If we dive a bit deeper into it, the purpose of the ballast resistor is to reduce the time it takes for the coil to be able to produce a strong spark, i.e. make the ignition coil work good enough at higher rpm (without using too much current at low rpm). Looking at this we see an example (electronic) ignition coil current in blue (and voltage in red), the current starts low and relatively slowly rises (milliseconds, but still slow in the electric world). If we wait long enough it would max out so the resistance decides the current, i.e. a 3 ohm coil would get 12V/3ohm=4A eventually, the problem is that we have more cylinders to fire, we can't wait - especially not at high rpm. This kind of "slow" current rise exists in EVERYTHING that creates electromagnetism, it takes time to change the magnetic field, and the current will "wait" for the magnetic field. The DC resistance may be low, but the impedance; resistance that effects AC or other rapidly changing current is much, much higher. So, even if your meter says that the coil has a 3 ohm resistance, the initial current may look more like it was 15 ohms. The problem is that if we make a ignition coil that works well at high rpm with many cylinders it will have to have a low resistance, it will work good enough at high rpm but let the engine come down to idle or cruising rpm and the current will be much higher than we want - overloading the points, overheating the coil and so on. Enter the ballast resistor, along with a low voltage ignition coil - say something more like a 3V coil in a 6V system or a 6V coil in a 12V system (it's not exact, but you get the idea). If we take a 1.5 ohm coil and a 1.5 ohm ballast resistor they would eventually share the voltage equally - on a 12V system they'd each have 6V, which would be okay for a coil made for about 6V and the current would still max out at 4A so the points are happy too. But, remember impedance? Assume the impedance causes the coil to initially behave as if it's 15 ohms, in series with a 1.5 ohm resistor most of the total voltage would be on the coil, and less than 10% over the ballast resistor. So, we have a "6V" coil that wants to start at a very low current and increase it slowly, we don't have the time to wait so we smack it in the head with nearly 11V making almost twice as much current flow initially. As the current rises the ballast resistor takes more of the voltage, but we have shortened the time it takes for the current to rise high enough to make a strong spark so it works much better at high rpm, and we still have a sensibly low max current for low rpm or engine off with ignition on & points closed. And by now all of you have probably fallen asleep, as this wasn't anywhere near what you wanted to know about ballast resistors... Oh well.
You may not have had any interest in electricity but without it you would be riding a bicycle or a skateboard.
I think for simplicity just run the S terminal directly to the coil, the R terminal to the ballast then to the same spot on the coil Pretty much how things were done for a long time
So, we've heard a lot of theory. Now does anyone have any practical advice to OP as he described his concern in post #1?
True, but don't run the ballast resistor and see how long the points last. The lower voltage helps. If the condenser prevented arcing, you wouldn't have any point pitting. Ballast resistors are a bit misunderstood. Yes, it's there to reduce voltage/current to the coil but it's really a band-aid to fix a different problem and that problem is dwell time. G-son got into the weeds a bit, I'll try to simplify it... It takes a specific, finite amount of time for the coil to 'charge' and reach 'saturation'. Once saturation is reached, any additional power into the coil sees in effect a dead short, causing heating to occur in the coil. Don't address it, you get a dead coil. Dwell time is the amount of time the coil is energized, i.e the points are closed. Dwell is defined in degrees (typically around 28-30 in a V8, give or take a few either way), not time. So to quantify it, we need to figure out a way to have an idea of the time intervals involved. In a V8 idling at 500 rpm, the points open/close 2000 times. So 2000 times in a minute means the points open/close every .03 second (60/2000 = .03). Increase the engine speed to 5000 rpm, now it's 20000 times and only .003 second, or ten times shorter. So the voltage/current that's adequate to produce a good spark at 5000 rpm, at idle is nine times too long. Compromises are made; in addition to the ballast resistor, the coil is beefed up to withstand higher temps/currents. Impedance enters into this, but there's other factors that affect that. If you drove at 5000 rpm all the time, you wouldn't need the ballast resistor... LOL Nearly all modern electronic ignitions now feature dwell control; they limit the duration of the power pulse to the coil irrespective of engine speed, and the coils are usually much smaller.
Mind . . . BLOWN! Can't say that I'd ever considered that before. This pretty much completely reverses my concept of the cause and effect of how resistors work. I just hope I can remember this the next time I try to measure a voltage drop in an open circuit. If you're new to troubleshooting older ignition systems and want to read up a bit on how they work you might want to look this over. https://www.jalopyjournal.com/forum/threads/hot-rod-technical-library-basic-ignition-systems.983424/
Yes. Old time mechanics call them condensers, old time radio and TV repair folks call them capacitors. I use both names, depends on who I am talking to. As a 76 year old, period correct vehicles have a condenser in the points distributor and electrolytic capacitors in the vacuum tube radio. This is similar to the engine vs motor debate (General Engine Corporation, Honda Engine Company). Get your popcorn ready, the anal-compulsive will arrive shortly to debate the issue... Russ
Yes, I agree; the reduced potential between the contacts will have less tendency to arc. I think Squirrel already did that in post #15
I just got back to this. I made sure the points were closed. With about 12.6 volts on the ignition switch side, I only get about 3.5 volts on the coil side of the resistor. That doesn't seem like enough to me. Should I not be seeing 8-9 volts? I'm getting a yellow spark, not the strong blue spark I was expecting. Because of the low voltage?? Now what? Wrong BR? What BR do I need? If you can't tell, I am lost.
Just go to the parts store and buy a ballast resister for a Chevy C-10. A stock coil and you are golden. You can spend hours thinking and rethinking it or just get something that you know works and think about something more profound like what color t shirt to wear with your stone washed blue jeans.
With 31% of the total resistance (0.9 of 2.9) in the coil you'll get 31% of the total voltage on the coil. Can't tell you what voltage is normal in those systems, but I can say that the voltage you see makes perfect sense with a 2 ohm ballast and 0.9 ohm coil. Porknbeaners suggestion to use things known to work well together is a good one. Mixing & matching things of unknown origins may or may not work.
Yeah, I figured that is what I had or something real close. The BR and the coil came out of my stash. They had to be from some stock sbc application because I never have messed with anything but old sbc stuff until very recently. Not sure where the coil came from, but I'm pretty sure the BR came from a '55 Chevy. This coil and BR has a lot of miles in this car before I tried, unsuccessfully, to go to an electronic system. I just re-installed the points system as it was previous to my failed experiment. It's running so I am just curious. Maybe it could run better with the proper voltage to the coil. But I don't know what the proper voltage is or how to get there. Sometimes I just like to know the why's and wherefores of what I am working on.
I am going from memory here but if I recall correctly your target is 9V to the points. You want to find a coil that reads "Ballest resistor required" on the side of it. A lot of the flathead guys use a GM coil that does not require a ballest resister. It keep the firewall cleaner.
Ohms law predicted this in post #19. I try to match the primary resistance of the coil I am using with the resistance of the ballast. Ideally I like 2 or 3 ohms each, and prefer 3. It keeps the circuit current to just over 2 amps so points last a bit longer.
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