Re: Any technical reason not to use isolation transformer to make DC?



Mike Silva wrote:
On Apr 27, 5:57 pm, Fred Bloggs <nos...@xxxxxxxxxx> wrote:

John Larkin wrote:

On Fri, 25 Apr 2008 09:52:32 -0700 (PDT), Mike Silva
<snarflem...@xxxxxxxxx> wrote:

On Apr 25, 12:48 pm, Fred Bloggs <nos...@xxxxxxxxxx> wrote:

If I want a source of 150-160VDC is there any technical reason why I
shouldn't use an isolation transformer to feed a rectifier & filter?
As opposed to a purpose-designed 120VAC secondary "power transformer"?

The designs of the isolation transformer and the "purpose-designed"
power transformer are different. The power transformer is designed to
accommodate the kinds of peak short current conduction phase angles that
you have with rectification while the isolation transformer is not.
Therefore, if you intend to drive the usual rectifier filter combination
for a DC output, the power transformer is the one to use.

What exactly are the internal differences?

One is made from iron and copper; the other is made from copper and
iron. I forget which is which.

John

Normally the design is optimum when copper and core loss are the same.
Since the copper and core loss are frequency dependent in a very
nonlinear way, it would be obvious to all but the most dimwitted that
the rectifier transformer going for a typically high crest factor load
would be different...The rectifier transformer can handle the isolation
transformer VA but not vice versa, the isolation transformer must be
derated.- Hide quoted text -

- Show quoted text -


That brings up the question, what loads are isolation transformers
optimized for? Wouldn't transformerless line-operated equipment be
one of their biggest targets?

I think it would be an interesting experiment to run an isolation
transformer at its rated VA into a resistive load, measure the
temperature after some hours, then run the transformer into a
rectifier/filter combination and load that DC voltage down to an
equivalent VA rating, taking into account the published derating
multiplier (0.87 for ful-wave bridge and capacitor input filter) and
check the temperature in that use. I think I'll round up the parts
and give that a try.

Mike

The transformer can take a lot of abuse. I think the DC available for the rectifier filter under load will be lower for the isolation than the rectifier transformer by a few percent.

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