Re: Tesla!

> As for current limiting, the "standard" method is to use an arc welder
> with the output leads shorted and wire it in series with the step up 
> transformers...perhaps you could use an old shorted out neon sign
> transformer.

I got out my collection of commercial inductors and transformers last 
night, and ended up testing 2 inductors and 3 transformers (including 
my 20 Amp variac).  I computed the inductance (L) of each component 
by hooking it up in series with a variac, measuring the voltage across 
(V) and current through (I) each device for a number of test voltages, 
and using the formula L=V/(2*pi*60*I) for 60Hz house current to 
compute the inductance (in general, L=V/(2*pi*f*I) where f=frequency in 

What I found was interesting, although unexpected.  The inductance of 
the chokes stayed pretty much constant over the range of voltages I 
tested (which I expected), but the transformers were a different matter 
altogether.  The inductance of a low-voltage, high-current transformer 
monotonically decreased as the test voltage increased, but the 
inductance of a high-voltage, high-current transformer increased up to 
about 50 V, then decreased as the test voltage was increased further.  
The variac behaved like the low-voltage transformer.  I assume this is 
due to some magnetic saturation effect, but a definite explanation 
would be more than welcome!

When I shorted the output leads of the low-voltage transformer, even at 
a fairly low voltage, the input current  shot way up and the voltage 
across the inputs dropped paractically to 0.  It seemed like shorting the 
output terminals effectively shorted out the input terminals also.  It's 
possible that shorting the output leads on a welder would essentially 
remove the transformer from the circuit and just using the current 
limiting part of the welder to control the current, in which case you 
might be better off just using the inductor or whatever the welder uses 
to control the current.  I have no idea what shorting the output terminal 
on a neon sign transformer would do (maybe that's part of tonight's 
adventure).  In any event, I would certainly hook up a meter and test 
things out at low power before I cranked anything up.

On a positive note, the variac was wonderful at limiting the current.  At 
full inductance, the short-curcuit current at 120V was well under one 
Amp.  I imagine that a 10 Amp variac would work just fine to control the 
current, you would just have to keep an ammeter in the loop to make 
sure you don't exceed 10 Amps.

Steven Roys (sroys-at-radiology.ab.umd.edu)