Automotive Ignition Coils

Conventional coils

The conventional (Kettering) spark ignition system in automotive applications uses a coil which performs the dual functions of energy storage and voltage step up. It is typically about a 100:1 turns ratio. DC power is applied, producing a current of about 5 Amps through the 8 mH primary inductance, storing about 100 mJoule of energy. When the current is interrupted the points opening, (The points are a set of contacts mechanically driven by a multilobed cam synchronized to the engine), the voltage rises (L di/dt) to around 300-400 volts, which is stepped up to around 30-40 kV (open circuit) in the secondary.

The di/dt is limited to around 50 amps/millisecond by a capacitor (called a condensor, for historical reasons) across the set of points. If this capacitor isn't there, the rapid voltage rise as the points open causes an arc across the points, which absorbs some of the energy, and more important greatly reduces the di/dt, reducing the output voltage. (The points contacts also typically melt). Typical capacitances are around a microfarad ( i.e. it stores about the same amount of energy as the coil inductance does).

In typical use the coil is connected in series with a ballast resistor of a few ohms to reduce the voltage, which in turn reduces the current through the coil so it doesn't burn up. The ballast resistor isn't used during cranking (when the battery voltage is usually reduced to around 6-8 volts). A typical DC coil resistance would be around an ohm, and at 12 Volts, the DC current would be 12Amps, dissipating more than a 100 watts.

Approximate parameters from Motorola MC3334 ("high energy ignition circuit") data sheet (online data sheet at The data sheet describes the chip as being used in aftermarket Delco (GM) five terminal ignition applications.

Ignition coil specs: Leakage L - 0.6 mH, primary R = 0.43 ohms +/- 5% @ 25 C, primary L = 7.5-8.6 mH @ 5A

This seems to have very low resistance, which would be desirable in this application, where the control IC uses the pass transistor to limit the coil current.

Measurement data

You can measure the R,L, and C of the windings using a RLC bridge or other devices. However, you have to watch out in interpreting the results, because the core losses and distributed capacitance can throw things off. Measuring a real device, particularly one with two windings, is a non trivial matter, if you want to get real answers..

HEI Coils

Some measured data (BK Precision RLC meter) from HEI type coils (supplied by Robert Wroblewski). Wroblewski also measured the DC resistances using two different Digital Multimeters, getting different results (which is not entirely unexpected).

Wells C834 HEI Ignition Coil, new with core.

Computed Rpri (from Q) = 3.7 ohms, which doesn't mesh with either AC measurements. However, the core losses are probably significant and contribute to the difference. Compare the without core measurements below. The difference in the primary DC resistance is probably due to the fairly low resistance (test leads make a difference) and possibly due to the inductance fooling the DMM.


Without Cores...

Carquest HEI Ignition Coil, without core in clear silicone RTV.

Wells HEI Ignition Coil, without core in mineral oil.

Calculating Rpri from Q and X(1.28 ohm) again, you get 0.68 ohm. The difference between the cored and uncored values is probably the core losses. Especially when compared to the typical numbers from the MC3334 data sheet, the latter resistance seems more believable.

The increased apparent resistance at 1 kHz is probably due to the distributed capacitance within the coil. Some of the C cancels the basic L of the coil, reducing its measured inductance. Note the 120 Hz measurements are very close to the DC measurements.

Copyright 2000, Jim Lux / igncoil..htm / 21 April 2000 / Back to HV Home / Back to home page / Mail to Jim
revised 24 April 2000, added new measurements from R. Wroblewski