Peter, obviously I agree with what you are saying here, as it is similar to what I said, but you included more detail. However, to stop there, IMO is stopping too short. One must include the self regulating aspect of the ballast resistor, as that is very important, and may be even more important than the starting aspect. There are ballast systems that do not bother to bypass during the starting mode, so the starting mode is clearly NOT their only function.
Tom
This is true in the general sense of “ballast systems”. For example, the ballast system in a florescent light system can be like this, and will contain capacitive and inductive components as well as resistive. However, as I understand it, the ballast in the 1970+ Series 2 ignition system is simply a series resistor, and it’s function is just what Peter described. Nothing more than a way of compensating for low battery voltage when cranking as far as I know.
David, that clearly is not what I believe to be true. Unfortunately, I am not sure where to find documentation on the Lucas system to confirm or deny. Do you have any to deny my understanding? I would be interested. I typically do not doubt much of what you say.
Tom
That’s the bit I’m not seeing.
Looks to me like the ballast resistor is only in the circuit while the starter is operating, then it does nothing once the starter is released (and hopefully the engine is running).
Exactly. If it’s not in the circuit it is doing nothing. [Edit] But in this case, (whoops!), it’s in the circuit EXCEPT when you are cranking, so it has the potential to do something other than drop the coil voltage when you are running, but as far as I know, that is it’s sole function…
You should be glad I sopped short Tom. I do not always stop when I reach the end of my knowledge, and the resultant mess can take some clearing up!
I loved playing with my Philips Electronic Engineer kits as a kid, but even around 8 or 9 I knew that having to ask mum or dad the colour of the resistor bands would be career-inhibiting, ruling me out of electronics success. My knowledge stopped at university first year physics. The way I read your extra info I couldn’t work out why the heat-dependent ‘coil regulatory’ aspect was the target rather than a side effect of the primary cranking purpose we both described.
Other way round George
[Edit] and David…
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Of course. I didn’t just get it wrong, I got it exactly wrong.
Tom,
This is the best documentation I could come up with. It is an excerpt of Jaguar TSB 1G8 dated May 1970, which describes the changes made to address some of the emissions regulations for MY1970.
Ballast.pdf (614.6 KB)
It describes the component as a “ballast resistor” (not a ballast network, or any other term that would suggest it is anything other than a simple resistor), provides it’s resistance value, and indicates that the coil used in the circuit must be used “in conjunction with the ballast resistor (or an equivalent resistance) in the circuit”. The only possible hint I see that the ballast circuit serves any other purpose than to compensate for voltage drop during cranking is in the initial statement in the Description which states “The purpose of the ballast ignition system is to provide maximum spark efficiency at high engine R.P.M. and assist engine starting under very cold conditions.” The italicized part (my italics) is the cranking part we’ve already discussed. What is meant by the “maximum spark efficiency at high engine R.P.M.”, I can only guess, and probably wrongly…
All,
Thank you all very much for all of your suggestions. Over all, I think that this discussion has been very valuable in learning more about our ignition systems and the ballast resistor.
In summary, as of right now my friends tach works great. It is very smooth, the needle no longer jumps up and down. When I accelerate past 20 mph, the tach needle holds beautifully.
With all of your post replies, it got me to thinking and rescanning the wiring diagrams. Going over all that I had done for the ignition system reinstall.
I then pulled out the one sheet of paper that Nisonger sent me along with the rebuilt tach. This sheet has several different instructions depending on your ignition setup.
As I re-read the instructions for using their upgraded tach with a standard points based distributor, I realized that the tach issue that I was experiencing was “me”. I had not followed their instructions! There instructions say to run a new white wire from the tach directly to the distributor side of the coil, not to the ballast!
So I removed the white wire from the ballast (which runs directly to the tach) and plugged it into the negative side of the the coil, the side that connects to the distributor.
Thanks again for everyone’s help!
Bob
This may, or may not, be helpful.
https://www.hotrod.com/articles/mopp-1110-ballast-resistor-guide-ballast-blast-off/
and
and
Thanks David, that is some of what I was hoping for.
Let me try some more specific information:
The term “ballast” in ballast resistor is used because the resistor is a “regulating” resistor. When the engine is turning at low speeds, the ignition points are together for a longer period of time. This allows more current to pass through the circuit, including the resistor. This increase in current causes the resistor to heat up which increases its resistance which then decreases the voltage/current to the ignition coil. The result is less current to the coil to keep it cooler and less current through the points to provide longer point life. (Previously I said I did not think that was the case, but checking I was wrong.). When the engine speed increases, the points are not closed as long, less current flows and the ballast resistor cools. Its resistance lowers allowing more voltage at the coil providing hotter spark at higher rpms. This is all independent of the starting bypass circuit that provides more voltage at the coil for starting, a circuit that is often but not always used.
Now one might think since point dwell is constant at all rpms, then average current would be the same at all rpms. But the ignition coil is not a resistor, it is an inductor. In the case of an inductor, the reactance (similar to resistance) of the coil changes with the frequency of the points opening and closing. My following description my not be technically correct, but it is the way I visualize it in my mind. The current in the coil “ramps up” after the points close. Let me try an example- not actual numbers. The points close, and for the first .001 seconds, let’s say X amount of electricity flows. Since it is an inductor, after .002 seconds, it is not 2X electricity flowing, but maybe 4X. After .003 seconds, it may be 10X. Therefore, if the points were closed for one pulse of .003 seconds, 10X has flowed. But if the points were close for 3 pulses of .001 seconds, only 3X would flow. Therefore more total current flows when the points open and close slowly than quickly, even though the total closed time (we call it dwell) is the same. Best I know.
John, the three sources you gave, while clearly not wrong, do not IMO get to the real reason a ballast system is used. Not that the term ballast is necessarily conclusive, but it is not simply called a dropping resistor, or a starting resistor, it it was named a ballast resistor for a reason.
Tom
Well, the basic equation for an inductor (lets just ignore the resistance for now) is V=L*(di/dt), so if V and L are constant when the points are closed the current will start at zero and increase in a linear fashion. So at T time it will be I, at 2T it will be 2I, and so on. So if the engine is running at low speed S the dwell will supply a time of the points closed of T, resulting in a final current of I. If the engine is running at twice the speed (2*S) the dwell will be T/2, the current will rise to a peak value of I/2. The average current will of course be a function of the on and off time of the points, but will stay fairly constant versus RPM, so a ratio less than one (not that this is really important to the conclusion).
So, this is a long way of saying that your conclusion is correct, the average current will drop as a function of engine RPM, but your middle paragraph is perhaps a bit suspect. They do make coils with internal ballast, where the connection between the coil and resistor is not brought out, so clearly not used for voltage compensation on start.
I will also note that the temperature coefficient for most resistors is fairly small, engineers tend to use different names than resistor for components with a large, predictable temperature coefficient (wirewound resistors are frequently on the order of 5ppm/degree C), so while I suspect this limiting characteristic you are talking about is there, I also suspect it is a second order effect. I can’t help but wonder if it has more to do with damping out the ring after the points open, getting the capacitor voltage back down to 12V before the points close (harder as RPM increases). But I would have to push a few equations around and break out the Mathcad to know for sure, and to be honest it is getting to late in the day for that. Switch mode power supplies are one of my areas of expertise, auto ignitions are not (although they are just a flyback power supply), so if someone with the expertise would like to chime in and clarify please do.
Nice thread gents. I’m learning new stuff, thanks.
If you guys get any cleverer, you’ll be telling me soon how to understand women!
Hi All…just to give a reason for the word Ballast…Dictionary meaning “to add stability” so either in a electrical Cct the word Ballast can be added to a resistor to clarify its use to add stability to current or voltage…or in a boat to make it stable…or in a building ballast added to sand/cement to make concrete
A ballast makes something heavier or increases the load on something. Structures, ships, balloons,… it also ‚loads‘ the circuit like the sand bags in a balloon do and as a result the voltage drops. And the voltage must drop so the coil doesn’t overheat, so the ballast is switched in and the voltage lowered to protect the coil in non-cranking situations.
Last night my wife got angry with me for spending too much time on this site. Something about how I should move to the garage and sleep with the Jag. She also made a comment about us all just being geeks. I defended us. Good thing I was not under oath at the time…
(She yells) “What are you doing in there? You’d better be looking at porn! Don’t let me catch you shopping for another car.”
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Geo,
Now that is good a good laugh!!!
My wife gave up a long time ago!
Bob
Hahahaha! That’s funny. I told my wife, it either the cars, or a mistress!, she likes the cars.