Car Ignition coils (there are other kinds, such as motorcycle and boat coils, but the ones made for automotive engines are the largest) consist of two windings: A primary and a secondary, wrapped around a core (usually multi layered silicone steel) and encapsulated in some insulating material (epoxy or oil are normally used). When a current is pulsed into the primary winding (which consists of a couple hundred turns of relatively thick wire), it sets up a magnetic field around it and the coil’s core. When this current is removed, the field collapses and induces a current on the secondary coil, which consists of several tens of thousands of hair thin wire This induced current appears as a high voltage pulse (a product of the ratio of turns between primary and secondary and of the field’s rise time), which in engines is used to fire the spark plug and ignite the air/fuel mixture inside the combustion chamber. In a car engine the coil may be pulsed by up to 312 volts (for high performance ignition systems. Voltages in the 200Volt range are more common for lower end systems) some 500 times a second when the engine is running fast. The pulse is provided nowadays by an ignition system which drives the coil by a capacitive discharge timed through a magnetic pickup in the engine. Before the advancements in solid state switching technology that made the current electronic ignition systems possible, the coil was driven by a special rotary breaker that was directly spun by the engine and switched the power mechanically.
This page is devoted to the construction and development of special solid state devices intended to switch a coil electronically at as high rates as possible, and as high powers as possible. When switched in such manner, induction coils make excellent high voltage power supplies, which can be used for several purposes, such as charging capacitors, driving Jacob’s ladders, Tesla Coils, Plasma Globes, and so on.
Here you can see a reconstruction of my first High Voltage device. It is an ignition coil driver; I built the original when I was 11years old from plans I found in an old electronics magazine. It taught me a lot, and it also gave me some memorable shocks (sometimes strong enough to knock me over and make me go pale).
As I recall it, it had a 2.2uF capacitor pulsed through a SCR. The first design ran at some 30 watts RMS, and made 3Cm long sparks about once a second. The device below uses two ignition coils (the other one is inside the box. Note that the original only had one) and runs some 50W RMS. The blue arc seen there is about 6cm long.Here is a more recent version of the this same driver. Both ignition coils have been fitted inside the box, and their cases are used as a common ground. The SCR is on a heat sink, and the capacitor bank was upped to 4uF (Any more and the operating frequency drops below resonance). I ran this with an inductive ballast pulling 20Amperes off the 220V line (nearly 5000watts!). The main capacitor was charged to 312V (due to the full wave rectifier in the input), and pulsed through the SCR into the coils (which are wired in anti parallel) at 6000Hz. I got close to a 20Cm electric arc from this, which had enough current to melt the wires and set their insulation on fire. The coils would hum and vibrate loudly and often the runs would have to be stopped because the oil was boiling inside them.
Integrated Circuit & transistor based:
Here is the final version of a 555-based ignition coil driver I built. This one uses the IC to produce square wave pulses at a variable frequency, and applies the pulses to a 2N3055 transistor, which connects and disconnects the ignition coil to the power supply. Since there is no capacitive discharge involved, the arcs produced are thin and blue. I got up to 3cm long arcs from a single coil with this circuit. It makes a high pitched whining sound, correspondent to the frequency it is running at.
The square looking thing in the foreground here is a microwave oven transformer. Those are usually rated at 2.5kV, 500mA and whereabouts, but due to their lousy current limiting, they will output as much as 2Amperes on the high voltage side (5000W). Here you can see one such transformer arcing from two iron electrodes. If the electrodes are separated quickly once the arc has formed, it can be drawn to over 10cm length. I was using those electrodes to cook (ehm, carbonize more like) a hotdog, and the carbon left over in them can be seen burning on top of the plasma: Normally plasma does not exist outside a current flow.
Not related to Ignition Coils, but this is a portable 25kV DC generator of my own design. it employs a relay to switch the power on and off from a 9V battery at up to 3000cycles per second. The pulses from the 9V battery are applied to a small 9V / 220V step down transformer that was potted in epoxy and is run in reverse. The transformer produces up to 10kV pulses by inductive action (since the power is being pulsed into it), and those pulses are fed into a TV cascade (the square green thing in the picture), which produces a steady 25kV output at a few miliamperes. I use this for charging small capacitors and for small scale ionic experiments. It could also be used as a stun gun, as I found out when I accidentally touched its output and was knocked down by the shock…
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