HE COIL This would be perhaps the least understood part of the system. A good deal of confusion surrounds the entire ignition system, and especially the subject of the polarity of ignition coils in cars with positive earth systems. Most cars these days have -ve earth, but our cars still have +ve earth. Coils these days are marked + and -. Conventional wisdom is that, in the case of positive earth, you should connect the + terminal to ground and the - to the live (negative) terminal of the battery. In this chapter I argue, with some diffidence, that the conventional wisdom is wrong, that a number of papers which I believe are poorly researched and misleading have been printed and disseminated, and, worse, believed, and that the errors have been reproduced and reprinted so many times that they have acquired the patina of truth. I am aware of the dangers of contradicting the dominant paradigm, because "... in the multitude of counsellors there is safety" (Proverbs 11:14 KJV). On the other hand, "The simple believeth every word; but the prudent man looketh well to his going" (Proverbs 14:15 KJV). Moreover, "There is nothing so absurd that it cannot be believed as truth if repeated often enough" (William James 1842-1910), and "The fact that an opinion has been widely held is no evidence whatever that it is not utterly absurd; indeed in view of the silliness of the majority of mankind, a widely spread belief is more likely to be foolish than sensible" (Bertrand Russell, 1872-1970). These philosophical insights have been verified experimentally in scholarly works: have a look at Solomon Asch's work on conformity, and Hasher and Goldstein's work on how the repetition of more or less plausible statements affects one's judgement of their truth and validity.
So I thought I would look at the whole issue again from first principles.
Long ago, when I first when I first had to come to grips with the mysteries of these ignition systems, some cars had positive earth systems and some had negative. All coils were marked SW and CB: SW went to the battery via the ignition switch, and CB went to earth via the contact breakers. Easy. They didn't have separate coils for +ve or -ve earth, and it didn't matter whether you had positive or negative earth: you bought your coil, and you wired SW to battery via the switch, and CB to earth via the contact breaker. If you had a positive earth, the current in the primary went one way, and if you had negative earth, it went the other way. It didn't matter. What did matter was how the internal windings of the coil were connected relative to the battery and contact breakers.
Nowadays, although some cars, mostly British classics like MGs, still have positive earth, almost all cars have negative earth, and instead of ignition switches and contact breakers they have things called engine management systems. If the coils were still marked SW and CB, the engineers wouldn't know how to connect them, so to make it easy for them modern coils are simply marked + and -. All the manufacturers have done is replace the SW symbol with +, and the CB with -. This has lead to the confusion that exists for those of us who still have cars with positive earth (see eg mgaguru's 2009 paper and about 1000 others). When I went to buy my new coil, I was told by everyone either that it didn't matter how it was connected, or that the - terminal should go to the negative battery terminal via the ignition switch and the + terminal should go to ground via the contact breaker. Unanimously. But they were wrong: if it didn't matter, the manufacturers wouldn't have bothered to mark the coils at all, either with + and -, or with SW and CB. There is clearly a difference, and you can see the difference by drawing the wiring inside and outside the coil, and consider how it would look if it were connected firstly - to earth, and then + to earth.
The coil is a step-up transformer. It consists of a laminated iron core surrounded by two coils of copper wire wound in series. The primary winding has relatively few turns of heavy wire. The secondary winding consists of thousands of turns of smaller wire, with a turns ratio of perhaps 100:1. The two coils are wound in the same direction, and the top end of the primary and bottom end of the seondary are connected internally and brought out to the - terminal. The windings are insulated from the high voltage by enamel on the wires and layers of oiled paper insulation. The entire assembly is usually sealed in a metal can with insulated terminals for the high voltage and low voltage connections. The can itself isn't part of the circuit, and although it's usually grounded by its fixing, this is not necessary. One of the low-tension leads is connected to the battery via the ignition switch, and the other to the contact breaker points in the distributor - which lead goes where is a vexed matter for those of us with positive earth cars.
In this diagram below, the battery is marked + and - in red; the + and - in black refer to the markings on the coil, and the red arrows show the direction of conventional current in the primary. The first diagram below shows what most people will recognise: the battery connected -ve to earth, the + terminal on the coil connected to the ignition switch and the - terminal connected to the contact breaker points.The second figure shows how it looks with the battery reversed to give a +ve earth, but the coil still connected with its + terminal connected to the ignition switch; and the final figure shows the coil also reversed, with its - terminal connected to the ignition switch.
Consider the "conventional" case first, with negative earth, and the + symbol on the can connected to the positive battery terminal, via the ignition switch. While the contact breaker points are closed, current flows from the battery through the primary winding of the ignition coil. This current builds up relatively slowly, due to the inductance of the coil, and builds up a magnetic field in the core and in the air surrounding the core. At some stage, once the primary current has built up to its full level, the contact breaker points open and the current is interrupted, and as we've seen the timing of this interruption is critical. The magnetic field collapses rapidly, thanks to the capacitor, and generates a back emf in the primary of about 300V, that is to say the voltage across the now open points increases rapidly to 300V. But here's the thing: because the magnetic field is collapsing, not expanding, it cuts the the primary coil in the opposite sense, and the back emf is negative, not positive. Because the turns ratio in the coil is about 100:1, it generates a much bigger voltage at the secondary - about -30kV (30,000 volts) (source: Wikipedia). And something else: because the secondary winding is wound in the same direction as the primary, the voltage at the primary is added to the 30kV on the secondary. But because the turns ratio is about 100:1, the voltage increase is only about 1% - not really material.
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