[R-390] Bucking Transformer / Autotransformer Question

[Barry]

I tend to disagree with that.  There are two basic ways to wire a bucking transformer.

One way is to wire the secondary in series and out of phase with the primary to then feed the load through the secondary.

The other way is to wire it as an autotransformer (wiring the primary and secondary in series and feed the load at the tapped point).

If you have a 125V primary and a 12V secondary and configure them as an autotransformer, then the primary is essentially wired as 137V but being fed with 125V.  Of course, if the primary is 115V and the secondary is 10V, then that would be closer to correct but  the secondary may not be exactly suitable for primary usage.  All that may not really matter that much in practice but it is worth mentioning.

In my case, with mains voltages running around 125V, then using the 125V tap on the primary and feeding the load from the 115V tap may very well be the better method.

All that said, I'm not a EE and don't I don't pretend to be an expert on transformer construction/theory but knowing that a true autotransformer is designed to do exactly what I'm wanting to do, then I think it may be the better overall choice.

Comments welcome.

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 9:57:00 PM
> Subject: Re: [R-390] Bucking Transformer / Autotransformer Question
> 
> Hi
> 
> There is *no* difference between a filament transformer wired this way or
> that
> and an autotransformer …. none.
> 
> In both cases, if you are “dropping” voltage, the primary current back feeds
> the
> line.
> 
> Bob
> 
> > On Jul 15, 2020, at 10:50 PM, Barry <n4buq at knology.net> wrote:
> > 
> > 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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> 
>