

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Old 25th July 2006, 10:33 PM   #4
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If one just unwinds a couple of turns, the winding will no longer be evenly distributed around the core, but cover only some percentage of its length. Will that make any difference, or doesn't it matter if the secondaries cover the whole 360 degrees or only, say, 90 degrees?

I think this has been asked before, but I don't remeber it being answered satisfactorily.

An analogous case is where we add some extra secondaries, typically with lower voltage and current draw. Will it matter if these are evenly distributed around the core or just wound more tightly locally somewhere along the core? In this case I primarily assume these extra windings use only a small share of the total VA capability. Of course, the total power draw of all secondaries must still be within the VA rating.

Actually, this latter method is also an option for the original question. Instead of unwinding turns, you could add a suitable number of turns that cancel the corresponding number of turns of the original secondaries, and connect these new secondaries in series (in the right direction) to the original ones, so you subtract the voltages.
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Old 25th July 2006, 10:45 PM   #5
poobah is offline poobah  United States
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Partial layers can have a minor impact on leakage inductance... very minor... especially on a 'roid.

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Old 25th July 2006, 11:12 PM   #6
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Quote:
Originally posted by poobah
Partial layers can have a minor impact on leakage inductance... very minor... especially on a 'roid.

So, nothing to worry about then?

No problems with local heating of the core or anything like that? (Maybe a stupid question but the magnetic fields and I have never been quite friends with each other. ).
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Old 25th July 2006, 11:17 PM   #7
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naw... just a little leakage flux.. therefore inductance... no biggy.

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Old 26th July 2006, 12:17 AM   #8
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If you want to lower the voltage you can do it by adding wire, a much easier process than removing it. You simply wind additional wire in the opposite direction of the existing windings. Put equal no of turns on either side of the center tap and you will have equal voltages.

I_F
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Old 26th July 2006, 12:33 AM   #9
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Old 26th July 2006, 01:45 AM   #10
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Thanks PoobaH, it sounds like it could be easy depending on the method the manufacturer used to wire it. But either way, can it still be unwound or only one way?

Quote:
Originally posted by I_Forgot
If you want to lower the voltage you can do it by adding wire, a much easier process than removing it. I_F
I find this counter productive and likely to reduce the VA rating. Have you done this and can you show more input with pictures and a Toroid model this worked on? What size was it? What size was it when you were done? What was the application?

Quote:
Originally posted by tomtt
http://variac.com
I looked but only viewed content regarding "variacs". Is there some hidden content regarding un-winding toroids or are you just taking a **** on this thread?

Cheers,

Shawn.
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