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where can i find very powerful transformers powerful enough to make a 7-12 inch jacobs ladder? Answered




Best Answer 7 years ago

Generally diy models use a neon sign transformer.

so far the only person that nearly answered my question was frollard. if i dont get and answer that says "WHERE" i can find it and that doesent mean ebay or something like that. im talking like any kind of buildings that would throw them away or an old house, the one i used to live in is very large we still own it but its too much to even try and sell its in that bad of condition. if no one answeres my question frollard will get the best answer


7 years ago

Do you mean
  1. a 7 to 12 inch wide spark gap? (Quite a stretch.)
  2. Or 7 to 12 inch tall Jacob's ladder?  (Just a typical setup.)
You need enough voltage to create a dielectric breakdown in the air between the two sides of the ladder.  An approxiamte value is 3,000 V per millimeter, which converts to roughly ~76,000 volts per inch of gap, or ~500,000 V for 7 inch gap.

However, Jacob's ladders "cheat" this by spreading an arc across a bridge of floating plasma, requiring much less voltage.  What happens is the spark originates at the bottom of the ladder at the narrowest point of separation, typically less than 1/4 inch.  When the air breaks down, and electricity sparks across the gap, the air around the spark becomes superheated plasma, visibly glowing.  This plasma has a much greater electrical conductivity, even lower than the original gap.  So the electrical arc follows the plasma "bridge" as it floats upwards.  (Actually the super hot air has a lower density and is pushed upwards by the heavier, cold and denser air surrounding the plasma...) So you don't need the full 500 kV to bridge a 7 inch gap.

How much is enough, once the spark forms?  The equations are complex plasma physics, beyond my grasp.  However, coming at the problem sideways, there are documented examples of very large arcs.  One video shows a 500,000 V breakdown with an estimated 100 foot length!  That's ~100 times what you are hoping for, so a very rough estimate is 5000 V, after the spark starts.  Bear in mind this is very rough, I have no real faith in this number.

However, the real problem you'll face is the drop in voltage once the spark starts drawing all the current you can supply.  This will lower the effective voltage, eventually restarting the spark at the bottom.

Like frollard mentioned, Neon transformers can be used. Flyback transformers will also work for a typically Jacob's ladder.  I think I've seen at least 5 inch arcs, so maybe a 7 inch gap is possible.

I also suggest stable air- creating stable plasma bridges will contribute to large spark lengths.

  1. High voltage transformers are potentially lethal.  High voltage sparks are potentially lethal.  Since a Jacob's ladder is a free spark, safety-proof the device so no one can touch the spark.
  2. Ionized air is super hot.  Hot enough to start fires and cause serious burns.  Treat the device as an open flame and safety-proof accordingly.
  3. The ionizing of air creates ozone, the odor is easily detected.  Ozone is poisonous to inhale and bad to expose any organic molecule to.  The device site should be properly ventilated to remove the ozone.
  4. Plasma arcs give off many frequencies of light, including invisible forms such as infrared and ultravioletStrong emissions of UV light in particular can be created that could cause blindness (sunburn of the retina).  The device should be shielded with filters that absorb dangerous levels of light, particularly invisible frequencies.
  5. Sparks are fully open circuits drawing tremendous current, limited only by your power source.  Large current flows can heat up any / all parts of a circuit, to potentially dangerous levels.  Be sure to carefully include redundant current limiting devices in your set up to ensure power safety.

Great answer, but where are your breakdown strength figures from ? I always reckoned on 25kV/inch.


Lost the original reference-URL, but ~same value is mentioned on Wikipedia page "High voltage":

Sparks in air
The dielectric breakdown strength of dry air, at Standard Temperature and Pressure (STP), between spherical electrodes is approximately 33 kV/cm.[2] This is only as a rough guide since the actual breakdown voltage is highly dependent upon the electrode shape and size.

[2] A. H. Howatson, "An Introduction to Gas Discharges", Pergamom Press, Oxford, 1965, no ISBN - page 67

That value is approximately what I first referenced, 3 kV/mm.  Normally, I'd try to quote something from NIST, but I forgot where to find it in their pubs.  The other source that came up in my Google search had same value.

So, that would convert to ~75 kV per inch, definitely different than your value.  I'd bet your value relates to a specific situation or is intended as a safety rule-of-thumb (hence more conservative).  Do you know where you get 25 kV/inch?

For the record, main reason I answered was to cover the safety issues!  Such a simple device is actually a complex safety concern, particularly the UV emission and Ozone creation.  Had a friend in undergrad that sunburnt her eyes working in a lab.  She was viewing slides on some sort of light-table that was outputting unsafe UV.  She had to take a medical withdraw for the entire semester!

I inherited my value at my father's knees.....

Its bound to be a strong function of the shape of the electrodes, which I vaguely remember experimenting with in HV lab at university


I'm disappointed I couldn't find a rule-of-thumb in The Art of Electronics! That is exactly the advice Horowitz and Hill packed in there... :(

HH is THE book, but it doesn't do high voltages !

Found a reference that quantifies the voltage need to maintain a spark after it occurs:

Once an arc is initiated, only about 20 V/cm or 50 V/in is required to maintain the arc regardless of the magnitude of the arc current. [19]

[19] J.D. Cobine, Gaseous Conductors. New York: Dover 1958.

So only 50 V per inch! is necessary to keep the arc, leading to a second estimate of 500 V for a 10 inch spark, even less than my first approximation of 5000 V.  Now I'm starting to believe the numbers...

SO... for the initial question, it looks like no exotic transformers are necessary to achieve a 10 inch spark.  Enough to overcome the initial spark formation is the major voltage hurdle.

  1. this leads me to re-conclude that stable air is a critical component to large spark gaps.
  2. Voltage drop on the short-circuit may not be a problem, but I'd still investigate ensuring good current supply while the circuit is arcing.
This question is actually getting rather interesting...I think an Instructable is warranted...

Yes, once it becomes an ARC or SPARK, its VERY low impedance, as you have discovered. The arc also exhibits negative resistance.