Jacobs ladder project schematic

Hi all

My next project is going to be making a jacobs ladder. Most people have it done using a high voltage source such as a neon sign transformer, but im want to do it using a high voltage car ignition coil which should do the job. Below is my rough schematic i have drawn. Any opions?

The open wires is wire the voltage jumps (would this provide enough resistance in the circuit to cancel putting in a resister?
I am buying a BOSH 716 ignition coil would this be enough? (12v dc in voltage out=???)

Picture attached

An ignition coil needs to have interrupted DC to output high voltage. And it's extremely difficult to transfer enough energy through an ignition coil to maintain the plasma arc necessary for the Jacobs Ladder.

Thanks for that? Any ideas on how to do it using a car ignition coil then? Schematic would aslo be helpful

spitso said:

Thanks for that? Any ideas on how to do it using a car ignition coil then? Schematic would aslo be helpful

Click to expand...

Ignition coils are not manufactured to create an arc larger than a spark plug gap (40-50 mils). Anything larger and the secondary voltage rises above specification and can cause arc within the transformer, damaging it.

I have seen coils survive large spark distances when resistive plug wires deteriorate. However the coils are not guaranteed to withstand it.

Hey, if it's just for fun anyway go ahead and use it - I would.... just be aware.

ok thanks for that explanation, i now understand allthough ive seen many people over the internet do it. i might ditch that idea and try and find a high voltage transformer. what would the electrical design be for that?

I've run ignition coils with 1" long spark gaps long term with no problem. They're even fine when being run and not having any discharge (max V spike on secondary). My scope showed that I was hitting 15kV spikes on the output. The coil I was using was an MSD Blaster 2 IIRC.

so ignition coils will work? im currently reading a website on how to make a jacobs ladder using a ignition coil now. It uses a dimmer switch (Variable resister????) to control the sign wave. What i dont understand is, isnt the output voltage from the ignition coil DC not AC, so there isnt any sign wave?

Below is the website
**broken link removed**

The output of the ignition coil is a pulse train, with each pulse a sharp spike that decays exponentially. It is AC, but only on one side of 0V.

You won't get a very big spark. You're going to be limited to about 1" of arc length with an ignition coil. Controlling them to get max power through them is something best done with a PIC as a control source and a HV probe to see what you are doing. You want to use a low-side FET to turn on 12V to the primary and, as soon as current saturates the primary, turn the FET off. You will get a positive kickback voltage that the FET needs to be able to handle (get at least a 200V FET and some transient suppressor zeners that react fast). Once you get the primary charging time figured out, then you crank up the pulse repetition rate and see how fast (kHz) you can go to get as much power through it as possible.

You also don't have to run them only at 12V. You can go higher. You risk damaging the coil, but remember cars idle at 13.7V anyway, and these things have some safety margins built in so they can go 50-100k miles. I wouldn't go over 18V or so though.

Up until the 1970s, when a car ignition coil had 12 V written on it, it meant that it was designed to run on 12 V and the resistance of it limited the current to about magnetic saturation.

Those of us who drove cars built then learned the technique of spinning the engine with the starter, then letting go so that the battery voltage could recover, to give a good spark and the engine would fire.

Then the manufacturers started making the coils with lower resistance so that they would saturate with more like 6V on them. Initially the current was limited with a ballast resistor (which was shorted when starting) then the limiting became electronic.

My point is that any modern coil will stand over 200 V on the primary, because it has to survive that when the current is interupted. However, if you feed it with DC, the current will rapidly build up and saturate the coil, beyond which extra current is pointless. The current will eventually overheat it.

The only spark ignition car we have is a 1994 Fiat, with two double-ended coils and no distributor. The primary resistance is 0.5 hm: to 0.6 hm: so the power is something like 240 W if fed from 12V. That is obviously not how it works. The current is electronically limited to somewhere around 5 A and the time is also controlled by computer.

If you either feed an ignition coil with very short pulses at around 200 V, or monitor the current, and turn off when it gets too big, you can use almost any voltage you want. Using mains and a light dimmer might well work fine, with possibly a capacitor in series.

In the Jacobs ladder the spark voltage is obviously in the 10 kV range, but it is the small gap at the bottom that limits the voltage. The sparks get longer an more impressive as they rise, in the tube of hot plasma that is rising by convection. However, the voltage must be less than what would be needed to jump cold air at the small gap at the bottom, because the current would go that way if it was easier.

A car ignition coils looks like a perfect pulse transformer for that to me. They are certainly cheap when second-hand.

I found two websites which demonstarts succesfull using a car ignition coil to run a jacobs ladder. It uses 240vac to a dimmer switch and a 3uf motor cap.

A question..... whats a motor cap??? the circuit symbol looks exactly like a capacitor????? is there any difference?

here is the website which shows the schematic
**broken link removed**

Here is another one that uses an ignition coil and dimmer:

**broken link removed**

I think you would have to be very careful using an ignition coil though.

spitso said:

A question..... whats a motor cap??? the circuit symbol looks exactly like a capacitor????? is there any difference?

Click to expand...

A motor cap is a low-capacitance, medium-voltage nonpolar capacitor used to start and/or run certain types of AC motors called capacitor-start, capacitor-run or capacitor-start/capacitor-run motors. Any appliance repair shop should have some on hand--they are commonly used in refrigerators and air conditioners and other appliances with motors which must spin up under high initial load.

But wait in the schematic which picasm recomended it just has straight 240v AC going into the car ignition coil...... Arnet ignition coils meant to run on 12V DC???

But wait in the schematic that picasm recomended it has 240v AC going into the car ignition coil. Arent the ignition coils meant to run on 12v DC????

schematic atttached

spitso said:

But wait in the schematic which picasm recomended it just has straight 240v AC going into the car ignition coil.

Click to expand...

You don't see the lamp dimmer and the capacitor in series with the mains voltage?

so your saying a dimmer switch and a capacitor transforms ac to dc aswell as reduces the voltage???

Dosent a AC dimmer switch vary the herts the sign wave ocilates and a capacitor holds/filters current? Or am i totally off topic

A lamp dimmer doesn't vary the frequency (measured in Hertz).

It works by turning on the lamp for only part of each half of the main cycle. Twice each cycle the dimmer turns on the lamp voltage very quickly rises from nothing to the full mains voltage.

The capacitor is filtering in this application. It lets the fast rise through to the coil, but stops the main frequency current which would overheat the coil.

Have a look at my post about coil voltage. Coils have to work with large peak voltages and 240V is quite normal for an igniton coil.

so theoretically speaking...the above schematic should work?

Aswell is a 700w dimmer switch ok? if not how many watts are used??