Technical Ballast resistors and points

I'm trying to understand how I fixed a problem I used to have with my points ignition setups. I run the big Accel super coils on my junk. They have 0.7 ohms of primary resistance. I realized later that on a points system you're supposed to use a second ballast resistor so that you don't burn your points. I learned this the hard way. After burning up several sets with a single ballast, I started adding a second parallel condenser to the system, still using the single ballast resistor. I left the stock condenser in the distributor and would attach the second directly to the negative side of the coil. This cured my problems. I'm trying to figure out if this is actually a better method than the recommended double ballast resistors. It's getting more power into the coil with less ballast resistance, so I would imagine a hotter spark, yet it's no longer burning my points. Did I accidentally do something intelligent or is the spark actually weaker now with too much condenser?

 

ballast resistors are for coil voltage reduction, 12 down to 7 for a 7 volt coil (7 or whatever they are) condensors are for points arcing reduction. So knowing that , what is it you did?

 

I'd say he done good.

 

To understand an explanation, you probably need to understand the difference between resistance, and impedance. The ballast resistor has to match the resistance of the coil. The condenser needs to match the impedance of the coil.

Resistance is more of a "static" thing. Impedance is more of a "dynamic" thing.

I've never heard of anyone doing what you did to improve point life, but it makes sense, if the impedance of the coil is different from that of a stock coil. Neat.

 

I usually hear that a 1.5ohm coil needs a 1.5 ohm resistor, but with the super coil it's confusing. It's a 0.7ohm resistance coil, yet Accel states that you need a 0.7-0.8 ohm resistor on top of the factory ballast. So they are going against that general rule, ending up with probably 2 ohms of total ballast resistance for a 0.7 ohm coil. Unless you are supposed to always end up with around 3 ohms total, including the coil?
The way I've thought of it, the resistor keeps the coil from overheating, and the condenser "cushions" the power spikes caused by the points opening and closing. With electrical stuff I usually fumble around until something works, now I'm trying to actually learn what's actually going on...
I'm thinking of getting one of those adjustable spark checkers, and seeing if the spark can jump a bigger gap the way I have it vs the way you're supposed to set it up. I suppose that will really show if it's making a difference. As far as I know I haven't overheated the coil, I drove 4000 miles through 8 states in 3 weeks with it like that at one point, without any issues.

 

>>>I'm trying to actually learn what's actually going on...>>>

>>>the condenser "cushions" the power spikes caused by the points opening and closing.>>>

>>>I haven't overheated the coil, I drove 4000 miles through 8 states in 3 weeks with it like that at one point, without any issues. >>>


While you apparently don't speak nerd talk, I think you already know what's going on. Your old condenser apparently wasn't a big enough cushion. So you added more cushion. You done good. Don't try to fix it. 8^)

 

The way I understand it, and I could be wrong, while there is a voltage reduction with a ballast resistor installed, what they are really doing is limiting the ampere or current flow through the primary circuit. Heat. Measure the entire primary. The primary coil winding itself is just part of it, the wiring, connections, switches, points, everything adds up.

An additional condenser connected in parallel on the negative side of the coil is simply doubling the capacitance of the ignition condenser. It's running about 0.44uF or MFD, and this is just spitballing, the quality of condensers is so bad these days, that is why the points were burning up in the first place? The condenser does keep arcing at the points minimized. What brand condensers are you using? Name brand, NOS? el-cheapo?

About 3 ohms of resistance is standard for a 12 volt contact point system in an 8 cylinder engine, or about 3 amperes through the primary circuit when engine is running. In a V8 the ignition coil is at about a 66% duty cycle, the coil isn't "ON" all the time, only when the points are closed.

This is why stuff can start burning up if the ignition key is left in the RUN position for any length of time without the engine running. The current flow is now around 5 amps, and it doesn't like that.

 

I would add to Jack's post, about "don't try to fix it".
If you have not added the extra resistor for the coil, I would add that. If you do not have enough resistance for the Supercoil, it can still die. I cooked two in two years, about a year apart (around 8,000 miles) with a Mopar electronic ignition until I had a face to face talk with the guys at an Accel booth at a car show. I added the extra resistor and solved the problem for the 3 or so years after while I had the car with coil #3 and the added resistor.

 

Wasn't there a math clause on the sticky page ?

 

My Corvette originally had another condenser at the coil. But I think it was for radio interference, and I don't remember which side it went on.

 

Right, I was going to mention that. For noise suppression or RFI it would be connected to the + or pos side of coil to ground.

 

Thanks for the heads up, with your experience in mind I think I will add the second ballast resistor on my cruiser for reliability's sake. Odd that it burned up when the instructions I found state that the additional resistor is only for points applications. I guess that brings me to my next thought, why, (in most cases?) does a points system need the extra ballast resistance, yet when you run a CD box they usually call for much less resistance? Does the CD box reduce the duty cycle of the coil so that it won't overheat with the extra current running through it?
Thanks for all the input!

 

I'm running points. Now just trying to understand how electronic affects the coil differently.

 

The two systems work in completely different ways.

Points, transistorized ignition, HEI and probably a bunch of other names used by someone as marketing wank connects battery voltage to the coil (sometimes through a ballast resistor) to slowly let the magnetic field build inside the coil, and store energy in it. All coils will resist change in current, so it takes a while for the current to rise to its maximum value, when the magnetic field is as strong as it gets.

Once the points open (or whatever electronics are used stop conducting) the current through the coil is interrupted. This is when we find out how badly coils really hate changes. The energy stored in the magnetic field will now attempt to pull current through the coil, and it can create a voltage spike several hundred volts high in that attempt if no current flows.

If it's a point system without the condenser the voltage will rise quickly while the points just barely have opened, the voltage will get high enough to keep a spark going on the points, so some current can keep flowing. The energy in the coil is burned off pushing current through that spark, and the voltage never rises very high.
With a good condenser a small current charges it just after the points open, delaying the voltage rise so the points have opened further once the voltage rises to hundreds of volts. No spark is formed on the points, all energy goes into making a high voltage spike in the coils primary winding.

As in a transformer, the magnetic field around the primary winding also affects the secondary winding, the hundreds of volts in the primary winding brings perhaps 50x that in a secondary winding with 50x as many turns. This gives us the spark on the spark plugs.

A CDI (capacitor discharge ignition) work in a very different way. It doesn't run a current through the coil to charge it, it uses other ways such as electronics or a separate high voltage coil in the generator (common on small motorcycle engines) to charge a capacitor to several hundred volts. When it's time to make a spark that charge is dumped through the ignition coil, skipping all the slow process to "charge the coil" and going straight to the high voltage spike on the primary winding and the higher voltage that causes in teh secondary winding.

So, the duty cycle of the CDI coil is basically zero, and there's only current flowing through it when it's producing sparks (or trying to).


The ballast resistor has multiple purposes.
The name "ballast" is there because it helps balance the load, i.e. it limits the current when the points are closed a long time (engine off, low rpm). Many modern electronic ignitions on new cars have the same function, but likely use electronics to control the max current actively instead. First feed the coil as much current as it wants, then once it's charged up with enough energy, limit the current to keep it from burning up.

This brings us to another important function, reducing the time it takes to charge the coil. As I said earlier, the coil hates changes in the current, and it takes time to reach the peak. So... what if we take a 6V coil, and start off by giving it a short kick with 12V at first, and then bring it down to 6V once the current has reached max? The high voltage start will make it all happen in half the time.
This is what we get with a (for example) 1.5 ohm ballast in series with 1.5 ohm coil - the coil starts out refusing taking any current, it basically has a very high resistance, and all the incoming voltage goes across the coil and none over the ballast. Once current starts rising, voltage over the coil goes down and up over the resistor. This means a system with a ballast can fully charge the coil in shorter time, meaning it can produce more strong sparks in a given amount of time. Not very important on a low rpm engine with few cylinders, a huge issue on a high rpm V8, or worse, V12.

And here I am, ranting away again. Oh well, I hope some of that makes sense.

 

Truck64 has got the basics right, I'll try to expand on this for more clarity...

The ballast resistor is primarily there to protect the coil, and the interesting thing is it's for LOW rpm operation. Any ignition coil will have a finite amount of time to build its maximum magnetic field up to saturation. Once it's reached that, any additional time that it's energized the current will act as a short and produce heat. As an example, on a V8 idling at 800 rpm, the coil will fire 400 times per minute. Your dwell number tells you how LONG it's on at each firing. Increase engine speed to 4000 rpm, now it's firing 2000 times per minute and the 'on' interval is now reduced to 1/5 of the time at idle. At 8000 rpm, it's 1/10. As long as the interval is long enough for the coil to reach full saturation, the loss of the additional 'on' time simply reduces coil heating. This the major flaw with a points ignition, the inability to control the 'on' time.

That still leaves the issue of arcing at the points. The higher the voltage, the more likely you'll have an arc, and the higher the current the more likely you'll see arc damage. So the ballast resistor will help reduce arcing by reducing the voltage and current at the points. But this isn't enough to eliminate arcing, so a capacitor (condenser) is added to further reduce arcing. This is where it gets interesting...

In a DC circuit, voltage/current are 'in phase'. To put it another way, they both arrive at a given point in a circuit at the same time. But an ignition system is a hybrid; it's a DC circuit while the coil is 'on', but as soon as the points open it switches to AC operation with AC rules until the points close again. A capacitor connected to a AC circuit changes the phase relationship between current and voltage, they are no longer 'in phase'. This causes the voltage to 'lag' the current, or in other words the current arrives first then the voltage. How big the interval is depends on the capacitance value and the frequency (rpm in this case) of the circuit. But the effect is the current arrives with reduced voltage (limiting arc damage) then the voltage with current already 'there', again limiting damage. This is a very simplified explanation as there are other factors involved, but covers the gist of it.

Does this mean you can ditch the ballast resistor by added another capacitor? Probably not. In this case it works because the OP has a coil that can tolerate the low rpm 'on' time, most won't. And by adding the second capacitor, he increased the 'lag' time between current and voltage, allowing the increased current with even more reduced voltage to reduce points arcing. Now whether or not that increases spark output, hard to say. I suspect you'd have to run the various combinations on a oscilloscope to see if there's any real improvement.

The more sophisticated electronic ignitions address this. The best use 'dwell control', divorcing the 'on' time from engine rpm. The coil 'sees' the same 'on' time at any rpm, hugely reducing coil overheating. Virtually all of the coil-on-plug ignitions use it, that's how they get away with those tiny coils.

 

Ultimately, by divorcing the dwell from RPM is what keeps everything cooler just as @Crazy Steve said.

Electronics have the potential for much better performing ignition systems. We all know that if it were available in the 50s it would have been used.

But points are what they are.. .a set of contacts on a mechanical cam. And so.e love them for their simplicity. But the drawbacks are weaker spark intentionally to keep the coil alive. It's much more complicated than that, but that is how I see the bottom line.

I don't post ignition stuff here as it is "OT".

 

Holy crap, thank you guys for taking the time to explain. It's going to take me a minute to wrap my head around everything but I'm starting to understand...

 

Right, the standard contact point ignition dwell is a compromise, because it is fixed but the engine RPM is not. So the coil actually tends to run a little hot at extended idling or low speed operation. The engineers chose a value that would work acceptably where a lot of driving is done.

I sat down and tried to figure it out when messing with the Pertronix and coils & the seemingly contradictory information people put out there. If you take the time to understand the "why" you can usually figure out the "what" yourself and be confident of the results before testing. A little Arithmetic will tell you whether any coil or ignition parts combo is likely to cause trouble.

Points and standard ignition coil don't like more than (about) 3 amps of current flowing through the primary circuit, this is achieved by a primary resistance of about 3 ohms, including the wiring, switches, everything together.

Pencil it out using Ohms Law, and the battery voltage - or better, the generator/alternator charging system voltage. Figure on 14.3 volts, not 12.6 volts.

The Ignitor module for example that can replace contact points is said to handle up to about 8 amps. A V8 specified 1.5 ohm ignition coil with no additional ballast resistor, a common setup, would flow almost 9.5 amperes - except for the fact the coil isn't "On" all the time.

At a ~ 66% duty cycle, it is now well below 8 amps. Like contact points, the bare bones Ignitor module is prone to permanent damage if the engine isn't running and current is flowing. One thing to keep in mind with a "hot" ignition setup though, the original points distributors were made of lesser quality materials in terms of dielectric,, and the rotor to terminal or tower spacing was different, they were intended for lesser secondary voltages - it
f there are defects in parts or materials the higher voltages will find it, and arcing or burning can result. GM was burning up rotors when they first introduced their HEI system, mainly because they had spec'd a spark plug gap of about .060 or more. The widest air gap sets the initial firing voltage of the ignition coil, and I expect they will run hotter with extra wide plug gaps too.

If you have a chance to look at your ignition system operating on an oscilloscope the traces can tell you a lot about how well all that stuff is playing together. Change even one small thing, and it just might bugger the whole shootin' match.

 

I think I am understanding this but have a unique application that complicates it a little. I am converting the ignition on a 1974 Chrysler Outboard motor. It is a two stroke four cylinder, which I think means it will be firing as much as an 8 cylinder motor. Chrysler uses a CDI system that is getting harder to find and is getting expensive. I want reliability and I think I want a 3 ohm 12v coil, with a condenser, the points, and I have a spark plug spec that will have a .035 gap. I am trying to avoid the extra expense of an MSD box as that would mean a 1.5ohm coil and different bigger gap plugs. Can you tell me if I am on the right track?

 

Sent you a PM....

 

I dumb it way down and follow Bubba’s advice (@GMC BUBBA the ignition guru)…. You want a total of 3 ohms. I use the Bosch blue coil on all my stuff that is already 3 ohms so no ballast needed. If you have a 1.5 coil, you need 1.5 ohm ballast. With a 0.7ohm could you need a 2.3ohm ballast. I’m sure there is more to the story but the general rule of thumb has worked for many of us on many different applications so far.