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Does this HV resistor exist?

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GammaRay

Fri Feb 28 2014, 04:18PM

Registered Member #5323 Joined: Fri Jun 15 2012, 02:14PM
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I've spent much time searching online for a 10 meg resistor @ 100-200 watts with a 30-40kV flash over spec - or anything close. Looked and looked, checked every vendor that I know. The flash over spec is the sticking point. Helpful guidance will be appreciated.

Dr. Dark Current

Fri Feb 28 2014, 05:01PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Nope, just make an MMR (multi-mini resistor).

Dr. Slack

Fri Feb 28 2014, 05:30PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

You can get resistors with a few kV very cheaply, so you don't need too high a 'multi' to get there.

Is this for constant disspiation, or for connection to discharge quickly? You might consider a water resistor for pulse discharge. A physically long polythene tube is good for voltage withstanding, water has a huge thermal capacity to absorb pulses, and the varaible value is not too important for energy dump duty. However, for continuous connection as a bleeder, water resistors are not good.

Are you really going to spend 160 watts in constant dissipation across yoour 40kV caps into a fixed bleeder?

GammaRay

Fri Feb 28 2014, 07:23PM

Registered Member #5323 Joined: Fri Jun 15 2012, 02:14PM
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To be clear, the recommended way is to buy several lower voltage, lower wattage resistors and connect them in series? (multi-mini resistor).

It is for an experiment illuminating 40 feet of florescent tubes powered by the bank of (10) 40kV caps totaling 70.5uf. Admittedly, the 200 watt spec is too high. I suppose I could come down on the desired resistor wattage to around 100 watts and still have enough of a safety buffer, yes?

Proud Mary

Fri Feb 28 2014, 08:41PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

GammaRay wrote ...

To be clear, the recommended way is to buy several lower voltage, lower wattage resistors and connect them in series? (multi-mini resistor).

It is for an experiment illuminating 40 feet of florescent tubes powered by the bank of (10) 40kV caps totaling 70.5uf. Admittedly, the 200 watt spec is too high. I suppose I could come down on the desired resistor wattage to around 100 watts and still have enough of a safety buffer, yes?

By Ohm's Law you'll see that I = V/R = 40,000V/10,000,000Î© = 0.004A = 4mA.

So no matter how many fluorescent tubes - none or many - you connect in series with your 10MÎ© resistor, no more than 4mA can flow when these series components are connected across a 40kV supply.

Unbalasted fluorescent tubes generally have very low conductance once they have ignited - a negligible resistance compared to 10MÎ© - so for practical purposes you can assume that 4mA will flow through the whole series string.

How much power will be dissipated in this long, fat, costly, smokey resistor? Watts = Volts times Amps = 40,000*0.004 = 160W.

This is a lot of heat and money going up in smoke - think how hot a 100W tungsten lightbulb gets (hence 'smokey')

Now let's think about this capacitor of 70.5Î¼F with 40kV upon it.

The total charge in Coulombs is Q = CV = 2.82 Coulombs, and energy stored is W = Â½ QV = 56400 Joules

If your 10MÎ© should go short circuit - let's say surface tracking bursts out into a blue-white arc snaking along the resistor - then this stored energy will be unleashed on your fluorescent tubes. An explosion with flying glass, molten metal, ruptured eardrums, hospital, mortuary costs, and so on.

Patrick

Sat Mar 01 2014, 03:14AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ha! mortuary costs, a mere nuisance...

testtest

Sun Mar 02 2014, 11:02PM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

GammaRay wrote ...

I've spent much time searching online for a 10 meg resistor @ 100-200 watts with a 30-40kV flash over spec - or anything close. Looked and looked, checked every vendor that I know. The flash over spec is the sticking point. Helpful guidance will be appreciated.

This could be of help for high voltage and high wattage.

have a look at the second example.

Several other examples on google.

Hydron

Sun Mar 02 2014, 11:44PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

GammaRay, the thesis I linked in your other thread also had a bit of a discussion about water resistors, may be of use.

Dr. Slack

Mon Mar 03 2014, 09:25AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

It is for an experiment illuminating 40 feet of florescent tubes powered by the bank of (10) 40kV caps totaling 70.5uf. Admittedly, the 200 watt spec is too high. I suppose I could come down on the desired resistor wattage to around 100 watts and still have enough of a safety buffer, yes?

<jedi hand wave> sounds like this is not the project you're looking for <\jedi hand wave>

I'm not saying that discharging your caps slowly through a big resistor and some tubes won't show something, but the only scope you have for drama is if something goes wrong and you blow all your tubes, which is not really the way to plan it.

Now if I had 10 off 40kV large caps, I'd build a marx bank, probably 2x5 stage bipolar, to reduce the peak voltage to ground, and to split the build into two. It's about the right sort of number of caps, and your voltage is high enough to make spark gaps very non-critical, low enough that you won't have insurmountable problems with corona, and you have enough energy there to make some really spectacular planned drama. The components to do it are easy to come by, this is one place where long thin tube water resistors would make excellent interstage charging resistors. One has been built before, have a look at lod.org, the LightningOnDemand website, and their Lorentz gun taser cannon project.

You already know that gloves and boots are little practical use at 40kV, so would be totally irrelevant at the 10 stage output!

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