[R-390] Bucking Transformer / Autotransformer Question

Bob kb8tq kb8tq at n1k.org
Wed Jul 15 22:57:00 EDT 2020


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
>>> ______________________________________________________________
>>> R-390 mailing list
>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>>> Help: http://mailman.qth.net/mmfaq.htm
>>> Post: mailto:R-390 at mailman.qth.net
>>> 
>>> This list hosted by: http://www.qsl.net
>>> Please help support this email list: http://www.qsl.net/donate.html
>> 
>> 



More information about the R-390 mailing list