Jacob's Ladder problem.

[bhauth]

I made a jacob's ladder with a 2 MOT stack, inductive ballasted to 40
mh shorted, bare 4ga wire electrodes, 13.9 Mohm Gabriel electrode
(yeah, I know that's too much). If you were wondering, I wear
sunglasses and I have some experience, though not with rising arcs.
Anyway, depending on how I bend the electrodes, the dang arc either
won't start, won't rise, or will rise until it is about 1/2 inch long
and then die, for no apparent reason, in a spot with the electrodes
diverging only slightly. Same things in an open and a closed garage.
Compared to my tesla coil with similar current draw, it is incredibly
lame. I'm feeling glad I went with the simple thing instead of my
originally planned audio modulated solid state one.

 

[Mike Harrison]

I made a jacob's ladder with a 2 MOT stack, inductive ballasted to 40
mh shorted, bare 4ga wire electrodes, 13.9 Mohm Gabriel electrode
(yeah, I know that's too much). If you were wondering, I wear
sunglasses and I have some experience, though not with rising arcs.
Anyway, depending on how I bend the electrodes, the dang arc either
won't start, won't rise, or will rise until it is about 1/2 inch long
and then die, for no apparent reason, in a spot with the electrodes
diverging only slightly. Same things in an open and a closed garage.
Compared to my tesla coil with similar current draw, it is incredibly
lame. I'm feeling glad I went with the simple thing instead of my
originally planned audio modulated solid state one.

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).

 

[default]

I made a jacob's ladder with a 2 MOT stack, inductive ballasted to 40
mh shorted, bare 4ga wire electrodes, 13.9 Mohm Gabriel electrode
(yeah, I know that's too much). If you were wondering, I wear
sunglasses and I have some experience, though not with rising arcs.
Anyway, depending on how I bend the electrodes, the dang arc either
won't start, won't rise, or will rise until it is about 1/2 inch long
and then die, for no apparent reason, in a spot with the electrodes
diverging only slightly. Same things in an open and a closed garage.
Compared to my tesla coil with similar current draw, it is incredibly
lame. I'm feeling glad I went with the simple thing instead of my
originally planned audio modulated solid state one.

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.

 

[Don Stauffer]

I made a jacob's ladder with a 2 MOT stack, inductive ballasted to 40
mh shorted, bare 4ga wire electrodes, 13.9 Mohm Gabriel electrode
(yeah, I know that's too much). If you were wondering, I wear
sunglasses and I have some experience, though not with rising arcs.
Anyway, depending on how I bend the electrodes, the dang arc either
won't start, won't rise, or will rise until it is about 1/2 inch long
and then die, for no apparent reason, in a spot with the electrodes
diverging only slightly. Same things in an open and a closed garage.
Compared to my tesla coil with similar current draw, it is incredibly
lame. I'm feeling glad I went with the simple thing instead of my
originally planned audio modulated solid state one.

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.

 

[bhauth]

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.

 

[Mike Harrison]

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.

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...

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

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

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

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

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.

 

[[email protected]]

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
http://www.repairfaq.org/sam/jacobs.htm

Rick

 

[default]

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

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.

 

[default]

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

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?

 

[bhauth]

Drawing seven amps of REACTIVE current. Reactive current doesn't do

work. What is the power factor and actual power to the arc?

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.

 

[Jerry G.]

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.
======

 

[bhauth]

The only issue with this type of project, is that it can be very

dangerous of touched while running.

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.

 

[Don Klipstein]

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.

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 ([email protected])

 

[Don Klipstein]

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

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!

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.

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 ([email protected])

 

[Don Klipstein]

Wow, I hope nobody new building one read that! How about the UV frying
your eyes?

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.

Starting fires,

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!

<SNIP>

- Don Klipstein ([email protected])

 

[bhauth]

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...

 

[bhauth]

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.

 

[Don Klipstein]

Don Klipstein wrote: text

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...

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 ([email protected])

 

[default]

Resistive losses don't do work other than
heating. Jesus.

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.

http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/circuitsphase.htm

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.

 

[bhauth]

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.