Jochen's High Voltage Page

Ignition coils

Ignition coils are used in cars to produce a spark which initiates the combustion of the fuel-air mixture in the cylinders. They can be had cheap at the scrap yard, or expensive from car parts dealers. Depending on manufacturer and type, their properties differ somewhat. In particular, so-called high-power types have a reduced internal resistance, allowing higher primary currents.

This picture shows three typical ignition coils. The "Bosch" type (numbers (1) and (2)) is quite common in Germany. The large plug at the center is for high voltage output. Coil number (3) has had its plug enlarged to prevent arc-over.

An ignition coils, like a transformer, consists of an iron core with a primary and a secondary winding. The turns ratio between secondary and primary is in the order of 100:1. Both windings are connected at one end, so that the secondary is automatically grounded through the primary circuit. Arbitrarily many coils may be paralleled for higher output current, but cascading secondaries is not possible due to the internal connection to the primaries. However, just like with MOTs, two coils may be anti-paralleled on the primary side. Through the internal connection, this automatically puts the secondaries in series, i.e. they produce different polarity output. The maximum voltage difference thus possible is around 60kV, which is already enough to jump about 10cm air gap.

The simplest ignition coil circuit, as used in many cars. It also demonstrates the internal connection between secondary and primary. When the switch opens, secondary and primary are in series, increasing the output voltage the few hundred volts appearing across the primary.

Ignition coils are usually operated on a DC supply, and just like flyback transformers they need a driver circuit. The simplest circuit is shown above. With the switch closed, an increasing DC current flows through the primary, producing a magnetic field inside the iron core, in which energy is stored. The final current is limited by the internal resistance of the coil, usually a few Ohms. When the switch opens, the current is interrupted and the magnetic field collapses, releasing the stored energy in the form of a large voltage pulse (a few hundred volts across the primary winding). This voltage pulse is multiplied the turns ratio, resulting in a peak voltage of around 30kV. The cap across the switch limits the ultimate peak voltage (see also the chapter on flyback transformers) by slowing down the collapse of the magnetic field, turning the singular transient into a damped high frequency oscillation. Without this measure, an arc would form in the switch after opening, possibly damaging the switch and slowing down the collapse even more, resulting in a much reduces output voltage. The additional resistor prevents welding of the switch contacts when it is closed again and the cap discharges through it.

The mechanical switch can of course be substituted by an electronic one, e.g. a transistor )bipolar, MOSFET, IGBT), which must however be able to withstand high voltages as well as high currents. Additional protection circuitry (varistor/VDR/MOV) is highly recommended. The circuits shown below use SCRs, which are well suited for high current switches.

Ignition coil circuit using SCRs. The driver circuit to the right takes a standard square wave (about 5V peak) and produces the two signals needed to switch the SCRs alternatingly. The component values given are meant as clues only, they provide about 30mA gate current to the SCRs. This kind of driver works well into the kHz range.

Another ignition coil driver, simple but with impressive results, is shown below. The cap is charged to around 300V directly rectified from the mains, and discharged through the SCR and the primary winding, producing a very strong high voltage pulse. The heater element serves as a ballast limiting the current drawn from the mains when the SCR is on, and in case something blows up. The single rectifier (in contrast to a two-wave rectifier bridge etc.) ensures that the SCR has sufficient time to return to "off" state after the caps has discharged. The coil is put under enormous stress in this mode of operation, especially discharges without secondary spark should be avoided.

Another ignition coil driver. The cap should be around 100uF (electrolytic) for a few pulses per minute (hand operated, driver circuit (a) on the right), or in the order of 1uF for higher firing rates (e.g. mains synchronised, circuit (b)). Polarity of the transformers is important and must be tried out.
This circuit works on mains voltage, and every point of the circuit is on mains potential, in particular the connectors of the ignition coil and the high voltage plug. A gate transformer to the SCR is a must. No point of the circuit may be earthed, as this would be a short circuit. Neither the high voltage plug nor the other two connectors may be grounded. Touching any of these (or any other part of the circuit) can be deadly. If possible, this circuit should be operated on an isolation transformer.

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