Rotary Spark Gap

jim.fosse-at-bdt-dot-com (jim fosse) wrote: 
>         I just ran a SPICE (Simulation Program with Integrated Circuit 
> Emphasis) analysis on a 7.5KV peak 60Hz sine wave generator, a 20K series 
> resistor, a 995H inductor and a 1 or 1000 ohm load resistor. I was
trying to   
> simulate my 7.5KV 20ma xformer which measures 19K dcR. I got a current of 
> about 19.45xx ma no matter what load resistor I used. When I actually 
> measured the current, I got 0uA thru my multimeter! When I switched over to 
> using a load resistor and using my miltimeter as a voltmeter, I measured 
> 20ma. I must conclude that 1: my understanding of my multimeter is wrong. 2: 
> my understanding of a shunt limited xformer is bad. 3: my understanding of 
> electronics/physics is bad. 4: all of the above ;)             

  I haven't seen any responses to this one yet, though I suspect that some 
of my mail gets dropped somewhere. My apologies if this has already been 

  A quickie back-of-the-envelope calculation shows that the 995H inductor  
has such a high impedance compared to your load resistor (more than a 
couple orders of magnitude) that the load resistors shouldn't have any  
significant effect on the current, and that the current computed is about 
right. The current deduced from your voltage measurement across the load 
resistor is just about exactly what you've computed, so that should make 
you feel good. The only inconsistency is the 0uA current measurement. 

  I haven't seen your apparatus, but the fact that you were able to make  
a measurement that does agree with the computed value would lead me to  
first suspect that the ammeter mode in your multimeter isn't working. 
Have you tried it in some different setup to verify that it's OK? Also, 
some multimeters only have a DC current scale. Is this the case, or  
could you have possibly set things up to measure DC current instead of 
AC? It would seem that a reading of somewhere in the range of 0uA would 
be correct there, too. :-) 

Wes B.