Slash five tuning help

[bbelk]

I am probably going to mess this up but here goes.

A wire passing through a magnetic field creates a current in the wire. If the wire is open, it produces a voltage. The more loops of the same wire you pass through the magnetic field, the more voltage. This is the way a generator works. The Magnetic field is static and the wires are moving.

In a coil, its just the oposite. The wires are static, and the magnetic field is moving.

There are two sets of windings in the coils. One with a few turns that is hooked to the battery on one side and ground via the points on the other.

The Points close, current flows, a magnetic field is producd that surrounds both sets of windings in the coil.

The second set of windings has lots and lots of turns. It is grounded on one side and separated from ground on the other by the spark plug gap. When the points open, the magnetic field collapses and when it does, it passes over the coils. There are lots of turns on the high side of the coil, so there is lots of voltage generated and it causes the spark.

Ta da

[Garnet]

Yep, I get that well explained example. But what's got Ken and I bamboozled is how the secondary side completes it's circut.

[bbelk]

Yep, I get that well explained example. But what's got Ken and I bamboozled is how the secondary side completes it's circut.

To a Direct Curent (DC) as in the 12 V system on the bike - a coil of wire (an inductor) offers no impedance. A Capacitor on the other hand looks like an open circuit to DC. However to Alternating Current (AC) an inductor offers impedance and the capacitor does not. The amount of impedance is a function to how fast the current is alternating (changing). To a very high rate of change, the impedence of the inductor is high and the impedance of the capacitor is low.

So - when the points open, the magnetic field collapses causing a rapidly rising voltage on the high side of the coil. To the rapidly changing voltage, the capacitor looks like a short circuit. So - the capacitor connects one side of the coil to the ground. The spark plug gap separates the other side of the coil from the ground. All those volts want to get to ground, so they jump across the gap and there you have your compleated circuit.

[Garnet]

OK now I get that part, the condensor does the job.

So, for 64,000 dollars, how does that work with my aftermarket electronic ignition that has bypassed the condensor?

[bbelk]

OK now I get that part, the condensor does the job.

So, for 64,000 dollars, how does that work with my aftermarket electronic ignition that has bypassed the condensor?

Thats easy!

Its magic.

[Ken in Oklahoma]

So - when the points open, the magnetic field collapses causing a rapidly rising voltage on the high side of the coil. To the rapidly changing voltage, the capacitor looks like a short circuit. So - the capacitor connects one side of the coil to the ground. The spark plug gap separates the other side of the coil from the ground. All those volts want to get to ground, so they jump across the gap and there you have your compleated circuit.

Hi Brad,

I wish you, I, and Garnet were sitting in lawn chairs with the happy sounds of a rally going on behind us, and our favorite beverages in hand. This would be a lot more satisfying discussion than talking to Duane about counter steering as a valid teaching method.

Sorry Duane

While I talked about the capacitor storing energy and reflecting it back to the coil secondary, I hadn't considered your point that a capacitor would look like a short circuit to a near instantaneous voltage spike from the points opening. Good point and I take it.

But my quarrel was with the often made statement that it is the points that supply the ground to the coil's secondary windings. Strictly speaking it would be the capacitor that completes the circuit, not the points. With the points open and no arcing across the points, they cannot be supplying the needed low side needed for the arc across the spark gap. And maybe that's the answer--it's the capacitor that does the job.

There's still one little thing that's nagging me, however. It's often mentioned that a bad capacitor allows the points to pit and destroy themselves. That suggests that a capacitorless set of points will still allow sparking, though of a bad quality.

And perhaps the answer to that is that when people talk about a bad capacitor causing the points to pit, "bad" doesn't mean a complete lack of capacitance. Just not enough of it.

Has anybody ever run an ignition with no capacitor at all, even if badly? If so then the question would be resurrected: Where does the low side of the coil's secondary winding get grounded.



Ken

[Garnet]

"Has anybody ever run an ignition with no capacitor at all, even if badly? If so then the question would be resurrected: Where does the low side of the coil's secondary winding get grounded."

I have tried that as a bench test for a coil. Hooked the posative side to a battery and used a pushbutton switch on the negative side instead on points. Then gounded a spark to the negative battery terminal.
The result was a very weak almost non-existant spark. I would have called it a bad coil. I then added a condensor into the circut and got a nice big fat blue spark.

[bbelk]

Ok - here goes.

The points close - the current ramps up through the primary (it can't change change instantly because inductors resist instant changes in current) and the magnetic field is built. The points are simply grounding a circuit that starts at the battery, runs through the primary side of the coil, and through the points to ground. The capacitor in parrallel with the points looks to the DC voltage of the battery like an open circuit.

Now the magnetic field exists. The points open, the magnetic field collapses over the great number of turns on the high side of the coil. Now all those volts exist in the coil and they are trying to get to ground.
A properly working capacitor looks to this rapidly changing voltage like a short circuit and some of those volts go to ground there. The rest jump the gap on the spark plug and go to ground.

Now if the capacitor is bad - the point gap looks just like the plug gap to all those pent up volts and you get arcing across the points.

I am sure that all of this terminology is wrong - but the phisics is much harder to explain without anthrophomorfising these things.

I am living proof that it is not necessary to be a good speller to be a good engineer.

[Frog]

Ken,

That is an interesting read, thank you for taking the time.

In your opinion, do you think that my issue could be caused by a poor ground? Or, are you saying that it is the spark plug that is grounded?

[Ken in Oklahoma]

Ok - here goes.

The points close - the current ramps up through the primary (it can't change change instantly because inductors resist instant changes in current) and the magnetic field is built. The points are simply grounding a circuit that starts at the battery, runs through the primary side of the coil, and through the points to ground. The capacitor in parrallel with the points looks to the DC voltage of the battery like an open circuit.

Now the magnetic field exists. The points open, the magnetic field collapses over the great number of turns on the high side of the coil. Now all those volts exist in the coil and they are trying to get to ground.
A properly working capacitor looks to this rapidly changing voltage like a short circuit and some of those volts go to ground there. The rest jump the gap on the spark plug and go to ground.

Now if the capacitor is bad - the point gap looks just like the plug gap to all those pent up volts and you get arcing across the points.

I am sure that all of this terminology is wrong - but the phisics is much harder to explain without anthrophomorfising these things.

I am living proof that it is not necessary to be a good speller to be a good engineer.

I like it Brad. My quarrel remains for statements that coils are grounded through the points. That's generally made in the context of somebody refuting a statement that the coil case needs to be grounded. Whether the coil case is grounded or not is immaterial.

For purposes of firing the plug the initiating event is the points opening. The capacitor immediately takes over the transient job of providing a ground, but the points are out of the picture until they close again.

I like that explanation better than the one I started out with.

And in any event neither the points nor capacitor involve the coil case. The case may be grounded through the coil mounting bracket, but it doesn't have to be for the ignition system to work properly.

What's sort of puzzling me now is all the coil mounting brackets I've seen with a metal projection on the inside, presumably to scratch paint from the coil body and ground it to the engine or chassis. Could they be grounding the case for the purpose of electro magnetic shielding, probably for the old AM car radios?

Sorry for the diversion Frog, but I'm glad for my better understanding of ignition systems.


Ken