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Ignition Coil resistance question

Eb74

New Member
Hi to all,

I am modifying a point ignition system using a different coil.

Original coil has a 4.5 Ohms resistance and the new coil would have a 3 Ohms resistance.

With a 12 V power source, amps are increasing from 2.6 Amps to 4 Amps.

I am concerned with the coil overheating or the ignition point taking too much of a load. Another person has stated those as issues but did not provide anymore information.

I am considering adding a resistance in series with the coil to reduce amps…

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?)
  • Would adding a resistance have an impact on the spark quality?

For the sake of discussion, if anyone want to share information, what would determine the quality or intensity of the spark? Voltage or amps? Or both?

My guess is that it is a matter of energy, and so I think it would be more determined by current into the coil... which of course is the result of the voltage and resistance of the coil...
 

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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 :D They generally lasted about a week of normal use in the field - as the battery died, you started to lose revs.
 
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?)
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 electronic ignition, your reply is exactly what I was expecting.

I will also be designing 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 with an 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
 

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I recently put together an electric fence unit that uses a modern 'Waste-Spark' coil, IOW it has two HV plug connectors, with the fence unit, one is connected to the earth gnd connector,
BTW Condenser is an old term for what now is a Capacitor. Usually 0.1uF
But I see you are using the older points version, see here:-
 
For safety's sake, add a 2.2-ohm/ 2.7-ohm, 20W or higher series resistor between the coil and battery to reduce the chance of the coil driver burning out from additional coil current. Replace the coil unit, not the coil driver.
 
To MaxHeadRoom78 and Nigel:
Ah ah ah, I am not that old... I am Canadian French... and the french word for "capacitor" is "condensateur" and I translated it to much word for word... Capacitor indeed!

To Shokjok:
Yes, I will test a Ballast resistor in series.

To all:
I was thinking that perhaps I could design an ignition cam with less dwell angle than the factory and avoid the use of a resistor and then maybe get a stronger spark through higher current in the coil...

As Diver300 points out, the inductance will fight current increase just as it fights the current decrease that produces the spark... I had kinda forgotten about this "other" side of the inductance. This is why I asked about the Capacitor size... could I help the current increase through a bigger or smaller capacitor (Stock factory is 250 uF) ? Thus allowing me to reduce the ignition cam dwell angle and avoid the use of a resistance?

Eb
 
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To Lightium:

Thanks for the link!

In my first post I omitted the capacitor in the circuits and it is possible that I have the secondary of the coil wired wrongly. I find quite different secondary windings connnection when I search the web.

That aside, Stock Harley Davidson (HD) "Dual Fire" ignition system use a single ignition point and a single coil with dual outputs, one for each spark plug, they both always fire simultaneously. Stock HD coils are about 4.5 Ohms primary resistance. The cam that activates the point has 2 bumps in it, which I show in my second post. I have "mapped" that cam with a degree wheel and a dial caliper to determine the "On" and "Off" time of the ignition point.

I want to replace the 4.5 Ohms primary resistance coil with a 3 Ohms primary resistance coil. Knowing that the 3 Ohms coil will draw more current, I have 2 options 1) Put a 1.5 Ohms resitor in series with the primary or 2) Reduce the "On" time of the ignition cam to compensate for the additionnal current... but then, option 2 raises the question of having enough "On" time to produce a good spark...

Since I know that the 3 Ohms coil is made to work with an electronic ignition I expected that the "On" time of the electronic ignition would be controlled way differently than the "On" time of a point type ignition (controlled through tthe cam profile). So I am worried about the coil overheating and ignition points wear at idle and low rpm when dwell time is at its highest.

Now, it is possible that option 1 may take care of it all, I will try that one first.

I have made a test bench that can turn the ignition timer up to 2000 rpm (equivalent to an engine rpm of 4000) and I recently purchased an oscilloscope with a 10kX probe to read the spark voltage. I will also read the primary voltage peak to see what it comes up to.

I can machine a few different ignition point cam with different dwell angle to see if I could use the coil without a resistor and perhaps get a stronger spark.

This is why I was asking if the insertion of a resistor in series with the coil primary could hurt the spark quality, I was wondering if making an ignition point cam with different dwell angle would be an option to get a good spark, avoid overhating of the coil, avoid ignition point wear caused by the higher current and avoid the installation of a resistor.

Then Diver300 mentioned the impact of the inductance of the primary circuit on the current rise and that made me think of the capacitor size. I find that the current capacitor size does not prevent arcing at the point gap...

Eb
 
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Let's do the math. A coil has a turns ratio of 1:100. 240 volts * 100 = 24KV. Now for the current to have a good spark it's around 20mA, so you would need 2 amps at least on the primary. It gets more complicated then this, but it is late. One equation for voltage of the primary would be, V=H*(I/t). V=primary voltage, H=inductance of primary, I=current, and t=pulse time.
 
Thanks Lightium,
What do you mean by "Pulse Time"? Wavelength? Or voltage drop slope?

The coil primary is fed a square wave, the wavelength will change with engine speed but the slope should stay about the same, I think the capacitor should have an impact there.

I'll try different capacitors and see the impact on my scope...

I found this article:

Not sure the link will work so I joinded the file.

Eb
 

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