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Hey quickdraw, I am going to set you straight on electrics and such. Coils and batteries use current not voltage to operate. Batteries charge from how much current is driven through them not how much voltage appears across the terminals. If your battery was open you could put a hundred volts across it and it would draw no current. The same thing applies to the ignition coil, it works on current. The reason why the manufacturer does not want the coil to have more than about 13.8 volts across it is because of the enormous step up in the secondary. The manufacturer has determined that if more than 13.8 volts is placed across the primary of your coil, when the secondary charges to the high voltage, it will break down the secondary side insulation and arc internally. (Not desirable) What the manufacturer should have told you is how much primary current is allowed and for how long. Unfortunately, most people think in terms of voltage and not current so the manufacturers have taken to specifying these things in terms of battery voltage and the ballast resistor value. If the voltage and resistance are known then the coil current can be calculated using ohms law. The primary side of the ignition coil has low impedance. (low ohms) so if you place the battery voltage directly across the primary of the coil it will draw too much current and the coil will saturate. This is where the ballast resistor comes in. This is getting kinda technical, (forgive me, I am an EE) but during the time that the points are closed, current flows into the primary and stores energy in a magnetic field inside the coil. The current linearly ramps from zero amps up to some value determined by the dwell angle and the ballast resistor. It is like pouring water in a bucket. When the points close, the bucket begins to fill with water. If the flow of water is too fast or the filling time is too long, the bucket will overflow before the points open. This is what happens inside the coil. When the points close, current begins to flow in the primary side of the coil and it stores its energy as magnetic field inside the core of the coil. This field has limits, too much current or too long a dwell and the core will saturate. When the core saturates, the excess current is turned into heat and therefore heats up the ignition coil. The ballast resistor "limits" the current to the primary side of the coil and prevents saturation. It really makes no difference what kind of coil you have, 6 or 12 volts. The only difference is that a 6-volt coil has half the primary resistance of a 12 volt coil. It has nothing to do with voltage. The ballast resistor is sized for the coil at a given battery voltage and a given dwell time. Here is a practical example: Assume the following: the primary side DC resistance of the ignition coil is .5 ohms, and the dwell angle is say 18 degrees and the battery voltage is 6 volts. Lets say the engine RPM is idling at 500 rpm. OK, the coil has to charge and spark 4 times per rotation of the camshaft or every 90 degrees. So 1 revolution at 500 rpm takes 120 milliseconds. Divide this by 4 and each firing cycle takes 30 milliseconds. With 18 degrees of dwell, this says that the coil charging time is (18 degrees / 90 degrees)*30ms or 6 milliseconds. So for the first 6 milliseconds of the cycle, the points are closed and current flows in the primary of the coil. Next, the points open, the field in the primary collapses and induces a large voltage in the secondary side of the coil and ZAP. Now for the remainder of the cycle, the points are open and the coil is COOLING off. The ballast resistor comes in because if you connected the coil across the battery without the resistor, then the current in the primary side would be simply (6 volts / .5 ohms) or 12 amps. WAY TOO MUCH. The ballast resistor limits the current to about 5 amps. For a 6 volt system, the resistor value would be about .6 ohms given a coil primary resistance of .5 ohms. Now lets run ohms law again with the ballast resistor. (6 volts / (.6 ohms +.5 ohms)) this reduces to (6 / 1.1) or 5.4 amps OK. With this setup, for 6 ms the current in the primary side will ramp up to 5.4 amps, then the points will open, the spark will occur and then for the next 24 milliseconds, the coil is resting with no current flowing in the primary. So why not just design a coil so the primary resistance is high enough that no ballast resistor is needed? Well, the turn’s ratio drives it. If they put lots of turns on the primary side of the coil so that its DC resistance was say 3 ohms then yes, you could eliminate the ballast resistor from the circuit with no problem. The issue is that if you double the number of turns on the primary, you have to double the turns on the secondary. A typical coil ratio is something like 5,000 to 1 meaning that there are 5,000 turns on the secondary for each turn of wire on the primary. You can see that if the primary side was set up to draw the right amount of current, then the secondary would have to have a huge number of turns on it to achieve the desired step up of the voltage. Back in the old days, this was impractical to manufacture so they decided to limit the current by adding loss to the primary side of the circuit. Also, the lower the primary current is, the less current has to flow through the points and that is a good thing. So to summarize, The Ballast resistor sets the current in the primary side of the ignition coil. If a “6-volt” coil uses a 0.6-ohm ballast resistor, that same coil can be used in a 12 volt system with a 1.7 ohm ballast resistor. The coil will still have the 5.4 amps in the primary and the ballast resistor would get larger to drop more of the battery voltage across it. Instead of the .6 ohm resistor dropping 2.6 volts across it, the 1.7 ohm resistor would drop 9.3 volts across it and keep the coil happy. A “12-volt” coil has twice the primary DC resistance as a “6-volt” one. There are other considerations having to do with the inductance of the coils and dwell time getting smaller as the rpm’s go up but that is beyond the scope of this discussion. I hope this helps everyone out there that has a smoking ignition coil or smoldering ballast resistors.
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