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ohms on coils
HAL 1 - 30/4/07 at 01:16 PM

Have just changed my electronic dizzy back to points system because of unsolvable problems, I'm now back to a basic ignition system.
The car has a 2ltr pinto engine
The coil ohms it should use for electronic are
Primary 0.72-0.88
secondary 4500-7000

The ohms for a points set up with ballast resistor are
Primary 0.95-1.60
secondary 5000-9000

Can anyone explain what these figures mean and what I should expect if for example i stick with the electronic coil on the points set up ?
these figures are all from Haynes manuals, sierra and cortina

BTW when i ran it for a while the ohms figure went up a few notches

hope all this makes sense to someone.

flak monkey - 30/4/07 at 01:27 PM

If you use an electronic coil on a points dizzy, you will destroy a set of points very quickly indeed.

Basically a points dizzy coil will give you less voltage than one designed to be used with an electronic one.

If you stick a multimeter on the coil, the primary and secondary coils should lie in the resistance ranges you have stated.

Coils work on the principal that a magnetic field builds up in the primary coil as there is 12v across it. When the distributor switches off this current (points open or equivalent), the collapsing magnetic field in the primary coil, induces a voltage in the secondary coil, which is sent via the dizzy to the spark plugs. The amount of voltage induced is basically down to the ratio of the numbers of turnss in the primary and secondary coils.

David

turbodisplay - 30/4/07 at 05:05 PM

Maplin used to do an electronic ignition coil driver.
It has a small current that flows through the points, and will drive a higher current using a solid state switch (transistor).
It will provide more current to the coil.


The reason that electronic coils have low resistance is that the current is limited by the dwell (coil charge) time being altered to suit the rpm. Points do not have this ability.

LINK:
http://www.maplin.co.uk/Search.aspx?criteria=electronic %20ignition&source=15&SD=Y

HAL 1 - 30/4/07 at 06:20 PM

Just been on howstuff works site and figured out that the higher the number the more resistance i:e 0.95 - 1.60 will give more resistance than 0.72 -0.88 ( i think )

Does this make sense, I will need more resistance through the coil on start up given the fact that i'm bypassing the ballast until running.

Or am i going mad !!

Might take a closer look at Maplins once i'm running ok

caber - 30/4/07 at 06:28 PM

I had a Maplin's unit on my Land Rover properly built in a metal bow with heat sink on the outside and it blew up in a big way, lots of components off the board, after I had used it for about 18 months. changed for Aldon Ignitor a much more useful bit of kit, all fits in the dizzy and has magnetic pick up no more points!

Caber

MikeRJ - 1/5/07 at 11:37 AM

quote:
Originally posted by HAL 1
Does this make sense, I will need more resistance through the coil on start up given the fact that i'm bypassing the ballast until running.


The subject of coils and resistance is quite complex, because a coil is also an inductor which is an energy storage device. The amount of energy stored (and hence spark energy) depends on the current through the coil and it's inductance. However, when you apply a voltage to an inductor, the current doesn't immediately rise to a maximum value, it takes a while to build up because a magnetic field is formed around the coil which oposses the current flow. The length of time it takes to build up to the maximum current is determined by the applied voltage, the resistance of the coil and the inductance of the coil.

Once the coil is fully charged, the maximum current that will flow through the coil is determined only by the applied voltage and the coils resistance.

With an electronic ignition, the system can control the length of time a voltage is applied to charge up the coil (known as dwell period), and can thefore limit the peak current to below the value you would get simply from using ohms law on the coil resistance. An electronic coil can therefore have a lower resisatnce which enables the coil to be charged up more quickly (to get full spark energy at high RPM), but without the coil overheating due to excessive current flow.

A points system has a fixed dwell angle, which means the dwell time (which is what the coil cares about) varies with engine speed. At low RPM the points will be for closed long enough that the coil will be fully charged, and current will be limited only by the coils resistance. If order to limit this current to prevent coil overheating and points burning, the resistance of the coil has to be higher.

The ballast in a points system is simply another resistor, which is added in series with the coil resistance. A coil designed for e.g. 6volt operation will have a much lower resistance than one designed for 12 volts, and using in a 12v system would cause it to rapidly burn out. By adding an external resistor (the ballast), the peak current can be limited to a safe value. During starting when the voltage on the battery is reduced, the ballast is switched out of circuit to allow the correct amount of current to build up through the coil:

Ohms law says: I=V/R

where:

I=Current
V=Voltage
R=Resiatnce

You can see that dropping the voltage lowers the resulting current, but if the resisatance is also lowered, the current can be restored to it's original value.

HAL 1 - 1/5/07 at 05:40 PM

Went and bought a coil for a cortina today, got home, tested it for the ohms only to find that it was 0.80 which isn't what the book recommends, the motor factors said that it was the one listed for cortinas, took it back and got refund.
The manual states that a coil of the 7 volt type be used, are these readily available ?
I'm now thinking that i should see a specialist about this as it's getting on my nerves now.
Anyone out there know who i can deal with ?

ed_crouch - 12/8/07 at 09:12 PM

Mike,

Thanks for this explanation: its the only time ive ever heard a decent explanation of coil charge time!

I hate not understanding stuff: thats another thing I can cross off the list...

Ed.