Electronics > Beginners

Negative Voltage spikes on scope. Automotive application

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Joe Dillman:
Hello everyone,  My very first post ever, I'm an automotive technician with a basic understanding of electricity but am eager to learn.

If I take the negative lead of my Pico 4425 scope and attach it at the battery negative post and the other lead to the negative side of an ignition coil why will it get voltage spikes below the ground level voltage? How is it possible for any voltage to ever be measured at negative levels when scope or meter is attached this way?  How could anything have a greater negative potential than the battery?

See attachments.

Thanks!

tautech:
Welcome to the forum.

With high energy/voltage discharges it's normal to see some waveform ringing.
This can be the measurement method or back EMF from the coil itself.
As the spark is discharged into a gap, it's a nonlinear load unlike a resistor or such.

Coils (inductors) are useful circuit building blocks and can be the core to generate negative or positive supplies. All you need add is an appropriately biased diode.

Joe Dillman:
Thank you for the info!  Attached is a current waveform of an ignition coil that has an open spark plug wire.  I've noticed that coils with open secondary will show a negative amperage spike when turned off.  Properly operating ignition systems dont produce the negative amperage spike.  Do you have an explanation for that? 

The attached is a resized screenshot from a presentation I made awhile back. Forgive me for the poor image quality.  As automotive technicians we tend to know how to ID a fault but can't always explain the waveform. Normally it doesn't matter to us why something looks the way it does on a scope.

ogden:

--- Quote from: Joe Dillman on August 22, 2018, 11:35:32 am ---Thank you for the info!  Attached is a current waveform of an ignition coil that has an open spark plug wire.  I've noticed that coils with open secondary will show a negative amperage spike when turned off.  Properly operating ignition systems dont produce the negative amperage spike.  Do you have an explanation for that? 

--- End quote ---

Inductance of ignition coil secondary with capacitance of connectors and spark plug wire forms LC resonant circuit which is causing ringing you already noticed. LC resonance in Laymans terms is energy bouncing between capacitor and inductor. When you connect spark plug - it's discharge takes lot of energy off LC resonant circuit so it does not ring so much.

Joe Dillman:
Thanks for the explanation and link. 

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