Ballast, Blaster II, ohms, current, friggin volts!

It goes beyond Ohms law, I will explain why. It is a long but true story. An ignition coil is an inductor and has resistance. I have a blaster coil and a good Ohm meter the DC resistance measured at 1 Ohm. I the 0.5V is added in there is 1.5 Ohms. There is other resistance to in the wiring from the battery .. ignition switch to ballast and ignition module. As a guess, it is likely below 0.5 Ohms so the total is now 2 Ohms. If the battery voltage is 12V the current would be I = V/R, or 6 Amps. By now it seems like a long story, but it is just starting. The 6A would be with coil powered and engine stopped, and the switch being points or electronic driver on. The engine being stopped, is not part of the original question, and the original question about the coil voltage has not been answered. I will get to that. First the electronics in an ignition system has a voltage drop, called saturation voltage. It is typically below 2V for a Darlington transistor and perhaps a 1V for an IGBT. The voltage at the + side of the coil, engine stopped could be 7V, by assuming a 2V saturation, and have the remaining supply voltage is split between coil, and ballast plus car wiring, each with 5V.

Now the story gets longer. We only considered the DC, or direct current measurements. The coil inductance changes all that. When an inductor is first powered, it opposes a change in current. The current starts at zero and increases with time, limited circuit resistance or other factors. The increases with an exponential formula with the time constant of inductance divided by resistance. I will not go into detail, but the point is, the current varies with time. It starts low, builds, when ignition happens the energy stored in the coil is used to fire the plug, and it starts all over again. It is an varying signal.

Some electronic ignitions control the coil charge time, and time that activity such the the coil is just in time for the ignition event. Some build current limiting in the solidstate switch. Those ignitions do not require a ballast. Ignitions including OEM mopar, Pertonix, Mallory unilite, and others need a ballast!

If you desire to measure the ignition current, use the meter on AC volts and measure the voltage across the ballast with engine running. For the 0.5 Ohm ballast, the current will be twice the measured voltage. The is because I=V/R. For systems without a ballast a 0.1 Ohm power resistor may be inserted in series with coil and AC voltage measured and multiplied by 10. Or better yet use scope and measure peak current. It is what matters.