Jacob's Ladder problem.

Ballasting is quite critical for good JL operation. My guess is you don't have enough. I also don't know if there may be issues with using inductive ballasting - never tried it but an entirely unsubstantiated gut feeling tells me that it may at least change the behaviour compared to resistive ballasting. Try running it using a 1KW heater element as ballast, if only to see if it makes a difference.

If you want to make it sound a lot scarier, adding a nanofarad or two across the HV supply works well but watch you don't get into resonant territory as this will probably kill the transformer insulation (if using a neon sign transformer I wouldn't go above about 470pf).

A Jacobs ladder requires voltage and current regulation to work well. What is your voltage? What does the arc look like?

Try a gentle radius bend at the bottom of the ladder and a very gradual opening as it nears the top. If the arc stays in one place is it melting the electrodes? It should with two transformers . . . if it isn't, that may indicate your current is too low

MOTs are high current but relatively low voltage. The arc should be thick with lots of plasma, but may not widen a great deal before extinguishing.. NST's have it all over MOT's with their higher voltage and current regulation when it comes to a ladder.

You get it to move until it is 1/2" - that suggests to me that you don't have enough current. MOT's don't have the voltage for a really good Jacobs ladder, but they can supply lots of current. With enough current, the arc will sustain itself and widen quite a bit. Decrease the ballast, or use something that provides a little regulation like lots of parallel high wattage tungsten bulbs. An electric space heater may work better than an inductor.

Yep, getting the electrode configuration on a JL is tricky. The gap everywhere must be less than the gap the HV can ultimately jump if you want it to work as a JL, but not too much less. And the gap must increase upwards at a smooth and very gradual rate. A sharp inflection will terminate the action.

For audio modulated spark, much better to have a stationary arc.

Don, of course, the arc would be stabilized at the top for an audio modulated one.

I should have noted what happens as the arc dies before - It rises slowly up to the point where it dies, and then the ends stop rising and the middle starts rising fast. The arc becomes more diffuse and seems to wrap around the electrodes slightly. Counting vertical length, the arc about doubles in length. Then, it disappears.

Decrease the ballast? I'm drawing something like 7 amps...do I really need more?

Resistive ballast drops the voltage by half - inductive ballast doesn't.

Increase the ballast? How is that supposed to give more arc?

A 2 MOT stack, if you didn't know, gives 4kv AC.

Don't go into resonance? Heh, I would need 350 nF.

I would imagine the phase shift would make it behave at least 'differently' to a resistive one - whether it would be better or worse I have no idea...

Too much current can sometimes makes the arc stick in one place - maybe as a result of local heating/vaporisation ettects.

My comment was from having seen a neon transformer with 2.2nf across it strike a spark of about 2.5 inches. Just tried measuring the inductance but it's off the scale of my meter (>200H). As you say, a MOT's secondary inductance will be a lot lower.

Yeah this takes me back. Years ago when I was a teenager I attempted to build one of these using the transformer from an old dental x-ray unit. Thing would produce a beautiful 2" arc, but only momentary "Jacobing" on the 2 lengths of coat hanger wire I had set up for the ladder. Sounds like you know a lot more about this then I did then, might check out this from a generally reliable source

Rick

Inductive ballast limits peak currents - it is a dynamic thing . . . Resistors work better for that application - anyhow you do this stuff to have fun and learn, so what is stopping you from trying it? Surely you must have figured that out building Tesla coils.

The arc stops and center of it rises - that sounds like the electrode isn't straight or bits of metal are vaporizing from a hot spot keeping the arc from following the hot plasma up the ladder.

Drawing seven amps of REACTIVE current. Reactive current doesn't do work. What is the power factor and actual power to the arc? Starting to see how a resistor might be a good idea?

Around .14 * V across arc. Resistive losses don't do work other than heating. Jesus.

Things I am considering doing:

- Smoothing out the electrode surface; however, I'm not sure how to do that very well - sandpaper would just make it more rough, if on a smaller scale.

- Adding a CW V doubler to one or both sides; this would restart the arc if just dies momentarily and the hot air is still easier to break down than the base, but I would need to order some (not too expensive) stuff. It may also make for more of a "crackle" and a weird sound waveform from the rectification if not that, which on the one hand is more impressive by itself but on the other hand isn't as good for putting the thing in a resonating tube.

- Adding a string of resistors to ground and finding the voltage; this would tell me the voltage across the arc, but I think the changes in voltage relevant to the problem would be too fast for me or my multimeter, and I would have to find a setup I would feel safe with - I do not want to stand within a distance where I could fall onto the thing, safety first, deadly voltages/currents, etc, and I'm not sure just what this would tell me about my problem. However, I am going to do this eventually anyway.

- Reducing the resistance on the Gabriel electrode - this would make it start better, but as it is unlikely to affect my problem, I'm planning on waiting until I decide whether I'm using a CW multiplier.

- Asking a different group or Pupman.

Many years ago, I made a Jacob's ladder using a 20 kV neon sign transformer. I used 2 metal coat hangers. The arc was drawn out to about 3 inches before it snapped.

The only issue with this type of project, is that it can be very dangerous of touched while running.

Jerry G. ======

Wow, I hope nobody new building one read that! How about the UV frying your eyes? Starting fires, blowing fuses, killing the dog, and ruining your ears (if you have big caps on it)? There are a million ways high voltage can destroy your family - although the most common way is for you to spend all your time on it until the wifey divorces you. If it's fun, it might kill you. This is the rule of life.

I consider this a clue. This voltage appears to me to be a little on the low side for a Jacobs Ladder. With some fussing around, I expect it will probably work - with the ladder's wires being of size around 10 to 14 AWG, preferably 14 or 12 rather than 10. Thicker wires like AWG 4 may heatsink a lower power arc to require the ladder to have especially smooth wires with divergence not exceeding a degree or two anywhere (possibly less), and the gap at the bottom may need to be only about 1.5 mm (1/16 inch) and at lower power the arc may break at 1/4-1/2 inch spacing, closer to or maybe less than 1/4 inch (6 mm) if the current is not maintained at higher (?) arc voltage drop like 3-4 KV. Heck, in a case that bad, the arc may require wire divergeance angle of half a degree to rise. Also, I give a slight chance that the arc stabilizes at a slightly to somewhat elevated position rather than break and restart at the bottom - in which case I suggest thinner wire and to a greater extent I suggest more power (especially more voltage, since 20 milliamps I have found to be plenty for Jacobs ladders).

- Don Klipstein ( snipped-for-privacy@misty.com)

You want higher actual power to the arc. You want heating of the arc. Heating of the arc makes it rise. It takes a few watts to make an arc rise just a few centimeters between 10 AWG wires diverging only a couple degrees. A couple watts can make an arc rise a few centimeters between wires of size around 20 AWG and diverging a couple degrees. Hope that the arc rises with enough momentum to become unstable and break so that a new arc starts at the bottom when you have barely enough power to make a stable arc at all!

I suspect Cockroft-Walton multipliers don't do Jacob's Ladders well - better to use neon sign transformers or oil burner transformers. Please keep in mind that oil burner transformers are often rated for only intermittent duty, at least in my impression!

- Don Klipstein ( snipped-for-privacy@misty.com)

Jacobs ladders don't produce much UV. Ever notice how much one causes fluorescence of various fluorescent materials? Fluorescent paints, papers, labels, white cotton fabric, inside surface of broken fluorescent lamps? Compare to an ordinary flashlight with fresh batteries and a Woods glass filter? Compare to a flashlight bulb placed inside a gutted blacklight bulb and fully powered? Answer: UV output of any usual Jacobs ladder is detectable but not hazardous.

Actual known hazard - keep flammable materials out of reach of the arc. Do not permit existence of flammable/explosive concentration of leaked flammable gas or vapors of exposed/used/spilled flammable liquids at the scene of a Jacobs ladder - nor any other source of flame or spark, such as gas appliances operating or starting or having a pilot light, where switches or motors could produce a spark, nor where anyone lights a cigarette, nor where anyone or anything produces a spark either normally or as a result of foreseeable malfunction or failure!

- Don Klipstein ( snipped-for-privacy@misty.com)

Don Klipstein wrote: text

.14 amps is pretty high powered for a Jacob's ladder. You're right about NSTs and OBITs being the easy way, along with the classic choice, ignition coils. I guess I'm just a sucker for MOTs - I made my TC off a single MOT too, with a 7 stage Marx generator. It'll be hard, but I'm giving them up for IGBTs and ferrites. Cold turkey. Right after this project.

I guess I'll try 12ga wire. I had been told that MOTs can melt 10ga, but I have a million feet of bare 12. In fact, I'm also going to try roughing up the 4ga and see if the arc will progress from point to point better!

Care to explain why you say CW doesn't do JLs? I'm not talking about a sizeable fraction of the power going into the caps...

Well, I admit that I haven't actually measured anything myself. My understanding has been that higher voltage lower current arcs (such as coil sparks) make less UV than lower voltage higher current arcs (such as coil spark gaps). I know that arc welding definitely requires good UV protection, and MOT arcs seem pretty arc weld like...and I know the spectral lines of nitrogen point to it giving off UV...well, I'm keeping my sunglasses on.

I never got good results with capacitive output.

However, I expect it will work if you have resistance in series with the ladder and you have enough current.

- Don Klipstein ( snipped-for-privacy@misty.com)

The point I'm trying to make is that a reactive element is probably the worst form of current limiting. Were the inductance in series with the arc - it would help stabilize it. In series with the transformer, it shifts the current to lag the voltage - not ideal.

Resistive losses? Are you plan on running it for long periods?

You could add active solid state current regulation to the input. Two transistors, a bridge rectifier, and a large heat sink. I did it with a 300 watt plate transformer used for a TC - takes a few good spark gaps and MOV's to keep the relatively expensive transistors happy. Tungsten lamps were easier, cheaper, and worked almost as well.

The basic SS current regulator works with two transistors but for power dissipation it took me 5 bipolar HV pass transistors . . . to regulate a 300 watt 9KV plate transformer.

There's a wealth of information on the web regarding dynamic and passive arc stabilization for welders.

I have some ferrite common mode chokes, if I add a cap I'd probably stick one end of it in the middle of the CM choke.