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Old 6th October 2014, 08:43 PM   #1
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Default Secondaries not to be used independently

I'm looking at some dual secondary transformers for a DAC project (I need 6x 6VAC transformers), but most of the ones in the size range I need say that the secondaries are intended to be used in series or parallel, not independently. Does anyone know why and what will happen if I do?
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Old 6th October 2014, 08:49 PM   #2
jcx is online now jcx  United States
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likely just that the insulation sec-sec isn't rated for any more V than seen in series operation

probably OK to use in floating sec/dual bridge type dual supply with common gnd on the bridges

but would exclude using for say separately isolated heater supplies in a DHT tube amp with 100s of V between secondaries

Last edited by jcx; 6th October 2014 at 09:17 PM.
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Old 6th October 2014, 09:19 PM   #3
kevinkr is offline kevinkr  United States
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It likely means they are wound on top of each other with no insulation beyond that on the wires to provide winding isolation.

In some applications where the windings are operating at different potentials the lack of additional insulation may be a problem.

I've not yet run into a transformer with this specific warning, although I have seen warnings to not parallel multiple transformers of a given type due to potential mismatches transformer to transformer.

What is the intended application?
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Old 6th October 2014, 09:20 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

It could mean a lot of things, why do you need 6 floating supplies ?

rgds, sreten.
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Old 6th October 2014, 09:30 PM   #5
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A strange warning. And vague to the point of being unhelpful.

I would take it to be saying not to use these windings to buck or boost the primary windings.

If your application has the windings at notionally the same potential, then I would just use it.
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Old 6th October 2014, 10:40 PM   #6
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This is one of the trafos I'm looking at:
https://system.netsuite.com/core/med...8ad2a&_xt=.pdf

They'll be used with bridge rectifiers and common grounds. The DAC setup has inputs for a separate transformer secondary for every power supply on the board, of which there are a lot (6x total of 3.3V and +/-5V supplies, one -15V).
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Old 6th October 2014, 10:54 PM   #7
Vasquo is offline Vasquo  United States
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I need say that the secondaries are intended to be used in series or parallel, not independently.
I think it just means that don't expect 25VA rating if you'll be using only one set of the primary or secondary windings. The 25VA is for the secondaries connected in series, or parallel.

Same goes for the primary winding... connect them either in series (for 220V operation) or parallel (for 110V) to get the 25VA rating.
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Old 6th October 2014, 11:00 PM   #8
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Originally Posted by Vasquo View Post
I think it just means that don't expect 25VA rating if you'll be using only one set of the primary or secondary windings. The 25VA is for the secondaries connected in series, or parallel.

Same goes for the primary winding... connect them either in series (for 220V operation) or parallel (for 110V) to get the 25VA rating.
That's fair enough, I think even 25VA is overkill in this application, though I'll have to check.
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Old 7th October 2014, 01:42 AM   #9
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could be wound bifilar so only wire insulation between windings
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Old 7th October 2014, 01:56 AM   #10
rayma is offline rayma  United States
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Originally Posted by Capt Grogg View Post
could be wound bifilar so only wire insulation between windings
Yes, all this means is that the two secondaries must be connected in series or parallel, not floated at significant voltages apart from each other.
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