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Making an oscilloscope probe for automotive ignition.

ClydeCrashKop

Well-Known Member
Most Helpful Member
Hi ETO Members
I am looking for info on making an oscilloscope probe / pickup / interface for use with automotive ignition.
Real automotive scope leads just clip over the coil wire and #1 plug wire.
How do I pick up these 20KV signals on a lab scope?
And a trigger signal from #1 plug wire?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I'd try a plastic clothespeg with the end of some insulated wire wrapped around one side, so it will be pressed agains the plug lead insulation for capacitive coupling when the peg is clipped on.

Add a fairly high value series resistor (10K?) from the wire and a couple of inverse parallel schottky diodes between that point and vehicle chassis ground.
Connect the scope across the diodes, ground to ground.

That should hopefully give a signal you can trigger the scope off safely?

If you want to view the waveform, try chains of several diodes rather than single ones each way so it clips at a somewhat higher voltage, and add a pot across the diodes to adjust the attenuation. You may need a rather higher value series resistor and a fairly high pot value, to minimise waveform distortion through the very low value capacitance of the clip on coupler.
 

Nigel Goodwin

Super Moderator
Most Helpful Member

ClydeCrashKop

Well-Known Member
Most Helpful Member
The one I linked to is a x10,000 capacitive attenuation

What am I looking at here? This is an automotive scope pickup that snaps on top of an old GM distributor. What looks like a wire wound resistor measures 335 ohms. It looks like a plate capacitor. The brass is about 2” square & ½ inch from the body. There is a little brass strip that can rotate. I guess for fine tuning.
It has worked fine for me and shows a detailed ignition pattern by just laying it on the coil wire.
I am trying to help an automotive group that I am involved with.
GM Dist scope pickup.jpg
 
Last edited:

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
It looks like it tilts and you can also vary the separation. I guess the box is one plate of the capacitor and the brass is the other side.

Separation varies the capacitance. So would a varying dielectric. See https://www.physicsforums.com/threads/capacitance-with-non-uniform-dielectrics.539825/

My guess is that you have a course (parallel plate distance) and fine (angle) adjust.

Then you have the standard scope probe compensation adjust and the divider similar to a scope probe. https://hackaday.com/2017/03/15/how-an-oscilloscope-probe-works-and-other-stories/
 

ronsimpson

Well-Known Member
Most Helpful Member
I think ALIE is cleaver.
1631624210686.png

I used two turns of copper tape around the wire. Then connected the scope to that. I can see using a piece of pipe, split to go around a wire. The problem with capacitive coupling is that the "home made" capacitor will be different every time. There will be no DC but I think that is OK in this case.

I have bought and made these. They will read DC and AC. Hard to make a good connection.
1631624358710.png
 

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