POS or NEG ground

Jim Huseby

Jim Huseby

Senior Member
There was a comment that an ignition schematic recently posted didn't show whether it was pos or neg ground. The fact is that the spark intensity and the engine's performance will be exactly the same either way, without switching any wires. The only difference it would make is which way the ammeter would read, which also wouldn't make any difference in how the engine would run. Or, to make the polarity situation simpler to visualize you could take the ammeter out of the schematic and connect its two leads together as one wire. If the charging circuit had been included, then the gen would have to be polarized when switching polarity. (Also, recently, it was posted here that though an A generator can be re-polarized, the field magnets will want to revert back to their previous polarity when left to sit for a short period. 'Seems plausible). After a bunch of you argue with me about this ignition polarity topic, and after I explain my comments, some of you will still argue. Here are three questions to get you ready: If your Model A was running a magneto, would the plugs be firing positive or negative ground? What is the function of the coil's primary winding, and where does the primary winding belong in the circuit? Which side of the condensor stores the electrons? Please answer these questions before commenting.
 
The questions posed are irrelevant to the issue. It makes no difference electrically if a system is positive or negatively grounded. The wiring of the coil relative to the system does make a difference however.
It is elementary physics that electrons move more readily from a hot surface to a cooler one, in a spark ignition system that is from the center electrode to the ground strap. The center electrode is hotter than the ground strap, that is where the heat range is determined. So the coil should be wired so the spark jumps from the center electrode to the ground strap.
If the system and the coil is wired so that the spark jumps from the center electrode, then reversing the system polarity, without changing the coil connections, will result in the spark jumping from the ground strap to the center electrode, requiring more energy to initiate.
If you change a stock Model A to a negative ground, and change nothing else, the spark will jump from the ground strap to the center electrode. However, if you then reverse the coil connections, the spark will then jump from the center electrode. It's more efficient that way. It does not make a noticeable difference ordinarily, but may make a difference in a marginal system or starting, or at high speed.
 
PetesPonies said:
PC/SR has it exactly right.
Click to expand...

Right, since the coil and generator have a soft iron core the residual polarity can easily be switched back and forth.

BTW, The residual magnetism in my powerhouse is only enough to generate 1/2 volt if I disconnect the filed wire. But a portion of that 1/2 volt is fed back to the field coils and builds the magnetism until the output reaches a much higher voltage. In fact, uncontrolled, the voltage can reach 40 volts.:eek:

Jim, you have one of the nicest speedsters I've seen, and I like the color scheme.:)
 
Let's see how good I remember my electronics engineering theory from a long time ago

On question # 1 you may want to restate this one as a magneto puts out AC not DC. So it is changing polarity based on 2x the frequency of the output. Plugs fire on AC not DC.

On #2 the function of the primary winding is to create a magnetic or flux field as current flows through the windings, when this field collapses it induces high P.I.V. in the secondary field, which in this case is called spark.

On #3 electrons are stored on the negative side, actually there are just more electrons on the negative plate than there are on the positive plate when the capacitor is charged.


Electrons like to be balanced or another way to think about it they like to be equal. So they will always flow from a higher source to a lower source.

Way to much theory here that will not help most work on their A.
 
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It is correct that the coil will work the same no matter how it is hooked up, but as mentioned the spark plug does care. A difference of 15 to 30 percent in voltage required to make spark "initially" jump the gap on the plug depending on which way it is going due the reason given above having to do with hot and cold surfaces.
 
I've also got to agree that PC/SR knows what he is talking about. I've run both positive and negative ground with the original generator. Keeping the generator polarized for negative ground is more trouble than it is worth unless it is rebuilt with NEW field coils and armature. If I changed to negative ground an alternator would be the easiest way. As for the coil, If I changed to negative ground, I would also reverse the coil polarity. With too cold spark plugs and a weak replacement generic coil , it will make a noticeable difference if the coil polarity is incorrect.. Whether the difference is large or small , I want the hottest spark. Good spark makes a difference in cranking, general performance and whether the plugs run sooty and the muffler collects unburned gas and backfires when shut down. I use the ignition coil and spark tester from Brattons, part number 17140. This tool shows if the coil is connected + or -. there is no risk of getting shocked and it works, no matter if you are running six or twelve volt.
 
This my attempt to help make it simple and not complicated for those that are tripped up and frustrated about a simple subject, so rather than throw something tricky at you, I'm just asking questions to get you relaxed and in a logical thinking mode. And I'm not trying to set myself up as some kind of guru, because I'm not. Think about my questions and their answers, as I asked. jmeckel posted excellent answers I was looking for, except his answer on the mag, though correct for a Model T, is not correct for an A with, say, a Wico mag. I'll just give you the answer to that one: two plugs would constantly be firing pos ground and two would be firing neg ground. I would like for someone who runs a mag to post why that that is. Now, a couple more questions that jmeckel has alluded to: Why is the primary winding given its name? Why is the secondary winding given its name? How do these windings differ? If you are running, say, neg ground, which spark plug electrode will be negative, the center electrode or the outer electrode? If you answer my questions correctly, you will probably figure out why I made my original statement.
 
I'd better ask the condensor question again because jmeckel gave a tricky answer. Which side of the condensor stores the electrons? Are condensors marked pos or neg ground? Why?
 
Yes, the thread is a little over my head. But I like learning new things even if I don't understand all of it.:) I run a Delco-Remy {'35 Chevy truck} generator on my '31 "A". This Delco was normally negative ground. However, I'm running it Model-A style positive ground and it works great.;)

Bob-A:D
 
Again, I'm trying to make this simple, not difficult. Here's another answer to a couple posts on this thread. Swapping the coil leads can't change whether the spark jumps from the plug's center electrode to the ground electrode or vice-versa. If the battery is connected positive ground, the plug's ground electrode will be positive and the electrons will have to jump from the center electrode. If the battery is connected negative ground then the spark plug's ground electrode will be negative and the electrons will jump from the ground electrode to the center electrode . Swapping coil leads will determine whether the current going to ground through the points passes through the primary winding or secondary winding and therefor how high a voltage is induced to fire the plugs. THIS IS NOT ROCKET SCIENCE. The battery current must pass through the PRIMARY coil winding and through the points to ground, so that when the points open and the field created by the 6 or 12 volts passing through the PRIMARY winding collapses, a high voltage(what should it be, 20,000?) will be induced in the SECONDARY winding which will then discharge across the plug gap to ground. It is the ratio of coil primary windings to coil secondary windings that determines the voltage available at the plugs. If you vainly attempt to change coil polarity by swapping coil leads, you will lower the voltage to the plugs. And that,folks is why the windings are named PRIMARY and SECONDARY. About all coils today have their low tension terminals marked (+) or (-). That confuses most people and it shouldn't. Back when pos and neg ground cars were intermingling, you would find coils with their terminals marked "BATT" and "SW" (for points). Today, since most all cars are wired neg ground, the semi-literate manufacturers label the coil terminals "+" and "-"., and therefor the "+" means primary and the "-" means secondary whether it will be used in a positive ground or negative ground system. None of the components in a Model A ignition system are polarity sensitive.
 
Jim Huseby said:
I'd better ask the condensor question again because jmeckel gave a tricky answer. Which side of the condensor stores the electrons? Are condensors marked pos or neg ground? Why?
Click to expand...

The short answer is neither. It all has to do with opposing fields.

The molecules of the dielectric material are polarized in the field between the two conductors. The entire negative and positive charge of the dielectric is displaced by a small amount with respect to each other. This results in an effective positive surface charge on one side of the dielectric and a negative surface charge on the other side of the dielectric. These effective surface charges on the dielectric produce an electric field, which opposes the field produced by the surface charges on the conductors, and thus reduces the voltage between the conductors. To keep the voltage up, more charge must be put onto the conductors.
 
Hello Mike V. I asked the condenser question in a way that could be treatad in the most simple manner in order to help answer the pos/neg quandry that unnecessarily agonizes and terrorizes many fordbarners. My intent is to take away the fear and terror and to demonstrate how simple my original subject is, and not to intimidate or alienate anyone. This is not a contest or demonstration of intellectual superiority. Discussions of break down voltage characteristics, inductance and resistance in a capacitor...well, we're just not at that level. All we need to know is that the condenser stores a charge. Now, the rest of that question was: is the condenser marked with a (+) terminal and a (-) terminal, and why?
 
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