Transformer question (only slightly OT)

[Roy J. Tellason]

On Friday 01 September 2006 03:01 pm, Chuck Guzis wrote:
> I have a power transformer that I scavenged out of a 15-year old UPS, so
> it's probably pretty close to being on-topic.
>
> At any rate, I want to use this transformer in a DC power supply.  I have
> three sets of windings:  two sets are 120/240v "universal" series/parallel
> type and the third is 24-0-24v center-tapped.
>
> I want to run this from 120vac.
>
> Is there any benefit to paralleling the two sets of 120/240v main windings
> (i.e. 4 120v windings in parallel)?  Will it increase the power rating of
> the unit or is that mostly a function of the secondary winding?
>
> Suppose instead of 24-0-24 on the secondary, I'd like 12-0-12.  Obviously,
> one way to get this is to hook the primary (-ies) up as 240v.  Is there any
> benefit to be gained in parallelling the two 240v primaries?

I would have to say that the exact answer to that would depend on the design 
of that particular transformer,  but it would seem to me that pretty many of 
them that I've looked at are limited by the primary winding,  particularly 
when it comes to output current,  so if it were me I'd use them both.  You 
could always apply some load to the secondary and try it both ways,  
increasing the load until some particular voltage drop happened.  I seem to 
remember some discussion in the yahoo "electronics_101" group about this very 
subject a while back.  Measuring the open-circuit voltage first,  you'd want 
to draw current until you got no more than a 10% drop or so,  and while doing 
that make note of just how warm that transformer is getting.  Try this with 
one primary winding,  and then with both,  and that should give you a pretty 
definite answer.

I also saw some stuff in the above-mentioned group that gave some rough idea 
of what sort of output current you might expect out of a given transformer by 
looking at the cross-section of the core.

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