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Re: [TCML] NST Measurements

Hi Greg,
Yes, interesting to discuss and "maybe" it won't change anything significant, but it has already changed some views (including my own). For example, the TCML in general has been recommending reverse feed for determining turns ratio for many years. Due to the very obvious non-linearity shown with measurement, this probably won't be recommended in the future for shunted transformers. So that is at least one change that is backed up by measurement. For NST's, your going to have to go off the faceplate ratings for these numbers for any accuracy. You can get close with high voltage probe measurements in a primary feed test (roughly 10% error).
I can't test your thesis either (missing the 500k high watt resistor). 500W is a bit much. More likely about 200W accounting for the drop, but even this capability wouldn't help much (helpful only at the specific load). I think a test that is worthy is to capture the output current during charge from zero to bang with some specific C sizes. That would tell something regarding load impedance and how the transformer performs over time.
In the end, when the cap is charged to breakdown at bps, the NST is equivalent to it's Voc x Isc ratings. Spark lengths as based on power for NST's and non-shunted transformers (including measured bps values) are too similar for the NST to be otherwise. If it were a purely resistive load, then likely a shift. Due to what we see with NST's and spark lengths, I would say we do know the power it's capable of in the right circumstance. I think Tesla Coils just naturally provide the "perfect" circuit for getting the most out of an NST. Strange but true. 

Take care,
Bart


G Hunter wrote:

Talking about NST behavior is a bit like discussing the weather--interesting chat, but not likely to change anything.  I'm not being critical--I'm as intrigued by this topic as anyone, since I have a pretty good collection of old NSTs.

Yes, the measured short-circuit current of every (good) NST I've ever tested was very near the faceplate value.  I don't own a high voltage probe, but others have assured me the open-circuit voltage equals the faceplate value.  This is not terribly useful for determining "real" power.  15kvac X 30mA = 450VA, but I doubt the NST can really support its full potential across a load that draws 30mA.  Anybody have a high voltage probe plus a 500k, 500W, non-reactive, high-voltage resistor to test my thesis?  Me either!

Besides, I don't know how useful a true dummy load test would be for either coilers or sign makers.  a neon sign doesn't present a pure resistive load, does it?  A Tesla coil cap bank certainly doesn't.  The sign presents a negative resistance load which feels like an open at first, but almost like a short-circuit once the arc is struck.  In its proper role, the NST is really just a self-starting lamp ballast rather than a true HV power supply.  Likewise, the Tesla coil tank cap presents a dynamic, highly reactive load which rapidly swings from a short early in the charge cycle, to an open at the end.  The cap repetively dumps its load, presenting the NST with a wild bucking bronco ride, with mains resonance effects thrown in for good measure.  The NST manufacturer never imagined such an abuse of their product.

Perhaps the most useful info on real NST power is right on the data plate.  I've seen more than one 15/30 NST marked "250VA" by the manufacturer.  Given their insider knowledge, this seems like a sensible figure.  I'm no sign guy, but I bet a 15/30 NST can fire 250W of neon lamps 24/7.

So how much real power can an NST deliver to a TC tank circuit?  We can only guess until someone comes up with a way to accurately instrument it.  Until then...make some sparks!

Cheers,

Greg

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