[R-390] Question about VARIAC

[Roy Morgan]

On Apr 19, 2009, at 2:23 PM, Barry wrote:

>> It's just an autotransformer with a variable tap. ...
>
> 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

It would let the smoke out instantly. (Assuming you are suggesting to  
hook the line supply across just a few turns of the variac.)

> ... If there were double, triple, etc., the amount of turns, this,
> too, would affect the input impedence, would it not?

Maybe what you have in mind is the impedance with no load on the  
thing. Yes, you have the right idea.  If you hook 120volts 60 cycles  
to the 120volt winding of a 400 cycle transformer, it will overheat  
fast.  A bit of algebra will show that you CAN run such a transformer  
on 24 or 28 volts 60 cycles.  Secondary voltages will be lower in  
proportion.  This can be handy at times.


> 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?

That is correct, but short of a  batch of variacs you don't mind  
burning up, don't try it.. HOWever.. if  you really want to, hook the  
line to the WIPER, set the wiper on one end and hook the other side of  
the line to the other end of the winding.  Hook an ammeter in the  
line.  Move the wiper to fewer and fewer turns until the ammeter  
begins to climb rapidly.  Do report your results.

Roy


Roy Morgan
k1lky at earthlink.net
529 Cobb St.
Groton NY, 13073