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MG TD TF 1500 - Ignition coils

I have a question about ignition coils that I wonder if people can answer, and it is one that I thought I already knew the answer. I always assumed that coils functioned by applying (in our case) 12 volts across the primary windings, plus the points in the distributor, and then interrupting this supply by opening the points as the engine rotated. This (rapid) interruption instantly generates a high electrical potential difference (voltage) across the secondary windings, which is transferred through the HT leads to the spark plugs via the distributor rotor arm, and thus each spark plug fires at the appropriate time.
However, I have suddenly realised that the wiring diagram shows one end of the secondary windings in the coil connected to the central HT connection, but the other end is actually connected to the CB terminal. This is the same terminal that one end of the primary windings are connected to, the other being the SW terminal. Thus no end of the secondary windings goes to earth, and this is easily confirmed by using a test meter. This means that the secondary windings do not appear to have a closed circuit that includes the spark plugs; so how on earth do the plugs fire? The closed circuit cannot include the wire going from the CB terminal to the distributor as it is too thin, and in any case the points have just opened. Saying that the HT is going to earth through the spark plugs does not seem to be correct, as the engine block does not appear to form part of the secondary circuit. I am aware that electricity is a flow of electrons, and that there must be a closed circuit for that flow to occur.
A further question is that the HT end of the secondary windings is also connected to the core of the coil; why is this necessary?
The above layout applies to most cars before electronic ingition, so why does the system work; what have I missed? Answers on a postcard........
Roger Wilson

The "coil" is an auto transformer (one lead of the primary and the secondary are connected together.
the high voltage is induced in the HT coils by the collapsing magnetic field of the primary when the points open. The system is a sort of AC not DC circuit so your DC concepts don't apply. The condenser across the points acts in part to complete the HT circuit as does the path back thru the primary, SW, battery to ground (lo impedence path). In addition the Capacitor, inductance of the primary, and the wire resistance act as a tuned high frequency LRC circuit to enhance the induced HV for the spark.
Don Harmer

To Roger Wilson, The thing you are missing is that the return for the secondary winding is through the primary and has the 12 volts of the battery added to the secondary voltage (can not return through the contact breaker points because they are open at that instant in time). I have made some oscillographic studies of the voltage across the points and condenser (capacitor) while the distributor is running,they are supprising. Let me know if the results would be of interest to you.
Bob Jeffers

Hello Don. Great to talk to you at Sebring. I wish I had physics profs that explained things as well as you did! George
George Butz

To Don and Bob, many thanks for answering my query, and Bob, I would be very interested in seeing the oscilloscope studies.
Anyway, your answers have lead on to a few other thoughts. If the secondary windings generate 15000 to 20000 volts, then adding the 12 volts across the primary seems hardly worth the effort, especially as it means that the secondary circuit then includes the ignition switch; why doesn't it get (very) hot. Each plug fires for only an instant, but they are firing all the time, for as long as the engine is running. Also, the rest of the secondary circuit would then include ordinary wiring, so why is special HT cable needed from the coil to the spark plugs; would ordinary wire be OK (not that I plan to change the wires on my car).
Thus; would the system work better if the coil secondary windings were NOT connected to the CB terminal (or the primary), but to the coil body, and so to earth (assuming the coil was bolted down securely). Would this arrangement mean that the condenser across the points could be eliminated. This condenser apparently has a rating of 1000 volts, and I have sometimes wondered why, when only 12 volts are across the points.
I do appreciate people's replies.
Roger Wilson

Hi Roger, You are asking the same question I asked myself when I set out to find out what the difference was between the original 12,000 volt coil and all the other higher voltage coils. I'll just give a verble description of the results since I'm not smart enough to set up links to pictures yet. First off, the reason for 1000 volt capacitor ratings. The voltage across the points right after they open is 600 volts peak-to-peak when the coil is open circuit, 400 Vp-p when the coils is loaded with a spark plug. The real interesting measurment was the voltage across the spark plug, only 1000 to 1200 volts. At the moment I don't recall the open circuit voltage from the spark coil when there was no spark plug attached. But it was of course signifigantly more, approaching the 12,000 volt specification of the original Q-12 Lucas coil. The reason the high voltage secondary is returned through the primary escapes me, I would have thought that it would be connected to the metalic case of the coil and therefore grounded to the chassis. Maybe, just maybe, that connection was done that way in case there was too good a coat of paint on the housing or the bulkhead and there was not a good connection from case to ground (earth).

The results of my tests will be the subject of an article in the TSO soon. I have to retake several of the oscillographs because they did not have a spark plug load on when the picture was taken.
Regards, Bob
Bob Jeffers

Bob, apologies for delayed reply, but I only check the internet at work, and although I saw your comments yesterday, I thought I'd think about them overnight. Anyway, I also was curious about higher voltage coils, and borrowed from my local library some nineteen sixties books (they still have them) about car electrics. Unfortunately, most books don't mention higher voltage coils, and those that do simply say that they exist, and are suitable for higher revving engines. The books do not give any technical details of these coils, but I assume that the coils simply contain more secondary turns.
I was also interested to see what the books said about the car HT system, but that was a disappointment. Most of them only show a simplified picture of a coil with separate primary and secondary windings, and some mention how the coils are actually constructed, but all fail to explain how the system really works. Anyway, obtaining only 1000 to 1200 volts across the spark plugs is a puzzle, I wonder where the rest of the 12000 volts is lost. The 600 volts across the points must come from the secondary; after all, the points are connected via a contorted circuit across the secondary. Also, there can still be a high voltage across the points even though there is no current flow (all the electrons flow into the capacitor and then out again).
I did think of asking the Technical Dept of Lucas, but the firm doesn't exist anymore. It was bought out in 1999 by TRW, who seem to have dropped the Lucas name, and don't appear to deal in car electrics anymore.
Best regards,
Roger.
Roger Wilson

Roger, No need for apologies, I like to think things over before I get foot-in-mouth desaese too.

First the 600 volts across the points, that is with the secondary open circuited therefore it is due to the inductance of the primary alone, (and it's collapsing magnetic field). There is no contribution from the secondary. As a matter of fact you can get quite a shock by putting your fingers across the points,(the 600 volts).

Next where did the 12,000 volts go? Well my notes say the the secondary has 26.9 Henrys inductance, to get that much inductance they had to wind on a lot of turns. They got 5,190 ohms of DC resistance in the secondary winding. The current flowing in the arc drops most of the voltage across the 5,190 ohms, some of the voltage is lost in the arc between the rotor and the cylinder peg in the dist. cap. ( I haven't tried to measure that yet). The primary has 11.33 millihenrys inductance. The DC resistance of the primary is 4.39 ohms. The turns ratio equals the voltage ratio. The turns ratio is the square root of the quantity of the secondary inductance divided by the primary inductance. For the original TD coil this came out to be 48.7!! For the TF coil (LA-12) this came out to be 70.9. The SP-12 Sport Coil came out to be 89.1.

Just to give you some sleepless nights as you try to assimilate all this, pertronix does not use a capacitor across their electronic points. They use a diode, works just as good, is more reliable, and a whole lot smaller. And I suspect cheaper in the quantities they buy.
Enough for now, Bob
Bob Jeffers

Bob, many thanks for reply, which I am still mulling over. I am still uneasy about why the system wouldn't work better if the secondary were connected to "earth" (rather than to the CB terminal), but I accept that my TD starts when I turn the key!
Best regards, Roger.
Roger Wilson

This thread was discussed between 14/04/2005 and 22/04/2005

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