Introduction: Super Simple Ignition Coil Drivers

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.

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.


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).