[R-390] Buck/Boost Transformer

[Bill Hawkins]

Barry,

The two connection methods are not quite equivalent.
You will buck or boost by the voltage of the secondary winding.
The first method puts full line voltage on the primary, for max
secondary volts.
The second method lowers the primary voltage (bucking) for less drop.
The second method will give you more boost if you are boosting, say from
95 VAC.

The secondary has to carry the full load current. The VA rating is less
than the full load VA because it is the secondary volts times amps.
A 12 volt secondary carrying 5 amps needs a 60 VA rating.
A 120 volt secondary in an isolation transformer needs a 600 VA rating.

So yes, a bucking transformer has a lower VA rating than a full voltage
transformer. Size and weight are proportional to VA rating.

Bill Hawkins


-----Original Message-----
From: Barry
Sent: Monday, May 04, 2009 9:53 AM

Gang,

I'm curious about wiring a buck/boost transformer.  There are,
apparently, at least a couple of different ways to do this.

I would have thought this to be the way to do this:

http://johnjeanantiqueradio.com/buck-boost.jpg

However, according to the R390A compendium, there's another way to do
this:

http://www.r-390a.net/faq-HiVolt.htm

Are there advantages/disadvantages to one over the other (or are they
essentially the same thing)?  
Although I understand (at least on a functional level) how both work,
the second one seems "backwards" (although I know it isn't).

I assume both require the secondary of the transformer to be able to
handle the anticipated load and I don't see anything else that would
change the requirements on the transformer.

I seem to recall a former list member (Dr. Gerald Johnson) talking about
a scheme where the bucking transformer only has to be rated in
proportion to the amount of voltage it is dropping.  In other words, if
it's bucking 5 volts from 125, then it only has to be rated at 5/125 of
the load.  (That wording may not be exactly correct, but I seem to
recall the bucking transformer did not have to be rated for the full
current load).

Am I wrong to assume that the bucking transformer in the above
schematics have to handle the full load (in amps) or does either one
allow the transformer to only be rated at an appropriately smaller VA
rating (I hope I'm making sense...).

Thanks,
Barry - N4BUQ