Resistor needed for glow discharge?

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Arun Luthra
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Resistor needed for glow discharge?

Post by Arun Luthra »

I want to do some glow discharge experiments in the less than 2000 V range (not for a fusor).

Thus far I have have applied up to 1000 V to low pressure air (I recall I have tried between 10^-3 torr and 10^1 torr). I haven't been able to get glow discharge, only brief arcs.

There is no resistor in my circuit, the only resistance is provided by the low pressure air. After doing a little reading and looking at a demonstrator glow discharge chamber design, it looks like people are using resistors to prevent current runaway. So I guess what is happening is that when a breakdown occurs, the resistance drops as ionization increases, until the capacitors in my power supply are rapidly drained (milliseconds).

Is my understanding correct -- I can't get glow discharge because I didn't use a resistor?

https://www.reed.edu/physics/courses/Ph ... %20Law.pdf
Screenshot 2022-08-07 1.56.13 PM.png
Based on this chart, it looks like 200 to 2000 kOhm is the plasma resistance to be expected in a "typical" glow discharge demonstrator apparatus. Is it similar for fusors?

It looks like a popular resistor value to use for glow discharge demos, and also for fusors is in the 50 - 100 kOhm range.
Arun Luthra
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Re: Resistor needed for glow discharge?

Post by Arun Luthra »

Are wire-round resistors with a high power rating the standard solution for this? Perhaps sitting in an insulating oil?
John Futter
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Re: Resistor needed for glow discharge?

Post by John Futter »

We use "Ballast" resistors
typically 25k ohm to 100k ohm high voltage wirewound types or special ceramic versions that are very expensive
Arun Luthra
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Re: Resistor needed for glow discharge?

Post by Arun Luthra »

In case someone finds it useful, here are some calculations (assumes Ohm's law). It is kind of fake because current is not actually an independent variable, but I want to see how the other variables would behave for a given current. Gas, pressure and electrode geometry would be the true independent variables but these are common current values for these devices.
Screenshot 2022-08-07 9.12.00 PM.png
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Liam David
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Re: Resistor needed for glow discharge?

Post by Liam David »

What does your setup look like, and what power supply are you using?

Without knowing more, I think that your synopsis is reasonable. Does the power supply shut off after an arc? Re-ramp? Does it have any built-in current-limiting features? Most of the power supplies you'll see discussed here are inherently current-limited, whether through magnetic shunting (NST) or through a high-speed control loop, or the inherent resistance of a transformer coil.

As resistors go, most fusors use ballasts that look like this:
https://www.ebay.com/itm/134147584396?e ... Swzn9e4Wqs

I use this one up to 70 kV / 20 mA:
https://www.ebay.com/itm/144169702256?e ... SwL1RcZK2e

At the voltages and currents you're running, you don't need something nearly as big. Find a resistor of sufficient physical size to dissipate the power you expect, and withstand the total voltage across its terminals. Your Ohm's law tables are not a bad way to start at this.
Arun Luthra
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Re: Resistor needed for glow discharge?

Post by Arun Luthra »

For this particular test, it was a large old DC supply (containing a large transformer coil) that does up to 1000 V and is current limited to up to 200 mA. It would arc at approximately 1 Hz, I presume it was recharging each time. It wasn't turning off, it was just supplying short pulses of current to feed each arc.

Something that I don't quite understand about the numbers in the table is that the ballast resistance is 9 times smaller than the plasma resistance. But, the plasma current is supposed to be undergoing positive feedback that causes a current avalanche. The math may be similar to viral herd immunity, where the current decays to a steady value if the overall resistance is too high? The analogy being that if viral R0 is small enough, then even a small amount of herd immunity (like 1/9 of the population) is enough to blunt runaway infection.
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Richard Hull
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Re: Resistor needed for glow discharge?

Post by Richard Hull »

Sounds like you have a 1Hz relaxation oscillator there.

Liam is right, much more info with images would be nice. We can't help shooting blind. The caps in your old supply might be the source of relaxation.

Richard Hull
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Rich Feldman
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Re: Resistor needed for glow discharge?

Post by Rich Feldman »

The limitation of "equivalent resistance" model should be clear in a I vs V chart.
Here's one example: https://en.wikipedia.org/wiki/Glow_disc ... nglish.svg
I couldn't quickly find one for NE-2 glow lamp, showing hysteresis.

Some power supplies (like yours, apparently) will give pulses if used without a ballast R. Others, as with NST's or high frequency CW multipliers, incorporate current limiting that makes glows easy.

Relaxation oscillators using neon glow lamps have been familiar for generations.
https://www.bristolwatch.com/ele/neon.htm
For my first one, I found a 90V battery in stock at an old electronics store. Today one can snap together a string of 9V batteries. Warning: a long enough string can deliver a lethal electric shock.
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Arun Luthra
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Re: Resistor needed for glow discharge?

Post by Arun Luthra »

I can confirm that I can get a continuous plasma when I slowly increase the current limiter. The current was high at 100 mA to 150 mA. When I added a 50 kOhm ballast resistor, the current was closer to 2 mA. 700 V minimum needed for breakdown, 8 mm spark gap, tested at 2.6 torr (pump off).
Screenshot 2022-08-10 5.15.15 PM.png
Given the measured currents, the plasma was approx 40 times more conductive without the ballast in this situation.
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Rich Feldman
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Re: Resistor needed for glow discharge?

Post by Rich Feldman »

Good lab work there.
As we see in things like fluorescent lamps: As current goes up by orders of magnitude,
plasma "conductance" can go up even faster; plasma voltage can go down.
You can chart the curve, if there's enough ballast impedance for a stable operating point, or other way to keep the current under control. As done commercially for more than 100 years. :-)

You got plasma voltage by subtracting ballast voltage from total voltage -- good. It doesn't work with AC power and inductive ballasts, as with fluorescent lamps for most of their history, because of AC phase difference. Then you need to directly measure the voltage across lamp.
All models are wrong; some models are useful. -- George Box
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