return to points

loudhvx wrote:

bountyhunter wrote: ...As for the arcing at the points: the voltage arc across the point is a direct consequence of the equation:

V = L dI/dt

Which is to say, the voltage across the points is proportional to the RATE of current change in the coil's primary when the points open, not the total magnitude of the current when they are closed. It's also dependent on the coil's inductance. Ergo, a lower inductance coil carrying more current may actually give less arc voltage. If the coil is eating points, it probably has higher inductance than stock.

v = L di/dt describes the voltage on the inductor portion of the coil (don't forget the coil also has resistance). If you ignore the condenser and the capacitance of the coil and the capacitance of the wiring and plugs, then it may give you an approximation of the voltage on the points before any arc. But we don't really care what the voltage is before there is an arc.

You care what the voltage developed across the opening points is because that voltage also appears across the transformer primary and is "stepped up" by the turns ratio of the transformer (which the coil actually is) so the stepped up voltage appears across the secondary side. The voltage builds up until the field strength is strong enough to arc across the spark plug gap, that voltage reflects back to the primary and is what appears across the open points.

bountyhunter wrote:

But the original poster wanted to know if the coil would be damaged by using points. If he is only worried about the coil, and can live with a slight reduction in point life, then he can save $36. That's the upside.

I don't agree. I come from an electronics industry and I don't think it's safe to overstress a coil unless the manufacturer specifically says it is "overdesigned" enough to allow it. ...

Of course, but as already discussed, we are under-stressing the coil.

bountyhunter wrote:

loudhvx wrote:

bountyhunter wrote: ...As for the arcing at the points: the voltage arc across the point is a direct consequence of the equation:

V = L dI/dt

Which is to say, the voltage across the points is proportional to the RATE of current change in the coil's primary when the points open, not the total magnitude of the current when they are closed. It's also dependent on the coil's inductance. Ergo, a lower inductance coil carrying more current may actually give less arc voltage. If the coil is eating points, it probably has higher inductance than stock.

v = L di/dt describes the voltage on the inductor portion of the coil (don't forget the coil also has resistance). If you ignore the condenser and the capacitance of the coil and the capacitance of the wiring and plugs, then it may give you an approximation of the voltage on the points before any arc. But we don't really care what the voltage is before there is an arc.

You care what the voltage developed across the opening points is because that voltage also appears across the transformer primary and is "stepped up" by the turns ratio of the transformer (which the coil actually is) so the stepped up voltage appears across the secondary side. The voltage builds up until the field strength is strong enough to arc across the spark plug gap, that voltage reflects back to the primary and is what appears across the open points.

Yes, of course, but that high voltage, with no current, does nothing. We only care what happens during the time when there is current through the points. That would be when the coil is charging (which is, presumably, not under debate), and when there is an arc at the points. So when there is an arc at the points is when we care about the voltage/current at the points. But when that happens, that high voltage will no longer be there, and you also can't use L di/dt to describe the voltage on the points because the circuit, then , becomes an RLC discharge problem where the R of the coil comes into play etc.

So the simple experiment was done to see which type of coil does more damage to the contacts. It was the low-resistance, low-inductance coil.

loudhvx wrote: We only care what happens during the time when there is current through the points. That would be when the coil is charging (which is, presumably, not under debate), and when there is an arc at the points. So when there is an arc at the points is when we care about the voltage/current at the points. But when that happens, that high voltage will no longer be there, and you also can't use L di/dt to describe the voltage on the points because the circuit, then , becomes an RLC discharge problem where the R of the coil comes into play etc.

That is not how a flyback converter works. When the points are closed, energy is stored in the core of the transformer in the energy form of:

1/2 L (I) squared

When the points just begin to open, the current flowing reduces (it does not stop instantly because that is impossible with an inductor) it actually has electrons flowing across the gap in the air as the points open. That high voltage buildup is reflected across to the transformer secondary where it is building up across the spark plug gap. The voltage buildup continues until it gets high enough to jump the spark gap, which it does dumping the energy stored in the transformer's core. The spark plug acts as a voltage "clamp" such that it limits the peak voltage of the secondary winding to whatever it takes to jump the spark gap. This in turn limits the voltage seen across the points.

I can't see how a few Ohms of resistance in the primary could significantly affect the discharge of the core's energy when the plugs fire: the transformer secondary is connected to the plugs through very high resistance wire (for EMI suppression) which would dominate whatever impedance the winding has.

The role of the condenser: the capacitor is connected across the points which creates a "parallel" path around the points for primary current to flow. As the points open and the voltage across the points starts to rise, the capacitor supplies some current into the coil primary which allows "tuning" the di/dt rate of the primary current fall rate in the transformer primary to get best spark. In cases where the condenser is bad, you get poor spark and eat the points because that effect is lost.

bountyhunter wrote:

loudhvx wrote: We only care what happens during the time when there is current through the points. That would be when the coil is charging (which is, presumably, not under debate), and when there is an arc at the points. So when there is an arc at the points is when we care about the voltage/current at the points. But when that happens, that high voltage will no longer be there, and you also can't use L di/dt to describe the voltage on the points because the circuit, then , becomes an RLC discharge problem where the R of the coil comes into play etc.

That is not how a flyback converter works. When the points are closed, energy is stored in the core of the transformer in the energy form of:

1/2 L (I) squared

When the points just begin to open, the current flowing reduces (it does not stop instantly because that is impossible with an inductor) it actually has electrons flowing across the gap in the air as the points open. That high voltage buildup is reflected across to the transformer secondary where it is building up across the spark plug gap. The voltage buildup continues until it gets high enough to jump the spark gap, which it does dumping the energy stored in the transformer's core. The spark plug acts as a voltage "clamp" such that it limits the peak voltage of the secondary winding to whatever it takes to jump the spark gap. This in turn limits the voltage seen across the points.

I can't see how a few Ohms of resistance in the primary could significantly affect the discharge of the core's energy when the plugs fire: the transformer secondary is connected to the plugs through very high resistance wire (for EMI suppression) which would dominate whatever impedance the winding has.

The role of the condenser: the capacitor is connected across the points which creates a "parallel" path around the points for primary current to flow. As the points open and the voltage across the points starts to rise, the capacitor supplies some current into the coil primary which allows "tuning" the di/dt rate of the primary current fall rate in the transformer primary to get best spark. In cases where the condenser is bad, you get poor spark and eat the points because that effect is lost.

The current doesn't change quickly in an inductor, but voltage certainly can.

If you've ever looked at the primary voltage on a scope, it's pretty telling. As soon as the arc starts, the voltage changes very quickly from multiple hundreds of volts down to 30 or so volts, but as I said, it is an RLC problem by then, so there is an oscillation associated with the change. But the average voltage that the "ring" is centered on, is around 30v, maybe higher with larger plug gap etc.

The role of the condenser (maybe not the only role, but at least one of the roles) is to act as a small delay so the points can move farther apart before the inductive spike on the primary takes effect. Without it, you get virtually no spark on the plugs because most, if not all, of the energy discharges through the points.