First, What’s a Coil
and a little explanation on what a coil is;
It is two lengths of wire wrapped around a metallic core. The wires are insulated from each other and the core. If the wires are the same length then it would be 1:1 and 10 volts in would get you 10 volts out. To get 40,000 volts out when you have 10 volts in, the output wire (secondary) would have to be 4000 times longer than the input.
That's basically a automotive coil. The two small screw terminals are each end of the primary winding. The case and the coil wire terminal are each end of the secondary and is approximately 4000 times longer than the primary.
As I noted the two wires are insulated from each other so how does power on one get onto the other?
Well when current is flowing through a wire it creates a magnetic field around the length of the wire (wrap a piece of wire around a nail and hook it up to a battery and you have an electro magnet).
If you recall the secondary wire is wrapped with the primary around the core so this magnetic field envelopes the secondary winding too.
There is stuff happening down at the atomic level I won't get into but as that magnetic field is created by applying voltage or collapses by taking the voltage away a voltage that is equivalent to the turns ratio develops on the secondary winding. I.E., 10 volts in 40,000 volts out.
So why do we need a ballast resistor?
It takes a certain amount of time for this magnetic field to build up (points closed or transistor equivalent in ecm). You have to wait long enough so the secondary windings are fully saturated to get full output. The amount of time available varies with rpm (more at low less at high).
So less time results in less voltage or lower output at high rpm's, just the opposite of what you want.
So your coil is designed to produce adequate voltage at high rpm's but at low rpm's (or when the engine is off and the points are closed) current flows for to much time and can over heat and damage the coil. This is where the ballast resistor comes in.
The ballast resistor is a variable resistor. As it heats up from current flow the resistance goes up and limits the amount of current. That is why you read a lower voltage at the coil at idle. At higher rpms when the current flow does not have a chance to heat up the ballast resistor the resistance is low and you have higher voltage to the coil and more current flow to give max output.
So in a nut shell the coil is designed to give max performance at operating rpms but would be over heated and damaged at low rpms. The ballast resistor will limit the current flow at low rpms to protect the coil from over heating and allow for max current at operating rpm's

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Rafa

Hecho en México

WOW thanks for the explanation!!

Great explanation, best I ever heard.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 225 nitrous
64 Valiant Signet
64 Valiant 4dr 170

That's an interesting explanation.

My understanding always was that the ballast resistor was designed to safeguard the car's ability to provide spark during demanding cranking situations with reduced battery voltage, by adding a power resistor that could be bypassed during cranking. There are some British cars from the same era that had coils wound for constant 12V operation that did not have this ability and did not start well under these conditions.

I can certainly see the logic in what you are saying. Wouldnt the ballast resistor be better called a "thermistor" or "thyristor"? I thought they had a fixed resistance value.

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

A thyristor is a solid state silicon device aka SCR often used in AC circuits. A thermistor is a low current device that changes resistance at different temperatures, often used in temperature probes.

The ballast resistor is a high current resistor. Any resistor changes its value when it gets hot, but this change can be either maximized (such as desired when making a thermistor) or minimized during its fabrication.

I'm not sure about a ballast resistors heat properties in relation to car ignition, but I always just thought of it as it provided hotter spark during startup for a short amount of time but lower during running conditions so the coil won't overheat.

Do modern cars even use ballasts any more? I haven't seen one on the handful I worked on. I got rid of mine as well when I changed over to an msd-6a and blaster2 coil. I know that deffinately provides for one hot spark when cranking (don't ask me how I know).

actually an SCR is a Silicon Controlled Rectifier, basically a gated diode that turns on with a small voltage applied to the gate, that switches off either by current reversal or interruption.

I cant remember what a thyristor does, but I remember they are often installed in the AC line coming into small appliances, for lightning protection.

What were we talking about again?

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

...greek

D/W

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If it ain't broke, fix it!

Don't mean to harp, but a thyristor is an scr.
http://users.deec.ist.utl.pt/~ineit/imd ... ossary.htm towards the bottom

Back to ballasts. Do modern cars just use an msd-style coil that doesn't need one? Many interesting ignition setups today, from ford dis to what I once saw on a Benz I think where they had one coil directly over the sparkplug (no sparkplug wire, just a small gauge wire to fire off the individual coils).

Most all cars, today, have COP (coil on plug). In between the older electronic ignitions and the COP, most cars used the electronics in the distributor, or the engine computer to control "dwell" and limit current through the coil.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 225 nitrous
64 Valiant Signet
64 Valiant 4dr 170

yup, youre right, dont know how I got it into my head, but thyristor and SCR are indeed one and the same.

I think the reason modern coils do not require a ballast is that they sit in an epoxy substrate rather than oil and do not have the overheat issues that the old ones did. Some of the later conventional coils had integral resistors and did not use an external ballast.

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.