Ignition Coil resistance question

Why a different coil?, is it to provide a better spark? - in which case the coil probably will be a different value. Either it's designed to be used like that, or the manufacturer will specify what series resistance (if any) it requires - you need to check the specs of the new coil.

Adding a resistor will obviously reduce the spark, less energy, less spark.

Going back to when I was a kid, we used to do a lot of old motorbikes and old cars in the fields etc. and we converted lots of them from magnetos to coil ignition - there were basically two types of coil, 6V or 12V - as long as you picked the right voltage for the battery, then it made no difference.

On motorbikes we generally just powered it from a battery, and charged it once it was flat - no need for charging circuits on a field bike They generally lasted about a week of normal use in the field - as the battery died, you started to lose revs.

Eb74 said:

I have the following questions:

• Does it matter if the resistance is placed on the positive side or the negative side of the power source? (On the drawing positoin A or B?)

Click to expand...

No practical difference. When the points open, the spark voltage is 15,000 - 40,000 V. The voltage across the resistor is about 3 V. I'm not sure if the resistance voltage is added or subtracted from the spark voltage, but it really doesn't matter as it's so small compared to the spark voltage.

On a traditional ignition system with points the coil current is controlled by the battery voltage, the coil resistance and the fraction of the time that the points are closed.

At high engine speeds the coil inductance also affects the current because the inductance slows the rise of the current each time the points close, so the average current is less.

To give a good spark at low battery voltages when cranking, some coils were designed for a lower voltage and a resistor was put in series. The resistor was shorted out during cranking. That was called "ballast resistor ignition".

More recently, especially when there is one coil per cylinder, the coil current is controlled by how long it is turned on by the electronic control unit, so the inductance (and the time) set the current. The resistance will make little difference. The electronic switch is closed for a constant time each cycle, rather than a certain fraction of the cycle time.

For example is the engine is turning at 1000 rpm, so sparking around 33 times a second, cycle time is around 30 ms. With points the coil could be turned on for 20 ms and off for 10 ms. At 4000 rpm, the coil would be on for 5 ms and off for 2.5 ms. With an electronic system using the inductance only, at 4000 rpm the times could be the same, 5 ms on and 2.5 ms off, but at 1000 rpm, the time on would be the same at 5 ms, and the off time would be 25 ms.

If the coil is designed like that, it probably shouldn't be used with a points system as it would not be designed to dissipate the amount of heating that would happen at low engine revs.

Thanks Diver300,

Without having the same knowledge that you have about electgronic ignition, your reply is exactly what I was expecting.

I will also be desinging the actuating point cam so I can set what the dwell time would be at lower RPM.
This is meant to be used on a harley shovelhead engine running 90% of the time at about 2500 rpm.

Also, I could build the coil mount out of aluminum for heat dissipation.

According to my Excel worksheet, with an ignition Cam Dwell of 75 degree I get the following "On" and "Off" time:
600 rpm : On 41.67 ms, Off 158.33 ms
2500 rpm : On 10 ms, Off 38 ms
4000 rpm : On 6.25 ms, Off 23.75 ms

And yes, the coil that I want to use is meant to be used for electronic ignition and because of the low resistance, I was expecting that the electronic ignition would operate at the same "On" time at all rpm... which I cannot do with points.

I have attached a chart that shows the stock point cam profile of a Shovelhead engine ('69 - '85). It is what they call a "Dual Fire" system, which is why it has 2 activating bumps.

You can see that the "On" time is not the same for both cylinders. I have not translated those degree numbers in ms but this profile is for a 4.5 Ohms coil, I'll verify that and perhaps I could start with the same and adding a 1 Ohm resistor in series with te 3 Ohm coil...

Would you know the lowest "On" time I could go to and still get firing at about 4000 rpm?

Thanks

Eb