hey EE guys Options

[bd1308]

i have a question to ask, but I have to ask it later in the day....

i just wanted to round up the troops....

dont laugh either.....



b

[jsteele22]

Hmmm... just thinking out loud here, so don't take this as definitive.

The role of the points is to make and break contact in the primary circuit (12-ish volts). Contact is made, current builds up and up until there's a good bit of energy stored in the magnetic field, then click - the contact is broken. Imagine there is no secondary coil. The primary coil is an iductor; it's windings don't like to see an abrupt change in magnetic flux : a reverse EMF (voltage) V = -L * dI/dt results. From the point of view of energy, the energy that was stored in the magnetic field is transformed to energy stored in an electric field between the points. If the field gets high enough, there is a breakdown of the air and a spark results.

One way to avoid the spark is to have a secondary circuit. When the primary current is cut off by the opening of the points, not only is a reverse EMF produced in the primary, as before, but also in the secondary (due to mutual inductance). Because of the huge turns ratio, the secondary EMF is much, much greater (it's a step-up transformer), so if the secondary is connected (via spark wires and an appropriately timed distributor rotor) to a spark plug, then the spark should appear in the spark plug gap first. Once the spark occurs, the spark plug gap has low resistance, and current can flow through the secondary circuit across the gap to ground. This current flowing through the secondary (actually, the time derivative of it) creates a time-varying magnetic flux in the coil that opposes the change caused by the points interrupting the primary current. So the reverse EMF across the breaker points is eliminated before it gets too large. From the point of view of energy, the energy from the original magnetic field is converted to heat in the spark gap.

So far, it sounds like the way to fry your points is to disconnect the spark plug gap from the circuit. The obvious way is to yank the plug wire. A less obvious way is to change the timing so the rotor is not making contact when the breaker points cause the EMF to spike. So when you say "running lean" do mean running with the timing too far advanced/retarded ? I'm sure the actual presence (or absence) of fuel in the air in the spark gap has some effect, but my hunch is that it's not huge.

Anyway, your question was about a different way to avoid frying the points. The idea is to hook up a diode so that when the EMF appears across the points gap the diode is reverse biased. As you mentioned, a diode won't conduct in the reverse-biased direction until a certain voltage (the "reverse voltage, or "peak inverse voltage", PIV) is exceeded. From the above discussion, it should be clear that one thing you DO NOT want to do is choose a diode with too small a PIV. Then you would short out the primary before the spark plug had a chance to fire. No harm there, except your engine won't work. The other thing to think about is the energy involved. Pretty much all of the energy that would have gone into the spark plug gap, or into the breaker points gap, will be dissipated inside the diode. I Better make sure the diaode can take that much power dissipation. I don't know how much (in Watts), but it's enough to toast stainless steel, or whatever points are made of. I bet there is a diode big enough, but I bet the cost is more than a new set of points.

An interesting variation on your idea is not to use a diode, but a "sacrificial" gap. Make a gap that is a little bit smaller than the points gap. Hook it up in parallel to the points. Under normal operation, it shouldn't have any effect (provided you don't make it too small), and under the no-secondary-discharge situation, it will produce a spark first, saving your points.

Good luck.


Jeff