[R-390] Question about VARIAC

[Roy Morgan]

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

A different angle:  If the variac is hooked up normally, for over  
voltage or line voltage only, and you put a short or very heavy load  
on the thing at a very low output setting, the first few turns will  
overheat.  For years I had a little bud box with a on amp variac, low  
voltage transformer and a rectifier/filter.  Made a fine variable DC  
supply.  Unfortunately, the caps or the diodes shorted and burned the  
variac's first 5 or 10 turns.  Sigh.


This is a good reason to put a FUSE in the wiper line.  Then if the  
load draws more than the variac is rated for (they'll do two times  
rating briefly and stay happy) the fuse will blow and you'll be  
temporarily inconvenienced only.


A further oddity:

There is a General Radio device called the Automatic Line Regulator.   
Basically, it has a center tapped variac, buck/boost transformer and  
error detection circuit with a thyrotron controlled motor.  For plus  
or minus 10 volts, it will regulate 30 amps, for half that voltage  
range it will regulate 60 amps.  I plan to run lots of stuff on mine.   
Beware of any for auction: they weigh some 85 pounds and shipping will  
kill you!

Roy

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