# Relating tesla coil power to plasma bulbs?

Discussion in 'Power Electronics' started by Folding time, Jun 23, 2018.

1. ### Folding time

5
0
Jun 6, 2018
Hi I had a question about the power necessary to drive about a gallon-sized low-pressure mostly hydrogen gas filled tube? There are other noble gases in the tube, but i am unsure which. I'm looking for the breakdown voltage and current correct? To light up the tubes. I know more power and i'll get brighter tubes. I'm looking for a general "light up" of the tube.

I know that neon sign transformers run at about 10kv and probably insanely low current right? Does current matter in this situation? I'm sure it does but slightly? If I reach the breakdown voltage necessary: that's what's important, right?. Then more current = brighter tubes? So what current should I shoot for?

2. ### hevans1944Hop - AC8NS

4,607
2,151
Jun 21, 2012
IIRC, neon sign transformers (NST) are limited to 7500 volts from the "hot" wire secondary to "ground". Some have two "hot" wires, representing opposite ends of a center-tapped secondary, with up to 15000 volts between the two terminals. The center-tap is grounded, limiting the voltage from either of the two high-voltage terminals to 7500 volts with respect to ground. The secondary current is limited, by design, to a maximum, or short-circuit, current that depends on the size of the transformer.

I have two "15 kV" transformers and the smallest is rated for 10 mA of secondary current. This is not "insanely low current" IMO, so I think you are wrong about that.

The VA (volt-ampere) rating of my NST is 450 at 115 VAC input, or a reactive current of almost four amperes in the primary winding. The power available in the secondary is a nominal (15 kV) (10 mA) = 150 watts. So a neon sign transformer, or "Luminous Tube Transformer" according to the Acme Electric metal tag that was affixed to the transformer case, is not very efficient. What it is, is current-limiting and fairly inexpensive... I obtained mine, for free, from a garbage dump while living as a teenager in Phoenixville, PA, in the late 1950s. Was able to garner another from a former employer in the 1980s as they just wanted to get rid of it. Lately, my wife has been interested in learning how to use high voltage arc currents to burn random patterns into wood saturated with a solution of water and conductive ionic salts... so-called Lichtenberg figures. A new (or slightly used) NST in the 15 kV, 30 mA, range costs less than \$200.

You probably need to become familiar with a phenomenon called electron avalanche conduction that occurs in a conductive gas at low pressures. Avalanche conduction creates a negative resistance where an increase in current is accompanied by a decrease in voltage between two points within an electrically conducting plasma. A common example is the neon indicator panel lamp, NE-51. It requires a large voltage across the lamp terminals, typically more than sixty volts, before conduction sufficient to "light up" the lamp occurs.

Once conduction begins, the voltage across the lamp terminals drops to a lower value, and the current through the lamp increases. If allowed to continue without hindrance, the current would quickly increase until the lamp exploded. To keep this from occurring, a "ballast resistor" is placed in series with the lamp, creating an ordinary positive resistance for the overall circuit, despite the negative resistance introduced by the neon plasma in the lamp. A neon sign transformer does the same thing by creating a magnetic circuit between the primary and secondary windings that serves to limit the current available to the neon tube. Here is a link to a PDF file that provides (probably) more than you ever wanted to know about neon lamps. It's a good start to understanding electrical conduction in gas plasmas.

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3. ### Hopup

253
36
Jul 5, 2015
I also do have couple of those heavy around 500VA NTS's. They do give quite bit of mA out of them. All of the small NTS seem to not be central tapped. If someone knows where to get central tapped small NTS it would be very useful.