Author Topic: Spark plug tachometer  (Read 11880 times)

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Offline RyersonBaja

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Re: Spark plug tachometer
« Reply #50 on: August 20, 2017, 07:06:40 pm »
There's a trade off with amplitude and ringing management from the shunting along with decreased sensitivity.
With scope measurement that's comparatively easy to work around as the scope will give frequency (RPM) at the trigger level setting where you can set this above the ringing to get a cleaner result.

This is why I suggested you use something at home like a lawnmower or motorbike to get the pickup part roughly optimised before moving forward.

Thanks for the small engine suggestion, but I'm stuck doing all the work at school. I don't have a small engine at home (don't own a motorbike, electric lawnmower), and the oscilloscope belongs to the school anyways. Not really much point in me spending money on my own test equipment when the Uni lets me use whatever I need, as long as it stays at school.

Still getting sorta the same problem as before, with the 120ohm in parallel with the diodes. Diodes are really acting more like capacitors. See:

For comparison, I replaced the diodes with a 1uF cap in parallel with the 120ohm.

Its definitely attenuating, as compared to 120ohm alone, but the diodes seem to be turning on too slowly to clamp the signal.

Do I just need a much faster diode? I didn't think I would, the signal seems to be on the order of ~50MHz, and 1N4148s should be good well past this.

Would the following be worth trying? (component values are just a guess right now, might be worth trying higher values of R2/R3)

My idea here would be to limit current through the diodes. C1 is probably not necessary, but could be chosen for additional attenuation. I'll try it monday and see.
« Last Edit: August 20, 2017, 07:15:48 pm by RyersonBaja »

Offline tautech

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Re: Spark plug tachometer
« Reply #51 on: August 20, 2017, 08:10:41 pm »
R1 value can be reduced further, even halved with a pair of 120's paralleled.
I'd fiddle with initial attenuation to get the ballpark value you need and then apply clamping to limit output.
1N4148's should be plenty fast enough but you could look at Shockley diode data sheets but with the disadvantage of needing two in series to give similar clamping value to a silicon diode.
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Offline Delta

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Re: Spark plug tachometer
« Reply #52 on: August 20, 2017, 09:42:03 pm »
 :horse: :horse: :horse:

There is a ready made signal just waiting for you on the Magneto's kill wire.  Why are you insisting on fannying around trying to inductively couple to an HV wire?

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Online Ian.M

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Re: Spark plug tachometer
« Reply #53 on: August 20, 2017, 10:29:09 pm »
Probably.  However if the 'magneto' has an integrated electronic ignition system, the kill wire may be a 12V logic signal.   5 minutes running, testing the line with a DMM on frequency and a 0.47uF HV capacitor in series with the test lead would tell if there is a RPM related signal there.

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