loudhvx wrote:
Oh and by the way, if you measure the instantaneous voltage on a stock coil primary, right as the current is cut, it easily reaches over 300v. I believe I measured it somewhere around 400v. This is self-induced by the coil, as opposed to being forced in a CDI system, but either way, the coil has to have a big spike on the primary to put out a really big spike on the secondary.
Correct, and that's why points gradually wear out and burn away in a stock points/coil ignition system. When the points are closed, the current flows in the primary. Opening the points abruptly will cause a large voltage spike to appear across the inductance (coil) by the law of inductance:
Voltage = Inductance X (rate of change of the current)
The points and coil primary are in series from ground to +12V, and whatever voltage appearing across the coil primary also appears across the points, minus the +12V at the high side of the coil.
Breaking points abruptly might seem to change the current INSTANTLY, but that's impossible as it would generate an infinite voltage. In reality, the voltage builds up until it arcs across the air gap of the opening points and that arc action clamps the voltage on the primary side, and the maximum available secondary voltage is the primary voltage multiplied by the transformer's turns ratio.
The capacitor (condenser) connected directly across the points also provides some small amount of current as the points are opening, to slow the change rate of current in the primary winding, which reduces the arcing across the points. That's why a bad condenser can cause points to burn away.
