Discussion in 'Power Electronics' started by Lol999, Jul 2, 2019.

1. ### Lol999

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Feb 16, 2017
Bit of a strange one but I would like to make a battery powered Jacob's Ladder or even have it run off a step down transformer.
Apparently part of the circuit responsible burns out if run continuously.

What would be useful would be the ability to switch the current on and off at a suitable interval so the circuit has longevity.

Is there such a pcb or component that would enable such a thing?

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The only parts subject to failure are the electrodes that carry the arc and the ionisation electrode if you use one.

We use galvanised rod for the main electrodes. The zinc coating is removed by the arc over time and you can get a bit of surface rust forming.

Our ionisation electrode is one lead of a high voltage resistor. This gets very slowly eroded (maybe), but we haven't had to replace or adjust it.

We have a timer triggered by pressing a button that runs out for maybe 10 seconds, so it's not usually left running for long periods of time.

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4. ### hevans1944Hop - AC8NS

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Jun 21, 2012
Sure, it's appropriate, but we need some information from you first in order for the correct circuit is to be retrieved. First, how many volts will you use for your Jacob's Ladder? What is the maximum arc length you anticipate between the electrodes? How will you initiate the initial arc that will "climb" the Jacob's Ladder? Do you plan to enclose the electrodes behind a glass or plastic cover or some sort? What dimensions will your Jacob's Ladder be after it is assembled?

Here is a picture of a line-power operated Jacob's Ladder:

5. ### Lol999

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Feb 16, 2017
First, how many volts will you use for your Jacob's Ladder? I probably won't go above 12v due to the size restrictions - hence no need for a huge arc.
What is the maximum arc length you anticipate between the electrodes? Something around 2cm or bigger would be nice

How will you initiate the initial arc that will "climb" the Jacob's Ladder? I would hope that it is self starting although I can always use, I forget the correct term for it, a stick with a metal tip?
Do you plan to enclose the electrodes behind a glass or plastic cover or some sort? I envision encasing the device in a wood and glass structure, the glass would be ventilated if appropriate.
What dimensions will your Jacob's Ladder be after it is assembled? Hopefully no taller than say 12" high, 18" as a maximum.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Ok, so 12V is way too low. Consider 12,000 volts a starting point.

Our Jacobs laddar uses a neon display transformer with an output voltage of around 12kV. The low voltage side (240VAC) is switched using a contractor that is controlled from a 24VDC circuit.

The entire device is enclosed in an acrylic box with approx 9mm thick walls.

7. ### hevans1944Hop - AC8NS

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Jun 21, 2012
I set my 17,500 VAC current-limited neon sign transformer (NST) in a window for Halloween. It runs off 120 VAC, 60 Hz. I usually scrounge up a pair of steel coat-hanger wires for Jacobs Ladder electrodes, which attach to "door-knob" insulated HV terminals located on opposite ends of the transformer. I have another somewhat larger and heavier NST that can be used in a similar arrangement, but it has somehow become too heavy for me to lift anymore, so it stays parked in the garage awaiting a suitable rotary-arc Tesla coil project.

8. ### hevans1944Hop - AC8NS

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Jun 21, 2012
@Lol999, based on your answers it is obvious that you know nothing about high-voltage arcs. I suggest you do some more research. It is a good idea to enclose the ladder electrodes with some sort of transparent cover to control air convection and ensure a stable, rising, arc. This may also offer some protection to wandering fingers too, although with the top open some idiot may venture to touch the ladder wires or the arc or both.

Always use a Jacobs Ladder with adult supervision, and be ready to access the switch that turns the transformer off.

9. ### Lol999

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Feb 16, 2017
Thank you gentlemen perhaps I misunderstood your question regarding voltage.
The 12v I was referring to was the input voltage, there are a number of youtube videos showing small devices being run from a 9v battery or 4 1.5v batteries.
They use something like this:
https://www.amazon.co.uk/Miniature-...coding=UTF8&psc=1&refRID=ZC0Z6XQRCCX6QZE7J58Y
to create the voltage required for an arc.
it is stated that these items are not designed for continuous use hence the need for an interval timer of some sorts, hence my original question.

10. ### Nanren888

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Nov 8, 2015
Sorry, just trying to make sure I have your question clear.
The device is not rated for continuous use, probably as the voltage conversion electronics need to cool down to not expire.
,
You are asking after a circut that will turn it on and off periodically to keep it within its safe operating time, each time it is on.
Likely you have no specification for the off-time.
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If you don't want to use an electromechanical relay, likely any solid-state switch you put between battery/supply and device will have some voltage drop across it which may affect things/need to be compensated for. But a solution should be possible.
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A simple timer with power supply connect/disconnect should be easy enough. FETs are a popular option for power switching. Or a low voltage solid state relay. But you will need to add something to periodically switch it. Do you have the ability to put simple circuits together?
.
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Can you confirm that I have the situation approximately correct?
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I think the more experienced here might have been effectively suggesting another alternative: a continuously-rated version instead. But definitately read those safety warnings. Would that appeal?

Last edited: Jul 5, 2019
11. ### Lol999

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Feb 16, 2017
Hi and thank you for your reply. I have no actual components yet, I'm merely trying to discern what I need.
I lean towards a battery operated version as there is no requirement for PAT testing if I use it in public.
I plan to encase the electrodes in glass so they are effectively isolated from nosey fingers.
I can assemble simple circuits, my soldering skills are not the best but I can get there.
My aim is to use this as a sort of advertising display when I stand on our trade stall so it needs to be fairly compact and not too lethal!

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The easiest option is probably too have a momentary switch controlling the power.

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13. ### hevans1944Hop - AC8NS

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Jun 21, 2012
@Lol999, the devices you are considering are cheap toys, not transformers, per se. They produce very short, not very intense arcs, that may make for a disappointing Jacob's Ladder display. They consist of a driver oscillator for the primary of a resonant fly-back type transformer whose secondary winding has thousands of turns of lightly insulated wire. That is where the "Tesla Coil" terminology, sometimes used to describe these devices, originates. By building up a strong magnetic field in the primary, using a low-voltage DC, and then rapidly collapsing that field by interrupting the current, high-frequency, high-voltage oscillations are created in the secondary. This is exactly the manner in which ignition sparks for internal combustion engines (ICEs) were generated, hence the name ignition coil sometimes is applied to such a coil, even when it is not used to fire the cylinders of ICEs. Other names, such as induction coil and medical coil have also been applied. Whatever you call them, they all depend on periodically interrupting a low-voltage, high current, magnetizing field in the primary circuit to generate from the rapidly collapsing magnetic field a high-voltage, low current, electrical field in the secondary winding.

I have seen these devices sold to schools as a means of creating short high-voltage arcs for various classroom demonstrations. The early ones used a simple magnetic reed switch, controlled by a knob on the handle, to select the interruption frequency and duration, thereby giving some control over the strength of the sparks produced.

Before CRTs vanished from television sets, to be replaced with "flat screen" LED, LCD, and plasma arrays, the high voltage that was needed to accelerate cathode ray tube electrons to high enough energy to illuminate the phosphor viewing screen was generated with similar circuits. The main difference was the CRT needed a DC power supply, and transformers can only produce AC. Some sort of high-voltage rectifier was necessary to produce a DC output, but this is not necessary to create an "arcs and sparks" Jacob's Ladder display. So your "toy" "ignition" transformers, although perhaps lacking a rectified DC output, may be adequate for your purposes of creating a very small demonstration Jacobs Ladder.

I will get around to advising you on how to keep from burning them up in a moment.

Jacob's Ladders have quite a history as special-effects props in science-fiction and horror films. Here is a film clip from an early effort. All of these Hollywood props were built from neon sign transformers, many if not all, probably salvaged from the constant re-modeling and re-building projects Hollywood is famous for. Anyway, these are very serious transformers capable of illuminating thousands of watts of luminous tubing with hundreds of milliamperes of current at tens of thousands of volts. That's more than enough to kill you.

Most of the smaller ones, like those used to light up beer signs in bars, operate directly off 120 VAC, 60 Hz, utility power and do so continuously without overheating. Other versions use 240 VAC, 50 Hz, but are also designed for continuous operation. In the USA, the secondary voltage must be current limited by the transformer design, and for secondary voltages greater than 15,000 VAC the secondary winding must be center-tapped and grounded to the steel case of the transformer. It is wise to also connect this "ground" to the utility neutral and to a purpose-driven earth ground, but AFAIK Code doesn't require it.

Since you haven't acquired any parts yet, I would suggest you look for a small neon sign transformer on eBay or Amazon. Purchase one with at least 5000 VAC and 30 mA current capability. Using a 12 VDC wall-wart for a power supply, Google a circuit to rig up a 555 timer and a relay with a 12 VDC coil and SPST (or DPDT) 120 VAC, 10A contacts to control application of power to the primary of the transformer. Buy a short extension cord with at least 14 gauge wire and cut it in half. Wire the wall-wart 12 VDC output and the 555 circuit with the relay between the two halves of the extension cord. The half that plugs into the wall goes to normally-open contacts on the relay. The other half of the extension cord accepts a convenience plug from the NST and is wired to the other half of the normally-open contacts on the relay. Pressing a button will charge up a capacitor and turn on the 555 which will operate the relay. Place a 1N2007 rectifier across the relay coil, cathode to the wall-wart positive supply terminal, anode toward the 555. After a period of time has elapsed, the capacitor will be discharged enough to turn off the 555 which will de-actuate the relay and turn off the Jacob's Ladder. Let us know if you need any help finding a circuit for the 555 timer. Maybe I'll breadboard this after you get the NST and show you how it works. Or maybe some other "hands on" type of member will do that for you. I have all the parts in front of me, just not enough time to work on it right now.

While you are waiting around, Google to find out what voltage is required to cause an arc to jump between two needle electrodes in air. Multiply that number by at least two to account for round wire electrodes in the Jacob's Ladder. A 5000 VAC NST should not require an auxiliary ignition electrode, but a sewing needle you hold on a plastic stick should work.

If you want to prevent the toy versions from burning up, a simple push-button switch to turn it on for brief periods should suffice just as well as a timer... but you will get tired pushing the damn switch while manning the advertising display booth. I would opt for the "professional" approach of a real neon sign transformer and a delay-off timer. Good luck, and please keep us posted on your progress.

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14. ### Lol999

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Feb 16, 2017
thank you for such a comprehensive reply. I guess I have a choice to make regarding "toy" or "serious" and seeing as this is a project for the near future (many other more important things to finish yet) I have time to ruminate on it.
If you or someone could provide a circuit diagram for such a "serious" device that would be great as not being immersed in electronics (I used to run an electronics factory years ago!) and yourself being across the pond some of the product terminology or acronyms may have different labels in the UK.
many thanks.

15. ### hevans1944Hop - AC8NS

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Jun 21, 2012
You go ahead and acquire the neon sign transformer and someone here will help you wire it up. It would of course be better if you could find a friend in Great Britain who could hold your hand to guide you through the assembly process, but we will do our best to help you once we know what we have to work with.

If you can find a NST with a nameplate attached, please record all the data on it, or photograph it, and post it here. A well-focused, well-illuminated, photograph of the entire NST showing its mounting hardware (brackets, studs, whatever) with dimensions would also help decide how to place the Jacob's Ladder electrodes.

The transformer electrical specs (line voltage, line frequency, primary current, secondary voltage, secondary current) are the minimum information needed to allow you, with our help, to safely... probably... construct your Jacob's Ladder. Of course, as others here have stated, you work at your own risk. If there is anything posted here that you do not understand, please ask for clarification. Take plenty of photographs as work proceeds and upload the photos here for our comments. Good luck with your "advertising" project.

16. ### Lol999

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Feb 16, 2017
Just as a further queestion as I am no further with this due to other more pressing concerns but would a T1D6 rectified power supply do the trick putting out 5v at 4A?

17. ### duke37

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Jan 9, 2011
As an alternative to the neon transformer, you could try an ignition transformer from a gas or oil boiler. I imagine that these are scrapped when the boilers corrode and leak. Go see a friendly plumber.

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Feb 16, 2017
19. ### duke37

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Jan 9, 2011
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