Jacobs Ladder "One Shot" Circuit? Using third leg and HV caps onsecondary... how did that work?

[Jeff Miller]

Hi, when I was in high school, electric shop teacher had a Jacob's
ladder made from a 15KV neon sign transformer. I've made some since, but
his had a special feature I never figured out the secret to. It was
strictly a one-shot device. Between the normal pair of electrodes, there
was a short "starter" electrode. When you pressed the power button, the
breakdown would start between the short electrode and the main
electrodes: I forget if it had a preference for one vs. the other. I
also forget how reliable it was, if you were guaranteed a start each
time. But I do remember it never restarted after the first spark had
extinguished itself, regerdless of the fact that the xformer was still
under power.

Inside the wooden box was one, or perhaps 2 high voltage caps about half
the size of a bar of soap: perhaps old mica transmitting caps. I don't
remeber what their values or even their voltage ratings were, or how
they were wired in. I failed to make a schematic and when I asked about
it the teacher was evasive. The only thing I can figure is that it
somehow took advantage of transients generated if and when it was
started some distance from a zero crossing: and that it would never fire
if controlled by a solid state relay with auto zero cross detection, for
example.

It's a nice feature if only because Jacob's ladders have a strong
tendency to overheat if they run continuosly. The electrodes get very
hot too.

Has anyone seen a schematic of this trick? With two caps, three input
terminals (arguably, if you conisder secondary midpoint ground) and
three discharge terminals there are only so many combinations to try
exhaustively, I suppose... but not knowing the values or the exact
spacing between the electrodes adds too many variables.

-Jeff

 

[Mike Harrison]

Hi, when I was in high school, electric shop teacher had a Jacob's
ladder made from a 15KV neon sign transformer. I've made some since, but
his had a special feature I never figured out the secret to. It was
strictly a one-shot device. Between the normal pair of electrodes, there
was a short "starter" electrode. When you pressed the power button, the
breakdown would start between the short electrode and the main
electrodes: I forget if it had a preference for one vs. the other. I
also forget how reliable it was, if you were guaranteed a start each
time. But I do remember it never restarted after the first spark had
extinguished itself, regerdless of the fact that the xformer was still
under power.

Inside the wooden box was one, or perhaps 2 high voltage caps about half
the size of a bar of soap: perhaps old mica transmitting caps. I don't
remeber what their values or even their voltage ratings were, or how
they were wired in. I failed to make a schematic and when I asked about
it the teacher was evasive. The only thing I can figure is that it
somehow took advantage of transients generated if and when it was
started some distance from a zero crossing: and that it would never fire
if controlled by a solid state relay with auto zero cross detection, for
example.

It's a nice feature if only because Jacob's ladders have a strong
tendency to overheat if they run continuosly. The electrodes get very
hot too.

Has anyone seen a schematic of this trick? With two caps, three input
terminals (arguably, if you conisder secondary midpoint ground) and
three discharge terminals there are only so many combinations to try
exhaustively, I suppose... but not knowing the values or the exact
spacing between the electrodes adds too many variables.

-Jeff

Shouldn't be too hard to do. Just make sure the gap at the bottom is too wide to break down by
itself.
The trigger electrode would generate a HV impulse to initiate the discharge- there a numerous ways
to do this - one would be a small 2-stage marx generator, or maybe a cockroft-walton multiplier. The
trigger would need to have a high-resistance path to avoid the main discharge current after the arc
strikes.

 

[Tim Wescott]

Shouldn't be too hard to do. Just make sure the gap at the bottom is too wide to break down by
itself.
The trigger electrode would generate a HV impulse to initiate the discharge- there a numerous ways
to do this - one would be a small 2-stage marx generator, or maybe a cockroft-walton multiplier. The
trigger would need to have a high-resistance path to avoid the main discharge current after the arc
strikes.

I was thinking an ignition system from a motorcycle or other small IC
engine -- should give you all the parts to make a nice fat spark if you
only supply 12VDC and a contact opening -- you could even time it from a
555 if you wanted to regulate power consumption.

 

[gwhite]

Hi, when I was in high school, electric shop teacher had a Jacob's
ladder...

Where is Aylward? He's the expert on the bible.

 

[Frithiof Andreas Jensen]

Shouldn't be too hard to do. Just make sure the gap at the bottom is too wide to break down by
itself.
The trigger electrode would generate a HV impulse to initiate the

discharge- there a numerous ways

to do this - one would be a small 2-stage marx generator, or maybe a

cockroft-walton multiplier. The

trigger would need to have a high-resistance path to avoid the main

discharge current after the arc

strikes.

The trigger may not need to be connected to the main gap in any way: A
decent spark about 50 mm (2 inch) can trigger the main discharge by UV light
alone (maybe helped by some ions flying about). "People" trigger cascading
spark gaps that way in Marx generators. It's safer that way.

 

[Frithiof Andreas Jensen]

The trigger may not need to be connected to the main gap in any way: A
decent spark about 50 mm (2 inch) *** distance from the main gap, not the

spark ;-) **** can trigger the main discharge by UV light