Lots of confusion about what ballast resistors do, so I thought I'd hook the oscilloscope to a coil without & with a ballast resistor to see what it does to the voltage, current, and most important, time. I did not use an ignition coil, I grabbed an electromagnet with just under 200 ohms resistance and a 200 ohm resistor for ballast, and a 11 ohm resistor for a shunt to measure the current flow. A coil is a coil so this shows the principle of what happens with or without ballast resistors on any coil, but the voltages, current and time to peak current may differ. Good enough to show what is going on. Blue trace is voltage at the coil, red is current through the coil. In the first picture we see the voltage (blue trace) at the coil go straight from zero to about 14V, and it stays there pretty stable - just as it should when hooked straight to a 14V source. (The initial "bounce" is actually the contacts in the switch I used bouncing. Not ideal, but we can ignore it for these tests.) The current (red trace) starts at zero and climbs to its max after around 30 milliseconds, as the coil builds a magnetic field. An ignition coil can supply a full strength spark once the current has reached its peak, so in this case a spark after less than 30ms dwell would have had less than full power. I think real ignition coils usually are faster than this, but again, we're just looking as the basic principle here, the actual time is besides the point, we just want to know the difference. Now I've added a ballast resistor, with about the same resistance as the coil. This time the voltage also goes up to full input voltage, about 14V, but as the current increases over time, the voltage goes down. When the current has peaked (after only 15 milliseconds, half the time) the voltage on the coil has dropped to about half the input voltage. So, the ballast resistor gives the coil a high voltage to start with, making the current flow reach peak quicker, then it reduces the voltage as the total input voltage is split between the coil and the ballast. The shorter ignition coil "charge time" is good on high rpm, expecially on a V8 (or bigger) as there's little time between each cylinder firing - if the coil takes too long to charge the spark will be far below max strength. The limited voltage once the current has peaked is good at low rpm when we have LOADS of dwell time, it keeps the peak current to a sensible level and reduces the heat loss in the coil (it's split between the coil and ballast) so we don't cook the points or coil as easy. I'm sure there's loads more that could be said, but this seems good for now. I just thought a few oscilloscope waveforms could make this more clear than just trying to explain it with text. Feel free to add info if you have anything that goes with the subject.
It's actually a lot simpler than that. It is essentially a strategy used to improve starting. A typical 12V points type ignition system uses a "6V" coil that would burn up if run continuously on 12V. The resistor drops voltage to an acceptable level. When cranking, the starter motor draws a lot of current and can drop system voltage to the point that the spark could be pretty weak but the resister is temporarily byp***ed so the coil sees full system voltage and produces a nice fat spark. Modern electronic systems are able to control dwell on the fly and achieve coil saturation without resorting to this strategy.
That is indeed also a feature of the ballast resistor, but it's more of a (almost) free bonus than the main reason to use it.
Can you elaborate on that? What makes a coil a 6V coil? Why would it burn up if run continuously on 12V? Voltage is ****ogous to pressure, right? Is it current that causes heat, or is it voltage?
A 6 volt coil will have roughly half the resistance of an equivalent 12V coil. If you think of voltage as pressure, you'll see that 12V will push twice as much current through it as it was meant to handle. The resistor effectively makes it a 12V coil except when it is byp***ed during cranking. ***uming the engine starts promptly, the extra current wont have time to heat the coil to a damaging level.
So, the purpose of the resistor is to limit current, correct? Which I believe is what the scope pics above are showing. It's not the voltage that will burn up the coil (though excessive voltage can pierce through the insulation on the wires and short out a coil), it is the current. Limiting the current reduces the heat in the coil, and the resulting lower voltage is a side effect, which I think is what @G-son was saying in his reply above. Take care guys and have a fun conversation about it. I'm off to a July 4th BBQ, I'll check in later and see what else is posted.
That's not necessarily true. The key is dwell time. When power is applied to a coil, it takes a finite amount of time for the coil to build up it's magnetic field, usually about 2 milliseconds. While the field is building, the coil is 'using' that power. But once it's reached 'saturation' (the maximum strength field possible due to it's construction), any additional power is essentially seeing a short-circuit and converts that energy to heat. Dwell is expressed as 'distributor degrees', i.e. the time the coil is 'on' when the points are closed. But it's not a static amount of time; cars are operated at varying speeds. So doing a bit of math, you'll see that... At a 600 RPM idle with a V8 (remember, it takes two revolutions to fire all eight cylinders), the coil fires 40 times per second (600 RPM/2 x 8 cylinders/60 seconds = 40 per second) or .025 second (250 milliseconds) between firings. The time the coil is actually 'on' is less than that as I'm not accounting for the 'off' time when the plug is actually fired. But increase engine RPM to 6000 and dwell time drops to 1/10 of the idle time, or only .0025 second (2.5 milliseconds), nearly ideal. This will substantially reduce heating in the coil. But unfortunately, few people can drive their car at 6000 RPM continuously so compromises have to be made. If you have a drag car that is pretty much only run wide open, you could ditch the ballast resistor and not have to worry about the coil. But don't try to drive it home.... These are just rough numbers and not particularly representative of any ignition system, I used these just to show the relationships involved. The better modern electronic ignitions use 'dwell control' to limit the coil 'on' time regardless of RPM so they can apply higher voltage/current without overheating the coil.
So does the " extra voltage " burn up the points? or it just the heating of the coil that one needs to worry about? How long can you run with out a ballast resistor? Ask because I don't believe my distributor machine is wired for one, it's an old '50s Allen machine that's been rewired.
Well... Points get BBQed because of (too) much current p***ing through them. The math is hard as this isn't stable, as shown the current starts at zero and then rises "slowly", I'll do some math here that only applies to steady current = the time after current has peaked, or a key on, engine off situation. The power developed in the points themselves could be calculated by multiplying the resistance (through the points) with the current squared. So, say, the resistance is 0.05 ohm (I have no idea what's normal, so it's a guess). If the current is 5A the math goes 0,05*5^2= 1,25W Not a whole lot, but points are small and that would cause some heat. Now, let's double the current, i.e. 10A 0.05*10^2= 5W All numbers for constant, steady voltage and current as i said first, so not like a running engine. Just to give an idea. Double the voltage, double the current, now there's four times the amount of power heating those poor points. They're not going to like that very much. And a dirty or oxidized surface on the points increases the resistance, so probably more heat caused by the heat that oxidized the surface on the points... Should go downhill from there pretty fast. No idea how long you could run w/o a ballast resistor, but if you have a coil made for use with a resistor things won't like running w/o one forever. But... On old cars there can be an "unintentional" ballast resistor. If the old, oxidized wiring, switches, connectors, ground points etc. has a higher than normal resistance this can give the same effect as a proper ballast resistor. Probably not the right amount of resistance, but I wouldn't be surprised if that's what keeps some points & coils alive despite running w/o the ballast they're made for.
Looks like I better get my machine fixed so I don't burn out the new points in the rebuilt distributors I work on. Thanks for taking the time to figure all this out.
Actually, it's not the current per se that kills the points, it's the arc generated when the points are opened under load. Virtually any mechanically-operated switch will produce an arc, both on closing and opening, but the 'opening' will generally produce a larger one. Any arc will produce some erosion/material transfer of the contact surface (even if it's only a few molecules) and all else being equal the larger the arc the more erosion you'll have. One of the reasons the 'condenser' used on points ignitions is there is an attempt to reduce the arc size and extend point life. Most switching contacts are rated in number of 'operations', i.e. how many times it's turned on and off. 'Common' switches like a wall light switch in your home can be rated at 100,000 operations in their 'lifetime', which would allow ten operations per day for 27 years. As current switched goes up, the number of rated operations goes down. To put that into perspective, a V8 running at 1500 RPM will have 100 operations per second, in 17 minutes you will exceed 100,000 operations. All things considered, points hold up fairly well...
It will only matter if whatever circuit you have connected for checking the dwell uses a fair amount of current. If you're talking a milliamp load, it won't matter for your purposes.
Ah, yes, you are right of course. It's way too easy to go one rabbit hole and compeletely miss the important bits. Oh well, you got it this time!
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