Picture of Ignition Coil High Voltage Display
A common automotive ignition coil can generate an output voltage on the order of 30,000 volts. This is a sufficient voltage to produce arcs which jump an air gap of an inch or more.  Driven properly they can also be used to create a Jacobs Ladder display

Ignition coils are inexpensive and readily available, and constructing a circuit to drive the coil is straightforward.

This article provides an overview of ignition coil operation, as well as a drive circuit design and system capable of driving the coil to produce arcs for your entertainment and experimentation.

Of couse, as with any high voltage project, safety warnings are in order.  Excecise caution and good sense when working with high voltage.  Remove power from the system before making adjustements.  

Don't touch the arcs.  Be mindful that the potentials generated can jump a significant gap, and that insulation on tools like regular pliers may be inadequate to prevent you from getting a shock.

Keep in mind that some parts of the system can get hot.  The arcs can be hot enough to to ignite paper and plastic, so operate it in a safe location.

High voltage discharges generate Ozone gas, which can cause irritation if breathed in.  High voltage discharges also generate some ultraviolet radiation, so limit your exposure and don't stare directly at the arcs.

First, enjoy some video!

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Dillon123 6 months ago

flybacks are better at lower voltages and are cheaper

LED forrest6 months ago

it is asome

tunes3 years ago
I have built one of these using the 555 timer and I now see that a PIC based design allows a greater degree of fine tuning to get to the resonant frequency of the coil. This seems important as different ingnition coils have slightly different capabilities in their output with the same circuit and voltage parameters. Some have used two coils in parallel/antiparallel to achieve even higher output voltages and longer arcs.

I would love to try your PC based circuit but I currently have no way of programming a PIC and have only Macintosh laptops at my disposal. I suppose I could invest in a PIC programmer and borrow a PC to just load the files on to the PIC. There are other projects such as the Aurora 9x18 RGB LED art (PIC24F08KA101) that also use a PIC from Microchip. Could I just buy the Microchip's PICkit 3 In-Circuit Debugger/Programmer? Would this work with the files you provide here?? Or could you sell me a few programmed PICs? I could also buy a few PICs and fedex to you for programming, if you would be willing to help me.

Also, could you send me a BOM for the parts list?

OK to email direct:
LargeMouthBass (author)  tunes3 years ago
Actually, ignition coils don't depend on resonance to generate the HV output.
There is a misconception among some that an ignition coil operates like a Tesla coil. An igntion coil is actually an iron core transformer, with a turns ratio on the order of 75:1. Switching off the primary current causes the primary voltage to rise to several hundred volts, which is in turn stepped up by the turns ratio.

What is important is to have an idea of the inductance and the resistance of the primary, as that lets you determine the time constant. From there you can configure your pulse generator to turn the switch on for an appropriate length of time.

I'm not familiar with the PICkit 3 In-Circuit Debugger/Programmer. If it is a product made by Microchip for general in circuit programming then it should be able to be used. I have actually never programmed any parts using in circuit programming before, but I know that there is an app note on Microchips web site explaining what you need to do regarding connections to the processor.

The files I have attached should work regardless of the programmer you are using. You will have to use the 12F683 with the files supplied, unless you know how to update the source code for a different PIC and then assemble to get a hex file for programming the different PIC. That isn't terribly difficult if you know a bit about the assembly language.

I don't have a BOM of the circuit per se, but the schematic has manufacturers PNs for all the more critical parts, like the processor and the IGBT. The other stuff is just common resistors and caps. 1/4 Watt resistors work fine. The caps you select must be rated to handle whatever maximum voltage you plan on applying to them. For example, the ones on the input to the +5V regulator will see the power supply voltage, so select them depending on the max power supply voltage you intend to use.
a question here:
if ignition coils do not depend on resonance, then why does one need a capacitor in an auto ignition system ?
Thanks so much for your comments and the files.

trn2la tunes2 years ago
Your comment got me thinking about something unrelated to the jaccob's ladder, but directly back to automotive applications. Many car modders are trying to extract every last little bit of power from their cars. One common way is with better ignition. Perhaps if coils such as these operate best at their resonant freq and that they all have a unique freq, then it may be beneficial for car enthusiasts to get their coil "tuned" to their optimal resonant freq. Perhaps with a little mass produced 555 IC based circuit built by you. I dunno, but there could be some dollars in a cheap little black box that goes between you coil(s) and power supply, with a potentiometer or something for tuning to the coils best resonance.Im sure you could elaborate further on that idea. Perhaps something is already available, but need simplification Comments :D
trn2la2 years ago
This will prolly sound quite dumb, but im going to ask anyway, as this will help me understand what the driving circuit does a little better.

If I use no driving circuit and just apply approx 13v DC to the coil and have 1 wire out of where a lead to a distributor would go, and 1 wire to the negative ( for the sparks, are these called anodes and cathodes?), then have the positive isolated with a switch.

Is it true that I will get just 1 quick spark just a split second after I turn switch off?.

What i am trying to understand is if the driving circuit just switches the supply power on & off very quickly!?!?!? and a spark is discharged when supply is interrupted. If this is true, does allowing an earth have the same effect as switching off current, except current is still on.

In other words will the mere act of presenting an earth then trigger a spark while un-driven power is applied? Or is it a case that absolutely nothing will happen if constant uninterrupted power is applied?
LargeMouthBass (author)  trn2la2 years ago
Nothing will happen if the primary coil is just driven with a constant current. It is the interuption of the current in the primary that causes the changing magnetic flux through the secondary, inducing a high voltage on the output terminal. The presence of an grounded or earthed conductor will not result in a spark under those circumstances.
maixy2 years ago
i wonder how much the value of the coil is ?
LargeMouthBass (author)  maixy2 years ago
If you mean the inductance of the coil, the one I used had a primary inductance of about 4.5 mH. The primary resistance was about 1.5 ohms.

The inductance of the secondary was too high to measure with my meter. The secondary resistance was more than 10k ohms if I recall correctly.
cool idea
infinity113 years ago
How would I make this in a negative voltage
LargeMouthBass (author)  infinity113 years ago
The polarity could be reversed by changin the wiring on the primary side so that the "-" connection of the coil connects to the power supply, and the "+" connection is connected to ground when the switch closes. I have not tried this, however.

I've seen projects where some people have driven two coils, with opposite polarity output, so that the potential difference between the two is double. If this is what you have in mind, I would suggest that you use two of the exact same coils. The reason being is that the discharge on the output is fairly brief, and the timing is determined by the inductances and resistances and turns ratios of the coils. If two different coils were used, the output voltage peaks may occur at different times between the two coils, and the output may no be as great as you hope. Again, I haven't tried this type of dual coil arrangement, so I am sort of speculating here.
VadimS3 years ago
Well designed circuit. I've seen a few designs that are best describes as shabby.
Noitoen VadimS3 years ago
Back in the 1980, we use to build cdi ignitions for cars with points. They had a pot core harting oscillator transformer to charge the 2 0,47uF capacitors to around 400v and a thyristor to discharge it through the coil. The circuit was designed so that it kept the original coil, points and capacitor so that in the event of cdi ignition failure, you would just switch a couple of connector to return to normal. The original coil on a VW combi ran cooler and the points only needed ajustment due to mechanical wear and the ignition never failed.
VadimS Noitoen3 years ago
I've built something similar for a tractor a few years ago. Made a vary noticeable difference in the idle and starting.
skowerr3 years ago
What would happen if I would connect a power audio amplifier (12V max) instead of the pulse generator... would I get a singing arch??
LargeMouthBass (author)  skowerr3 years ago
This type of setup won't really work for the "plasma speaker" type projects. Those use a much higher drive frequency than can be used with an igniton coil. The drive frequency is usually tens of kilohertz, and then the audio waveform is used to alter the pulse width to modulate the arc to produce the sound.

The primary of an ignition coil has a fairly high inductance, and so the current can't be driven with a high enough "carrier frequency" which could then be modulated to produce any kind of reasonable quality audio. Flyback transformers are usually what is used for plasma speakers.

At any rate, the drive to the switch needs to be a digital signal to turn the switch either on or off, and so you wouldn't want to apply an analog signal directly to the gate/base of the switch you are using.
Thank you for your comprehensive answer :)
I asked because it is easier for me to get for free a ignition coil than a flyback.
One more question. What type of material should be used for the electrodes, so they would not melt after a couple of minutes of arc production?
LargeMouthBass (author)  skowerr3 years ago
The electrodes in this project are just 14 ga copper wire. I had no issues of the metal melting, but the insulation near the ends started to burn after a while. Brass brazing rods work well, and that is what I used on my Jacobs ladder that was built using a neon sign transformer. The brass can be somewhat better than copper in that it is stiff and so it will keep its shape better. But the copper works fine, just strip off all the insulation off to be safe.
The wires may get a little warm after producing arcs continously for a while.

I've only run the setup here for about 3 minutes at a time. If you are planning on making something to run for much longer, its best to check it out and see just how hot it gets. An of course, don't leave it unattended.

RickO5 skowerr3 years ago
Fun idea, I may have to try that.
trewq73 years ago
HoldOnTight3 years ago
You have taken a complex subject and you have described all the information needed to produce one of the best Instructibles I've ever read. Great job and thanks for sharing!
stackerjack3 years ago
I was always of the opinion that the main reason for the capacitor across the contact points was to allow the current in the coil to collapse rapidly, thereby inducing a very high voltage for the spark.
static3 years ago
Well done. I have been threatening to use a GM HEI coil, with 555 to do something similar, for quite some time. I have an old function generator I could use for a signal. but I'd rather keep from HV.. The old tool ain't worth much, but it works. I ruin a 555, and associate components I'm not out of much
Eurober3 years ago
A good explanation, but you have omitted to describe the purpose of the capacitor connected in parallel with the "points" in the typical automotive application.

The method I have used in the pasts is to drive che primary by discharging a 1 /uFcapacitor charged at 400V with a thyristor. With hi-voltage coils one can reach 60,000-70,000V without any damage to the coil.
LargeMouthBass (author)  Eurober3 years ago
You are correct, I did not include a mention of the capacitor ("condenser") that is used with the mechanical points system. That part allows a pathway for the current to flow once the switch is turned off, and in the mechanical setup it prevents pitting of the switch contacts. My design doesn't use such a cap, as the IGBT is use has internal clamps that protect it from damage.

The system you describe, where a cap is charged to 400V and then discharged through a coil via thyristor is called capacitve discharge igniton (CDI). I've never experimented with such a system, but I belive they use a different type of coil. Anyway, the older inductive discharge "Kettering" type system that my design is easier to implement, as CDI needs a circuit that can generate the ~400V needed to first charge the cap.
Yes, you are right on the capacitor, but it serves also an other purpose. The 1 μF capacitor, combined with the inductance of the system, creates a resonant circuit with a decaying AC voltage with a frequency in the region of 50KHz, if I remember correctly; facilitating in this way the transfer of energy to the sparkplugs. As a matter of fact, I would avoid using the over voltage clamping of the IGBT. The reason is that any current that is shunted to ground represents wasted energy. I would use a suitable capacitor instead. Because it is a reactive component, no energy will be wasted. I bet that at the secondary one would get a more "robust" spark. Please note that obviously the voltage at the IGBT will go negative also during the inductive kickback.

To achieve the 400V in the capacitive discharge method, I used the common voltage multiplicator circuit (Villard circuit and similar) to go from the typical household AC voltage line to the desired voltage, making the circuit quite simple..
LargeMouthBass (author)  Eurober3 years ago
I have wondered what difference it would make is the primary circuit was allowed to "ring down" via a capacitor across the switch, like the arrangement used with mechanical points.

I'd like to try a capacitve discharge type circuit, but I think I would need to use a coil intended for a CDI system. The coils I used in the project here were all intended for inductive disharge systems, and have an inductance of about 3 to 5 mH.
Funny, I posted an other answer and it has misteriously vanished.

For a confirmation of the purpose of the capacitor please read the following explanation from:

At the same time, current exits the coil's primary winding and begins to charge up the capacitor ("condenser") that lies across the now-open breaker points. This capacitor and the coil’s primary windings form an oscillating LC circuit. This LC circuit produces a damped, oscillating current which bounces energy between the capacitor’s electric field and the ignition coil’s magnetic field. The oscillating current in the coil’s primary, which produces an oscillating magnetic field in the coil, extends the high voltage pulse at the output of the secondary windings. This high voltage thus continues beyond the time of the initial field collapse pulse. The oscillation continues until the circuit’s energy is consumed.

I personally designed, build and sold SCR ignition units in the early seventies.
One of the various occupations that help me to pay for my MSc EE.

The standard coil had no problem holding the voltage, because the spark at the sparkplugs acts as a voltage limiter. On the other hand, if one bench drives a standard coil with a large airgap, it is 100% certain that the standard coil will be damaged. As I said with a hi-voltage coil and a SCR unit, it is easy to achieve 60,000-70,000Volts
Eurober3 years ago
For the sake of completeness I have to correct you. The capacitor is a fundamental part of the ignition system and without a car will not start.
I am afraid there are no misconceptions. In any case job well done.

At the same time, current exits the coil's primary winding and begins to charge up the capacitor ("condenser") that lies across the now-open breaker points. This capacitor and the coil’s primary windings form an oscillating LC circuit. This LC circuit produces a damped, oscillating current which bounces energy between the capacitor’s electric field and the ignition coil’s magnetic field. The oscillating current in the coil’s primary, which produces an oscillating magnetic field in the coil, extends the high voltage pulse at the output of the secondary windings. This high voltage thus continues beyond the time of the initial field collapse pulse. The oscillation continues until the circuit’s energy is consumed.
(End of quote)