Tach input from spark voltage (idea)

[tom cloud]

>
>I've been working on various inductive tach schemes with some success.  I 
>want to try a new approach which would be to take the input to the tach 
>from the "points" or "tach" signal from a car distributor (or whatever).
>
>In case my description is lacking, from the input or low voltage side of 
>the coil (or cap or whatever).  What voltage range should I expect this 
>signal to be in?  Are we talking tens of volts or hundreds?
>
>I understand that the spark plug sees kV, but the input to the spark 
>circuit should be quite a bit lower, right?
[ snip ]

There will be several hundred volts at the primary of ALL ignition
coils, regardless of what method they use.  Without a complete rehash
of inductance and L-C (tank) circuits, just experiment with a coil
and a resistor.  Get a starter solenoid (for starters, heh - heh).
Measure its resistance.  Now, get a resistor of the same value.
Take some clip leads and connect the coil (starter solenoid) to
the battery several times.  Sparks, right???  Do the same thing
with the resistor.  If you see any sparks, they'll be LOTS smaller.
Try holding the clip leads (that have the 12 volts from the battery)
with each hand whilst you do this.  The 12 volts won't hurt you.
Touching the resistor whilst holding them won't hurt you.  I
can't say that you will feel anything doing this with the solenoid,
but you might (depends on the amount of inductance).

GUARANTEED:  If you do the above experiment with an ignition coil,
touching the primary only, you'll get a jolt.  Wanna BIG jolt (I
can personally attest to this 'learning from experience').  Put
your hand on the output terminal of the ole starter solenoid whilst
you're starting your car -- you're guaranteed a jolt stronger'n
Chicory coffee in Reserve, LA.

So.  What's the answer?  On points / coil ignition: +12 volts with
points closed.  Some large NEGATIVE (I usually estimate about
-300 volts) voltage when they open.  This voltage will "ring"
positive and negative and will "damp" (i.e. die down, just like
a kid's swing when no one's pushing it).  How fast it is damped
depends on the circuit conditions: capacitance and load (spark
plug).  CDI systems skip the +12 volt step and just knock the snot
out of it with the 300 volts from the git-go.

So, if'n you want to read this signal, it's reasonably easy.
That's what 'clamping' diodes are for (that's just a diode you
use to 'clamp' sumpin').

Put a resistor in series with your pickup (maybe 100-k ), tie
(clamp) the other side to something with two diodes.

Example:  to trigger a CMOS gate:  put 100-k R in series.
>From the gate input, put one diode with Anode connected to the supply,
and the other with the Cathode connected to GND.

I don't like to use any kind of MOS in an app like this.  I prefer
good old BJT (read: plain ole transistor) whenever there's large
noise spikes (worst case: ignition pickup!!!!).  So, I would
drive a simple NPN transistor with the circuit described above
(substitute the base of the transistor for the CMOS gate input).

Bottom line:  the noisiest signal you're going to have to deal
with in a car:  the ignition -- including (esp.) the primary.
Second noisiest: the alternator.  Actually, everything else is
pretty quiet (except the starter).


Tom Cloud <cloud at peaches.ph.utexas.edu>