[R-390] Bucking Transformer / Autotransformer Question

[Larry H]

Barry,  I posted an article on this subject on 10/4/2016 and here is the
link:  http://mailman.qth.net/pipermail/r-390/2016-October/056213.html .
You will find it helpful.  Unfortunately, the link in it to the article by
Mr. Elliot is nolonger valid, but here is a valid link to it:
https://sound-au.com/articles/buck-xfmr.htm .  He claims that the
autotransformer configuration is more efficient, and I agree with him.

Regards, Larry

On Wed, Jul 15, 2020 at 12:03 PM Barry <n4buq at knology.net> wrote:

> I'd like to employ a bucking transformer for some of my equipment -
> particularly a Fluke 760A calibrator.  I have a pretty hefty old
> transformer with two, separate 10V secondaries that can deliver 1.0A and
> 10.0A.  It also has a tapped primary with taps at 115V, 120V, 126V, and
> 132V (I think I'm remembering all those correctly).
>
> I was thinking that since my mains voltage is right around 126V, then I
> could connect that to the mains and use the 115V tap in an autotransformer
> configuration.  I'm unsure, though, whether that is the best way to do this
> and whether the primary windings will have to carry more current than it's
> capable of supplying.
>
> While I could connect one of the secondaries as a standard bucking
> configuration, I was thinking the autotransformer configuration might be
> better/simpler.
>
> Any thoughts on this approach?  I'm just unsure of how much current that
> small section of primary would need to supply.  I think it would be the
> total current drawn by the load but I might be thinking of that incorrectly
> so thought I'd ask.
>
> Thanks,
> Barry - N4BUQ
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