[R-390] Buck/Boost Transformer

[Tom M.]

I made one of these several years ago and packaged it in a blue wall box with power cord and a regular 120VAC receptacle (and a label on the front as to what it does, preparing me for old age so I don't use it as an extension cord for my welding machine).  I also have a fuse installed inside it.

I believe the secondary has to carry the full load.  Using a filamant transformer, should be ample capacity for a 390A.  I used a 12V transformer.  

Tom

--- On Mon, 5/4/09, Barry <n4buq at knology.net> wrote:

> From: Barry <n4buq at knology.net>
> Subject: [R-390] Buck/Boost Transformer
> To: r-390 at mailman.qth.net
> Date: Monday, May 4, 2009, 9:52 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
> 
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