Jacob’s Ladder

This is a quick and simple project that is a bit old but still always a delight to see. Who doesn’t love staring at those high voltage sparks moving upwards again and again on the background of a mad scientist’s laboratory?  Since we have a few high voltage transformers around here, I thought I could make a Jacob’s Ladder as a last minute project to add to the ones that would be presented on the Open Day.

The construction is pretty straightforward, you basically just need a high voltage power supply, in this case I’m using an NST (neon sign transformer) just like the ones I used before on the Tesla Coil and the Ion Thruster, with an output voltage of around 10kV.

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And then a V shaped spark gap where each electrode connects to one of the high voltage output wires from the transformer. For the spark gap you can use a number of different materials, generally stainless steel, but I just went with something I had close by, a bit of 2.5mm2 (AWG 13) rigid copper wires.

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To create an electric arc, you need to reach the breakdown voltage of the air, which is around 3kV/mm depending on humidity and temperature, among other things. This means, the distance between the electrodes will have to be adjusted accordingly to the output voltage of your transformer. For 10kV I’ll need to make sure that the distance between electrodes, at the bottom of the wires, where they are nearest each other, will be around 3 or 4 mm.

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For the base, I just used an old PC ATX power supply enclosure we had laying around. The connections themselves should be made using high voltage wiring because of the isolation, however I ran out of it, so instead, I cut a few bits of wires from flyback transformers. These I got from some old TVs and CRT screens, that’s why you can see the suction cups still there.

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That’s it! Now you just have to keep some distance and turn it on. You might need to adjust the distance between the electrodes, but be sure to always turn it off first and discharge the electrodes with a ground rod just to be safe. If they are too close to each other, then probably the spark won’t rise. If they are too apart, there won’t be a spark at all.

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At the top of the wires, the distance should be enough to interrupt the spark. The more powerful the transformer, the brighter the spark will be, because for all purposes, this behaves like a short circuit, it will draw as much current as the power supply can deliver.

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As the voltage breaks the dielectric strength of the air, plasma is generated. The ionized air will rise with the heat, and so will the electric current path. That’s why the spark always goes up.

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As it reaches the top, the distance between the wires gets too wide and the ionization trail becomes unstable, eventually breaking the arc. Then it starts all over again from a place where the distance is shorter.

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If you look closely, you can notice the beginning of a new spark, as the old one breaks in the previous picture.

Be careful working with high voltage, it can be lethal! Also, you shouldn’t keep any kind of electric arc working indoors for a while, because it generates ozone which can be dangerous for you and other animals, in just a few seconds you’ll start feeling the smell.

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