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Unwinding Toriodal Transformers
Unwinding Toriodal Transformers
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Old 2nd February 2004, 12:01 PM   #1
DJED is offline DJED  Australia
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Question Unwinding Toriodal Transformers

Hello All
I've just registered so bare with me!
I was thinking of building David Tilbrook's ETI-477 power amp and I'm unable to located the correct transformer. So I have 2 off 40-0-40V 300VA toroidals and was wondering if they could be used on this amp? The 477 requires 35-0-35V. Also is it possible to unravel some of the winding to lower the voltage to the required voltage.

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Old 2nd February 2004, 03:19 PM   #2
Centauri is offline Centauri  Australia
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Probably not, as one half of the secondary could be buried under the other - would be a hit and miss process to determine number of turns anyway.

Altronics in Perth has correct ones, also Harbuch Electronics in Sydney and Farnell - best to go with one of those.

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Old 2nd February 2004, 03:41 PM   #3
Peter Daniel is offline Peter Daniel  Canada
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Usually secondaries are winded side by side (at least on Plitrons) and it's not a problem. I did it few times.
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Old 31st March 2004, 05:40 AM   #4
pic60237 is offline pic60237  United States
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Default less work


You could also try selling the transformer ? and buy one at partsexpress. Just an idea.
they are pretty cheap and seem nice.

Price: $59.97 EA
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Old 31st March 2004, 01:00 PM   #5
protos is offline protos  Greece
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Recently unwound a 18v down to 12v.It is doable but I wonder about the efficiency since the secondary now only covers 70% of the primary.I did not want to unwind it completely and rewind it for obvious reasons.
What do you think Peter Daniel?
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Old 31st March 2004, 02:34 PM   #6
jneutron is offline jneutron  United States
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Originally posted by protos
Recently unwound a 18v down to 12v.It is doable but I wonder about the efficiency since the secondary now only covers 70% of the primary.I did not want to unwind it completely and rewind it for obvious reasons.
For first order approximations (iow, approximations that can be measured), the amount of primary covered is unimportant.

The toroid transformer works because the secondary winds surround the flux that is going through the toroid. The voltage on the secondary is proportional to the time rate of change of the flux, and where the secondary is on that transformer core has no bearing on how much flux travels in the secondary..

So, no it doesn't make a diff..

Parasitics, capacitance...they will be a little diff, but they are second order effects, and will be far less significant, and will not play in efficiency.

Since the VA rating of the xfmr is still the same, you can draw more current from the lesser voltage winding, and as the wire is same guage as before, there will be more IR loss..so you may not be able to use all the VA capacity...so efficiency will suffer in that respect.

Cheers, John
I hate all these smart gadgets..I refuse to buy things that are smarter than me. I've made a list of those things... Cabbage just made the list.
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Old 26th December 2004, 02:59 PM   #7
Ernst.Zimmer is offline Ernst.Zimmer  Australia
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Default RE ETI Project 477 power amplifier

To whom it may concern,

If you are still interested in details regarding the transformer required for the subject project I have all the information that you may require. I have built this amplifier about 20 years ago and it is still working perfectly and the quality of the sound is outstanding.

Best regards Ernst Zimmer

If you want further information please contact me on my email address ernst.zimmer@bigpond.com
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Old 26th December 2004, 04:43 PM   #8
rcavictim is offline rcavictim  Canada
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Instead of UNWINDING turns to reduce the voltage you could ADD some turns! Two options here. The idea here is to add a winding or two that can be hooked in series with one (primary) or two windings (sec) that will be out of phase and 'buck' the voltage produced by the existing windings. This is akin to placing a separate transformer in series with the primary winding of an existing transformer to effectively reduce or increase the voltage applied to the main transformer to correct for line voltage sag, etc.

You will have to determine how many volts per turn your torroid makes as is. To do this wind say ten turns of scrap hookup wire, guage not important for this test around the core of the toroid and hook up a AC voltmeter. Energize the primary winding as normal. You should see a voltage developed which will usually be less than a volt per turn on a transformer of this size. Perhaps 1/4-1/2 volt. This is the number you use to determine how many turns you need to add if you want to buck the primary, or buck each secondary winding with its own separate bucking coil.

For example if you want to add turns to buck the primary (in this case probably the easiest solution). Your desire is to drop the secondary from 40 volts to 35. 35 volts is 87.5% of 40 so you need to make the new input voltage to the primary 87.5% of nameplate voltage. If the mains voltage is 115 volts then this amounts 14.4 volts drop to 100.6 volts. If your test indicates that you get exactly half a volt per turn from this core then it is a matter of 14.4 volts divided by 0.5 volts = 28.8 turns, That should be easy to do by hand. Use a suitable gauge magnet wire to carry 300 VA. #18 or #20 AWG ought to do it. You can use plastic sleeved wire instead of enamelled magnet wire if you wish. Hooked in series one wat with the primary will give the proper output voltages you desire. Hooked up the other way (wrong) will give you an increase in output voltage.

This is an easy and virtually free fix to your problem.
"There are more worlds than the one you can hold in your hand." Albert Hosteen, Navajo spiritual elder and code-breaker, X-Files TV Series.
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Old 26th December 2004, 11:54 PM   #9
quasi is offline quasi  Australia
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Default Hey DJED

Go ahead and remove the windings. Most toroids are wound bi-filar because that is the only way for both secondary voltages to track correctly.

Work on approx 3 turns per volt. It doesn't matter if you end up with a couple of more volts than you wanted because from what I remember the 477 has margin built in.

I personally would run the 40v transformer as it is and take the extra free power.

I.e. a transformer with 35 volt windings will provide 49.5 volt rails and the transformer with 40 volt windings will provide 56.5 volt rails. This amounts to around 30+ watts of power for free.

Check voltage ratings of all the components just to be sure that you don't exeed anything and go for the higher rails.

If your worried about checking components, post or email the schematic to me and I'll check it.

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Old 2nd January 2005, 09:30 PM   #10
bremen nacht is offline bremen nacht  Europe
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Originally posted by rcavictim
Instead of UNWINDING turns to reduce the voltage you could ADD some turns!...

...This is an easy and virtually free fix to your problem.
Brilliant. I've been hanging onto a bunch of big toroidals for years now, hoping they'll come in handy one day - it never ocurred to me that it'd be that simple to 'tune' them.

Thanks! Happy new year, and stay away from those exploding automobiles!
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