# Current limiting for jacob's ladder?

I have a 10 kV 23 mA ignition transformer.

Apparently the way to make a jacob's ladder is to just connect the electrodes to the secondary, plug the primary into the wall, and call it good.

But of course, as a transformer this thing has no current limiting, and I have no way to calculate the effective resistance of the arc. (I'm guessing a theoretically perfect transformer has no current limiting, but a real life one probably might limit current in some manner I am not familiar with, magnetic saturation, etc, but I'm guessing also this is not the "correct" way to limit current when designing a circuit)

What's the best way to limit current here? I can use a power strip that will trip at 1800 W, but this thing will draw 230 W at the max of the spec.

Do I just fuse it at, say, 0.5 A on the primary side and use a slo blow to avoid blowing the fuse on the inrush?

Or do I use a single resistor on one half of the secondary side to limit the current?

Or use a variac on the primary side along with a killawatt meter to measure the current draw as I turn up the voltage?

What's the correct approach here?

Yes, I have worked with HV before and I understand this thing will kill me easily, I have built plenty of woodburners with scrap MOT, etc and I know how to handle HV.

• Some people call me crazy, but I used a neon sign transformer, a wire coat hanger cut in two, and some insulated stand-offs mounted to a piece of wood. Connected the wires that went to the neon to the coat hanger pieces, plugged it in and held my breath. It took less than two minutes to get the gap right and the thing arcing nicely. I AM NOT a trained professional and am just sharing my experience. Jul 18, 2018 at 20:43
• Any fluorescent ballast has enough inductance to define current limit. Since arc has negative resistance it drops in voltage as current rises and the “- ESR” depends on current while the ballast is high “+” reactive X(f) that limits the current and thus the holding current rises with gap as it rises and extinguishes at an electrode gap limited by the current threshold, Ih. Similar to an SCR. Jul 18, 2018 at 21:59
• The more current, the higher the ladder goes. Some use a variation. Wear sunglasses 🕶 Jul 18, 2018 at 22:05
• Just as a note: 10 kV accelerated electrons striking metal target implies "soft x-rays." Pretty soft. And yes, there is air in the way to hinder the final velocities. But x-rays still occur. See any of several papers by J. R. Dwyer from about 2003 to 2008. (Dental is somewhat 'hard' and is 70 kV, typically.)
– jonk
Jul 18, 2018 at 22:52
• Not to worry too much. Current-limiting is good practice, but older transformers are designed to run for hours igniting oil in the furnace across a spark gap. See pupman.com/listarchives/1997/may/msg00486.html Jul 19, 2018 at 1:19