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Ok to "Juice" my NST's to 140volts from a Variac for that extra bit I need?

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MikeT1982

Sat Jul 28 2007, 06:32AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
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Posts: 119

Would it be ok for me to "juice" the input of my 3 paralleled 4kV@30ma NST's up to 140 volts from there reccomended 120volt input (from France) with a small panel mount 6 amp, 0-140 volts variac I found? The current draw of my NST's according to France is 1.2amps each, so thats 3.6 amps total. I am thinking this may be enough voltage boost to get me away from encorperating a voltage doubler in my cap charging circuit yet may get me into the 5-6kV charge voltage I want. I am thinking I could lay on the variac untill I get up over 5kV and then leave it set and mount it inside my laser cabinet. I mean heck if one NST makes 4kV from 120 volts, thats 33.33 times the input as the output. So if I put in 140 I will get 4,666 volts which rectified should be 6532 volts...enough to make up for the parasitic losses of my voltage equalizing resistors across each cap I talked of in my last post. maybe.

*Edit* If you guys want to delete this post here please feel free, I think I will take the advice on making a voltage doubler AND use the variac. Then I could charge my caps from too little to make my flashlamp fire all the way up to detonating capacitors LOL!! (not dumb enough to turn the knob that far LOL!)

Hazmatt_(The Underdog)

Sat Jul 28 2007, 07:25PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Mike I would suggest you put 6-12 volts into the primary side of each NST and measure the bushings with your volt meter on the 600V range. Once you do that you will find out that a 4kv NST isn't going to give you 4.000kv. I think you will be surprised to find that NST's often output 20% higher unloaded.

After you have the NST's pushing 90mA its going to be hard to cause them to 'sag' much, so you probably don't need to worry about input voltage, but do some homework and measurements.

Steve Ward

Sat Jul 28 2007, 07:32PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

If i were in a hurry and just wanted to get a few shots, i would probably push it to 140. But eventually i would find a more suitable solution (like the voltage multiplier, and changing the bleeder resistors).

MikeT1982

Sun Jul 29 2007, 12:54AM

Registered Member #621 Joined: Sun Apr 01 2007, 12:37AM
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Posts: 119

Awesome guys. Do you think higher than 25,000 Ohm bleeder/equalizers would still equalize the voltage of the serieses caps (they're Electrolytics unfortunatley for budget and I have 16 in series LOL!) I got these huge 20 watt 25Kohm Ohmite Brown Devil resistors. How high in value could I go and still have a safe Voltage equalization. I just went and checked the cap specs and they are 39.0 mOhm at 120HZ ESR....I don't know if ESR has anything to due with that. I will throw the NST's on a variac and due as you said Hazmatt!

mikeselectricstuff

Mon Jul 30 2007, 10:27PM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
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Posts: 253

at some point you may saturate the primary, and so won't get any more output - the trick would be run it without a load and gradually increase the voltage whilst measuring the primary current - at saturation there will be a sudden increase in current ( altough with a nst a sharp rise in current can also be caused by arcing on the secondary...)

Shaun

Mon Aug 06 2007, 05:45AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I would bet that NST cores are made with sufficient enough a safety margin that you wouldn't need to worry about saturating the core with only 20V above the rating.

cbfull

Tue Aug 07 2007, 12:54PM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

When I push the input voltage to any of my neons all the way up to 140VAC, there is a significant increase in the output arc's power (judging only by appearance), I don't think there is any significant rise in current draw since the spark power seems to increase without hinting that it's hitting a plateau. Good idea to measure current inputs, ideally we would all have digital storage oscilloscopes so we could do linear regression on the data and be quite precise. Is that excessive? I sure hope so.

I use two variacs to power them. One is wired as a variable inductor to control current, and the other controls inpuy voltage all the way up to 140VAC. I am not sure why I always feel the need for the variable current, I guess I just like being able to adjust every aspect of the output.

While I admit I do this to my neons, I would never tell someone else that it's safe for all neons. If a neon were pretty old and showed signs of being in a moist environment, I would first see if it works at all at 120VAC, and then unpot/repot it to eliminate the possibility of internal moisture or damaged insulation.

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