Transient protection?

[drZoidberg]

Oh man...
Now I see how the ignition coil works!

Screw the diode.

That would mess the whole circuit up. Thanks, Rufus, for pointing out how the circuit works.
I was thinking of the transformer as an ordinary transformer, with a separate primary and secondary side. That doesn't work at all, of course. The current flowing in the primary winding has to keep flowing. My solution would have stopped the engine altogether Hah!

Well well..

Also, C makes some good points about not putting diodes in very hot environments. To answer your question, I don't know. The diode would probably have burned up in the heat from the engine, unless it was placed sufficiently far away from the heat. I mean, there are some other electronics going in there also, and they need to be placed somewhere not so hot also. But as I said, your car would go nowhere fast.

I now see that IanB has pointed out the egg on my face =)

[drZoidberg]

Condescending??  Uh... sorry? How would you prefer I answered?

I would prefer you pointed out why you think I was wrong, like some of these other fine members have done, instead of rolling your eyes and saying:

Yeah... somehow I don't think attempting to dump the whole transient is a good idea.

Sorry if I wasn't super nice back at you. 

[c4757p]

I wasn't sure if you realized that's what you were implying, so I thought I would point it out. I'd have explained more completely, but I was typing from my phone - figured I'd at least point you in the right direction until I got to a proper computer or someone else took over.

There was no eye rolling. I was trying to be as helpful as the situation allowed, and possibly misunderstood your line of thinking.

[drZoidberg]

OK. I understand.
No worries then =)

[somlioy]

Just to clarify, this isnt something I'm permanently hooking up to the car. I just hook it up once in a while to check the dwell angle, which needs to be adjusted quite often with these kinds of ignitionsystems.
Plus and minus to battery, coil signal to the low side of the coil (between point and coil).

[Paul Price]

The video is almost right, but fails to explain the capacitor across the points. It doesn't just "absorb back emf", it allows the circuit to oscillate with the inductance of the transformer.
Without the capacitor the voltage from the coil, when the points opened would try instantly, in a few billionths of second, to rise to 1000 or more volts, this would cause the points, still only slightly opened to arc over.

With the capacitor, the coil's inductive flyback current must charge the capacitor to a high voltage at a much slower rate(over some fraction of a millisecond), so by the time the voltage is high enough to arc, the points have already moved too far apart for the voltage to jump that new point gap distance.  The result, instead of creating an instantaneous arc that will just arc across the point contacts and kill all the spark coil energy, you get a lower voltage but a high voltage, a developing 400V oscillation that creates 22000 or more volts in the secondary of the spark coil.

Connecting a diode across the ignition coil instantly absorbs the energy stored in the spark coil inductance, kills the spark energy but will protect your points. The side-affect  is no spark to the plugs.

A good dwell/tach circuit should just should sense voltage at the points-sparkcoil connection, not interfere with its operation, that's why I have suggested and offered circuits very early in this discussion to make an input buffer circuit.

[somlioy]

Havent had any time to work any on this but I checked out your circuits. I'm probably going for the CD4093 one, since I already got those somewhere. Is it necessary to protect the input of the CD4093 with clamping diodes or are the 100k and 1Meg doing the work?

[Paul Price]

The inputs of the 4093 already have clamp diodes and the 100k resistor limits the current into the 4093 protection diodes to do no harm to this circuit. Just to be sure you can place a zener diode (cathode to the 4093 pins 1-2 input, anode to ground) for additional protection. The primary voltage can reach in excess of 400V to present itself at the input to your tach/dwell meter., but the 100k resistor protects the input of the 4093 and while it isolates the dwell/tach meter from upsetting the spark.
A zener diode at pins 1-2 of the 4093 with the 100k resistor in the circuit will offer additional protection. A low power 8.2V zener will do.

You could also just put two 1N4184 diodes at the CD4093 pins 1,2. with the top diode connecting its cathode to the +9V, its anode connecting to the cathode of the second 1N4184 and pins 1,2 of the 4093. The anode of the bottom diode connects to ground.  You already show something like this in your original drawing, but again, don't bother with schottky diodes.