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# [TCML] Measuring or calclating transformer current

```I have a small older iron core transformer (I suspect came from an old bug zapper) with no label.  It works and I want to make a cute little baby TC around it, and I have the parts to do so.
I was able to figure by air gap spark length of 1.8mm is about 5400 volts (if air breaks down at 3 kV/mm).

Now is that 3 kV/mm applicable for AC?  Or is that DC?  If it is AC, is that rms or peak to peak?

What I want to figure out is the highest current that can be drawn from this little transformer.   With Ohm's Law and I = V/R  (peak to peak)  I used a little 1/4 watt 1M ohm resistor figuring 5400 V/100000 R = 5.4 mA, but across the little gap the DMM read 4mA.  The spark was a fainter purple color indicating current drop, but after closing that lossy spark and getting up to maybe 6mA on the meter, magic smoke and a stinky burned out resistor occurred!   Because P = VI = (.0054 A)(5400 V) = 29.16 watts!  Whoops!

Now I have a 75 Watt 12000 ohm resistor, but the current through that would be 5400 V/12000 R = 450 mA and there's just no way this little bug zapper would supply that kind of current. I do have other power resistors in my collection, but how do I find the max current draw from this little transformer so that I can build out a little TC from it?

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Brian Hall
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