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Registered Member #621
Joined: Sun Apr 01 2007, 12:37AM
Location:
Posts: 119
Sorry to bring back up my boring resistor crap. You guys helped me pick my voltage equalizing resistors, now I'm onto the current limiter. I've honestly been messing around with all of the equations and trying to figure this out. I didn't want to have to bother you guys. I think I figured it out myself but I just want to make sure I am on the right track.
I am using 3 paralleled NST's for 4kV @ 90mA to charge a cap bank and trying to decide if I really do need a current limiting resistor between them as I am using NST's which ARE current limited by nature of design.
With 4kV @ 90ma I get 360 watts. Now I am guessing that even after full wave rectification which brings me to a theoretical 5656 volts DC, I am STILL limited to 360 watts.....that the 90mA will drop proportionately down to keep me from exceeding 360 watts due to the NST's being inherently current limited. Now if my theorizing is correct, If I short them dead i will only get about 360 watts......so I shouldn't need a current limiting resistor right?
I mean in a xenon flash circuit isn't the purpose of the current limiting resistor between the rectifier output and the cap bank just to enable the flahlamps arc to extinguish (I doubt 90ma will keep a 40 somethign inch tube going anyway) and to keep the transformer from "pegging shorted" and caps from "getting hit hard" when the caps first start charging? So if I wanted to pick one, wouldnt I just go with say a 400 watt resistor that has an ohm rating that limits my current to a safe range...which would be my transformer's max 90 ma. However in my case with an NST I think this would be redundant no?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Just go with the current limiting of the NSTs. Their equivalent circuit is a transformer in series with an inductance, so they will limit current into a short circuit load, for instance an uncharged cap bank behind a rectifier.
BTW, the NST delivers a variable current depending on the terminal voltage, so it does not deliver "360 watts". The minimum output power of zero watts happens at either open circuit with volts but no current or short circuit with current but no voltage. The maximum output power happens at half open-circuit output voltage, half short-circuit current, which is only 25% of the product of o/c volts and s/c current.
As you have 3 parallel NSTs, you might want to start your experiments just using one, and work up from there.
Now there's something I don't know as I've never played with them, does a "30mA NST" deliver 30mA into a short circuit, or 30mA into its rated tube load? Is there an industry standard, or does it depend on the way the manufacturer chooses to rate it?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You are correct, it's 30mA into a short circuit, and the voltage on the nameplate is the open-circuit output voltage.
Of course, NSTs can deliver a good deal more than one-quarter of Voc*Isc when driving a Tesla coil, because the reactance of the tank capacitor they're charging partly cancels out the internal ballasting.
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Choosing a current limiting resistor.
