Super Simple Ignition Coil Drivers

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An ignition coil
(or spark coil) is nothing more than a low frequency auto-transformer with a relatively high turns ratio. The transformer typically has only a dozen or so turns on the primary but many thousands on the secondary. It is very comparable in design to the pulse transformers used in other appliances such as fence chargers, well suited for high voltage bursts. The original purpose of the coil is to produce a reliable and hot spark within an engine cylinder, and tens of thousands of volts open-circuit, or about a 2-4CM spark.

While it is possible to drive the coil with low-medium voltage AC, this will not produce the massively high output voltages commonly expected from such transformers. It is often driven with a train of square pulses, as each pulse will store energy in the coil, which gets released suddenly when the DC is abruptly disconnected. As the coil has inductance, this sudden change in current results in a massive voltage spike in both the primary and secondary.

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Step 1: Lets Start With the Bacics

Step 2: How My Schematic Works.

This is how my schematic works: Her is a schematic that shows what the capacitor does. This the first schematic. It is a good driver for making big, hot sparks.

Step 3: My Schematic (cheap and Easy)

this is my schematic its easy, but it, with my power source, only makes about 20 KV (1 inch arc) at a low fixed freq. it's also noisy but still very cool. this makes VERY small streamers due to its frequency. and makes cool plasma globes out of light bulbs.

Note: be sure to use high current capable relays, automotive relays are a good choice. This circuit will inevitably destroy any relay, due to arcing at the contacts accelerating corrosion and excessive heat eventually melting the relay. The power supply must also be capable of maintaining constant voltage at the frequency of operation. Adding a large (1000uF+) of capacitors is needed to minimize stress to the power supply. Do not power this circuit with lithium ion batteries or an expensive supply.

Theory of operation:

Upon the application of a power supply, current briefly flows through the capacitor and coil. This is the start of "ringing" due to the action of the capacitor and inductance of the coil. When the voltage across the capacitor reaches a high enough value, the relay engages, shorting the capacitor. Doing so causes the relay to disengage and another "ping" to this resonant tank circuit, continuing the oscillation. The frequency is set by the value of the primary inductance of your chosen coil pack or ignition coil, the impedance of the relay coil, the value of the capacitor, and the time taken for the contacts to close.

As can be seen, this circuit will result in large current pulses through the NO contacts as the energy of the capacitor is dissipated there, and large voltage spikes on the primary on the NC contacts, accelerating corrosion and damage.

Use-case:

Despite causing damage to the relay, this circuit is fun to play with, and is a quick an dirty way to test coil packs and ignition coils. The shocks from the circuit are very painful but generally not too dangerous (when operated at 12V) as the power output is limited. Of course caution should be exercised around such a circuit for obvious reasons.

Step 4: A Super Simple Driver

this ridiculously simple driver with no doubt simple,and exceptionally cheap and can easily produce 40KV! there are a few disadvantages with it, its mains powered, making it dangerous, and the frequency is not adjustable.


(due to my laziness and super slow Internet, instead of pictures, here's another instructable showing how to build it. it will have the pictures and schematic in it)

Step 5: 555 Timer Driver

Another popular coil driver is the popular is this 555 timer driver. The 555 serves as an oscillator at audio-range frequencies and drives a transistor that drives the coil. its a little more costly and complicated but its overall relatively simple.

While the circuit diagram is relatively straightforward, it is poorly designed and the transistor will see excessive stresses due to peak currents and voltages from the output. Even high-current BJTs generally have a Vce of 1V or more at high currents. This is also worsened by the power dissipation due to the additional base current and voltage, resulting in very high power dissipation. The HFE or current gain of the transistor at high currents is generally fairly low (between 10 and 50), which stresses the 555 timer. With 100 ohm base resistor and a supply voltage of 12V, approximately 100mA of base current is expected, so the maximum collector current that keeps it in saturation is only a few amps. The resistor will see large power dissipation, up to 1W depending on the duty cycle, so a 1W resistor must be used.


However, if parts are carefully chosen, and the components are not being stressed beyond there Absolute Maximum Ratings; this circuit can be fairly reliable and provide a stable unregulated source of high voltage. The frequency and duty cycle are be adjusted with the variable resistors.

Step 6: Another 555 Timer Driver

Here is another, better version of the 555 timer circuit. Several improvements have been made. The BJT was switched out for a high voltage, high power MOSFET. The diodes on the 555 side allow the circuit to operate at duty cycles at or below 50%, drastically improving efficiency, although I don't understand the purpose of the 1N4005 diodes.

The purpose of the series 1N4005 diode is to allow the coil's primary to ring at a couple hundred volts when the MOSFET enters cutoff (or turns off), allowing time for a spark to form at the output. Otherwise only a very breif positive-going spike will result, as the the internal body diode (and the other 1n4005) diode would clamp the ringing of the coil to -0.6V, greatly reducing overall performance. The other 1N4005 isn't necessary as the previously mentioned rectifier isolates negetive-going voltages already. However the 1N4005 is a very poor choice. A modern high speed diode (intended for switch mode power supplies) or 200V+ schottky diode is preferable.

Step 7: SCR / Triac Driver

Now, the SCR driver. This circuit is very straightforward. However, it requires a source of high voltage. The circuit uses the ignition coil as a pulse transformer, and the output is a single-shot high voltage pulse.

Theory of operation:

A high voltage (300V) power supply is used to charge up a small high voltage film or ceramic capacitor (do not use an electrolytic capacitor!) After it is charged, the supply is disconnected leaving the capacitor charged. The "fire" button is then pressed, triggering the triac or SCR. This hammers the primary of the coil, and the SCR continues to conduct until the underdamped ringing of the primary and secondary settles. SCRs remain latched until the voltage across their anode and cathode falls to zero or negative for several milliseconds. That high voltage ringing on the primary results in massive ringing on the output due to the high turns ratio of the secondary and a lot of the energy in the capacitor is turned into high voltage at the output.

This circuit is great when you need a very high voltage pulse at the output to ignite a flammable mixture, implementation of a powerful fence charger, etc. The circuit is also reasonably robust as SCRs are both cheap and can deal with very high powers (triacs are used in a variety of high power switching applications).

3 People Made This Project!

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127 Discussions

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KennyK30

Question 5 weeks ago

Anybody please help me upgrade this circuit please? Cause when i build up this circuit in prototype and test with 12v ignition coil, it always ends up with 555 times fried just few seconds........HELPPPPPP.......PLEASEEEE.........

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-max-KennyK30

Answer 5 weeks ago

Begging for help isn't going to help you.
If you actually want help, please provide as much detail as you can about your circuit.

Check your circuit, you probably have wired it incorrectly.

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KennyK30-max-

Reply 5 weeks ago

Already checked twice, the connection and wiring of the circuit all correctly....... i think there is probably something wrong usage with that 1w 100 ohm resistor as it ends up with might be a high voltage from ignition coil flow back into 555 timer via this resistor.........although after that i already replace that 1w 100 ohm resistor with 3w 380 ohm resistor, 555 timer also fried up in couple of seconds but this time it can last more few seconds before ends up......... i wonder can i replace that 2pc 1k ohm resistor from 0.25w to 3w type, and 1w 100 ohm resistor to 3w 1k ohm type for more durability..........

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-max-KennyK30

Reply 5 weeks ago

It is a poorly designed circuit, especially if you use an under-rated transistor like the 2N3055. Consider building the other MOSFET version instead. Use a MOSFET rated for 250V minimum and 10A continuous current.

That resistor burning up is a *symptom*, not the cause. You need to find the cause.

The base of the transistor should appear like a short circuit to ground, with about 0.7V when biased on. If any high voltage appears at the transistor base then something is VERY wrong. Your transistor is already dead at that point.

Build the circuit slowly and test each part. Just build the 555 section. Verify with an oscilloscope that the 555 is functional and pin 3 has a square wave on the output. A speaker or headphones can suffice as an alternative to a scope if you set the oscillation to a frequency within audible range.

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KennyK30-max-

Reply 5 weeks ago

When i try with another new 555 timer with just bulb type test light, it just working normally for a long time, so i wonder could it be the high voltage of the ignition coil flow back to 555 timer via transistor base.......to prevent this happen again, i wonder that could we add a Schottky diode across transistor collecter and ground to release it's stress...........

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KennyK30KennyK30

Reply 5 weeks ago

And also a capacitor across transistor collecter and ground......If so, may i ask you what type of diode and capacitor did you recommended......... Thank You!

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-max-KennyK30

Reply 5 weeks ago

OK, great! So you know that the 555 circuit is working on it's own. Now add the resistor and add the BJT, but replace the ignition coil with just a 12V lamp. It should be about half brightness, maybe flickering a bit depending on the frequency of the 555. Set the resistors on the 555 such that it's as close to 50% as possible. One resistor will be much larger than the other, like 1K and 100K.

Also it helps to use a current limited lab power supply because the 555 timer can only tolerate a maximum of 16V before it burns up. Add a 7812 or 7809 or 7808 voltage regulator if your power supply exceeds this limit.

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-max-KennyK30

Reply 5 weeks ago

Not sure how good you are at math, but the equation for voltage across an inductor (the primary of your coil) is V = L*Δi/Δt (technically di/dt to be more precise, it's a 1st order homogeneous linear differential equation). In other words, voltage is proportional to the rate of current change. The circuit makes a square wave. When the transistor turns on, the coil has 12V applied across it, so the current ramps up at a constant rate. When the transistor turns OFF almost instantly, the current is suddenly dropped to zero from some peak value. This sudden drop in current causes a big voltage across the collector of your transistor (theoretically infinite because di/dt is close to infinite), but in practice it's many hundreds of volts, limited by parasitic capacitance between windings and avalanche breakdown within your transistor.

Using a snubber diode will fix this issue, but you will not get any output from the secondary either. Remember: whatever voltages you can achieve on the primary, the secondary will see 100 times more because of the turns ratio. By adding the diode, you give the energy a path to ground. By limiting the maximum voltage on the primary side, the current decays much slower because of that relationship between current and voltage. The slower change in current over time means slower change in flux in the core and finally; less voltage induced in the secondary.

This is the fundamental problem with this type of circuit. It took me years of messing with the singing arc flyback project to realize this fundamental limitation. The only solution is to use a better circuit.

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KennyK30-max-

Reply 5 weeks ago

OK! After think again, i understood right now, so our mission in finding why 555 timer fried out is focus on rebuild a 555 timer with pin3 0.7V out when biased on and find a way to prevent ignition coil high voltage from flow back to 555 timer....... Thank You!

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apillai5

4 years ago on Introduction

Hey Max , Do you remember me ? . Anyways I need your help . , i made this circuit and i have gotta problem , The 100ohm Resistor is getting Too hot and the 555 Timer got blown The Transistor is also getting too hot . The circuit works when i pulse the Dc , And I have not connected the capacitor as i do not know the value of the capacitor . SO PLEASE HELP ME MAXX

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KennyK30apillai5

Reply 5 weeks ago

Hello apillai5, may i ask have you already make some improvement for your failed circuit you mentioned earlier.......is after the improvement everything operate normally?????

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-max-apillai5

Reply 4 years ago on Introduction

Pin 3 output is limited to I think about 200mA sink and drain. With 100 ohms, that means there has to be a potential difference of 20V across it. How much voltage are you feeding this circuit? The ne555 will not tolerate anything more than 16V, so do not feed it more than 12V w/o a voltage regulator.

To me that appears to be a 1/4 watt resistor, so you cannot dissipate more than 1.4W though it. Knowing that the 555 is capable of supplying enough current. 106mA @ 12V-1.4V(transistor's saturation voltage), totaling over 1.12W will completely fry that sucker, and since you only need a few mA's to saturate the transistor, you will need to choose a different, higher value resistor. Try a 220 ohm, or 470 ohm resistor. Keep incrementing up or down to find the best value.

Also, I did not build this circuit, I just put it all the circuits I found online into one place! Sorry if I can't help you more than that.

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AdithyaP-max-

Reply 4 years ago on Introduction

I have mentioned in the photo of the circuit , 12V 2A wall adapter

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-max-AdithyaP

Reply 4 years ago on Introduction

OK then, I can now see many, many problems. First and most importantly, do *not* omit the capacitor, it is necessary for the 555 to oscillate properly. Otherwise the stray capacitance between the pins will result in a very weak and unstable UHF oscillator that cannot drive the transistor or the coil. The value of it I believe is 0.1uF, but some experimentation will be needed to choose the exact value. (the output freq. should be between 40Hz-400Hz.) A "104" numbered ceramic capacitor should do the trick.

12V @ 2A is most likely not enough to get good performance. Once you get the circuit to function, you will probably be disappointed with the output. As the schematic you have shown calls for at least a 6A power supply. You can probably use an SLA battery for power. 12V high amperage power supplies are also really cheap, I modded an old Xbox power supply so that it can output 12V @ 12.6A. (Before the mod, even small over current transients would cause the thing to go into a fail-safe mode and shut down until power is removed. There was a custom chip that was responsible for that protection, and I simply shorted the optocoupler to the +5V rail though a small resistor, effectively bypassing the whole protection IC. Now there is no current limit or short-circuit protection)

Once you get the beefy power supply sorted, mount the transistor to a heat-sink or else it will overheat. TO-3 package style transistors are becoming more rare these days, so that is why an MJE3055 or TIP3055 may be a preferred choice. heat sinks for these are more common and readily available. Then if you still have issues with the 100ohm resistor, either try a higher value, or get a 3W 100 ohm resistor. Hope this helps.

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AdithyaP-max-

Reply 4 years ago on Introduction

I am going to buy a 12V high amp adapter and try your recommendation :D
and i do have the heat-sink

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-max-apillai5

Reply 4 years ago on Introduction

If you still have problems, and you think you are killing the 3055, try a TVS diode across the collector and base that is reverse-biased (backwards so it does not normally conduct), and that it has a breakdown voltage of around 60-70V. A few zener diodes may also help, make sure they are beefy or paralleled up to handle the EMF transients well.

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-max-apillai5

Reply 4 years ago on Introduction

Your transistor should be mounted on a heatsink. This circuit is not the most efficient and as the transistor warms up, the worse it will perform and the less efficient it will become. That leads to thermal runaway (as it heats up, it makes more heats, causing it to heat up faster). The NE555 is not really the best chip to use with these types of circuits because it can be killed with strong EMF. Make sure use a separate power supply for it (from the main ignition coil power supply) or some beefy LC filtering and voltage regulators (with current limiting).

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PamelaM131

1 year ago on Step 6

I found this works the best. The capacitor for the 555 I found my design works best on 0.047 mF, 0.1 is too slow and makes the MOSFET heat up a lot, 0.022 consumes less current but the sparks are weak.

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-max-PamelaM131

Reply 2 months ago

It depends a lot on your transistor, transformer, and parts selection. You do need a massive heat sink because if the transistor stays on too long, the current builds up to massive high levels and the MOSFET acts as a 0.1 ohm resistor when on, and dissipates that current as heat (current squared times resistance). There are also switching losses, although at ignition coil frequencies these are less significant.

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PedroF121

1 year ago on Step 6

I made mine with IRF640 Mosfet and recycled materials, except for the variable resistor knob with switch, the 555 and the iRF640. Here are some pics of the signal waveform to the gate of the mosfet and the drain of the mosfet. For the Drain of the mosfet i was using a 10x probe, so each 5V division on the grid is 50V, so the inductor kicks a very high voltage on the D terminal. I changed my car coil for a small 230VAC/9VAC transformer to make a recreational shock machine and it gives mild to high shocks.

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