Questionon isolation transformers

Fred Olsen fwolsen at WI.RR.COM
Wed Apr 13 11:17:44 EDT 2005

Ted Bruce wrote:
> What differentiates isolation transformers from other types?

Hi Ted,

Isolation transformers may (should) have things like electrostatic 
shields between the windings, or perhaps have the primary and secondary 
on separate bobbins.  Whatever is done is in the name of reduced leakage.

> If I were to use, say a 110VAC to 12.6 VAC and
> a reversed 12.6 VAC to 110 VAC, would
> that combination qualify as an isolation transformer,
> functionally?

Functionally, yes.  There is no wire connection from in to out.

One thing that's seldom mentioned though is that an isolation 
transformer would not ordinarily have a turns ratio of 1:1.  It will be 
more like 1:1+ a few percent to make up for losses.  In most cases the 
losses with back-to-back filament or control transformers are of no 
consequence in our typical BA applications.  It is usual however for the 
'driven' transformer to run somewhat hot.  Be aware and keep an eye on 
that at first.

There's no reason to go all the way down to heater voltage levels unless 
you simply want to use transformers on hand.  Control transformers with 
higher voltage secondaries will work fine, may run a little cooler, are 
designed for 24/7 operation, and are dirt cheap.  The two do need to be 
identical or darn close.

> Anyway, the isolation transformer
> schematics have a common winding,

No.  That sounds like an autotransformer and is definitely not isolated.

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