[R-390] Question about VARIAC

[sdaitch at kuw.ibb.gov]

Bob and Barry,

Looking at Barry's concern again, 

"If I were to tap the input at one turn 
(or two or three), then I assume this 
would appear as a very low impedence 
and wouldn't work very well (most likely 
incurring a very heavy current draw and 
hopefully trip the circuit breaker before 
burning out the transformer."

I am almost thinking Barry is looking at
running the INPUT to the variable transformer
in the first few turns, making it a quite
large step-up transformer.  Short (and pun 
intended) of either modify the unit to do 
so, or feeding the input into the variable
contact (not normally recommended, I beleive) 
there isn't enough winding in the transformer
to make it operate as a transformer, but more like
a very low inductance coil.

Certainly that isn't the way the variable transformer
is designed to operate, but when Barry discusses
a few turns, "very low impedance" and high current
draw, it is almost like the discussion is on the
theoretical, "if you used only a few turns" on
the variable transformer as the primary.

Barry, correct me if that is not where you were
thinking.

73
Sheldon



----- Original Message -----
From: rbethman <rbethman at comcast.net>
Date: Sunday, April 19, 2009 9:37 pm
Subject: Re: [R-390] Question about VARIAC

> 
> Barry,
> 
> The impedance of a transformer "usually" does NOT come into play 
> UNTIL 
> there is a short-circuit condition.
> 
> Variacs/autotransformers, have a current rating based on conductor 
> size, 
> "primarily", and can be run throughout the variable ranger that 
> they are 
> built for.  The number of turns used, determines the voltage out.  
> You 
> are not matching an impedance as we are accustomed to with RF.  
> Power 
> transformers are really a different animal.
> 
> Bob - N0DGN
> 
> Barry wrote:
> > I guess what I was getting at was impedences.  If I were to tap 
> the input at 
> > one turn (or two or three), then I assume this would appear as a 
> very low 
> > impedence and wouldn't work very well (most likely incurring a 
> very heavy 
> > current draw and hopefully trip the circuit breaker before 
> burning out the 
> > transformer.  If there were double, triple, etc., the amount of 
> turns, this, 
> > too, would affect the input impedence, would it not?
> >
> > I guess what I was trying to say is that I figure there's a 
> point where the 
> > number of turns matters but just wasn't sure where that point 
> is.  Is that 
> > incorrect?
> >
> > Thanks,
> > Barry - N4BUQ 
> >   
> 
> -- 
> Bob - NØDGN
> 
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