[R-390] Bucking Transformer / Autotransformer Question

[Barry]

As I replied to Roy just now, I'm thinking the autotransformer configuration may be the better way to go.  I could get fancy and set it up with switches to that all of the tap combinations could be used for inputs and outputs to do both bucking and boosting with a few different values.  Might be a fun project.

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "Bob kb8tq" <kb8tq at n1k.org>
> To: "Barry" <n4buq at knology.net>
> Cc: "R-390 Forum" <r-390 at mailman.qth.net>
> Sent: Wednesday, July 15, 2020 3:18:05 PM
> Subject: Re: [R-390] Bucking Transformer / Autotransformer Question
> 
> Hi
> 
> The load current goes through the secondary. If you have a randomly chosen
> 12V filament transformer, it likely
> will do a fine job.
> 
> The math:
> 
> 10A load current
> 10A secondary current (in the 12V winding)
> 1A primary current (to feed 10A at a 10:1 ratio into the 12V winding)
> 
> Obviously, 10A is a pretty big load ….
> 
> Bob
> 
> > On Jul 15, 2020, at 3:02 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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