Negative Voltage spikes on scope. Automotive application

[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]

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? 

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. 

[james_s]

Careful poking around the ignition system with a scope, even on the primary side you can get spikes that can potentially damage your equipment under the right circumstances. I think the other posters answered your initial question.

[drussell]

Careful poking around the ignition system with a scope, even on the primary side you can get spikes that can potentially damage your equipment under the right circumstances.

Indeed.  The voltage spikes on the primary can often be in the 400-1500 volt range, depending on the ignition type.  You should probably use at least some kind of divider with some clamping to protect the scope from transients.

I usually use the magnetic inductive pickup from my timing light connected to the scope when I'm trying to probe the various signals on the actual spark plug wires when there is a distributor since you can watch either one specific cylinder or all of them by using the main coil-distributor wire depending on where you clip the pickup and can get a reasonably good representation of what is going on in the actual wire, though that obviously doesn't work when we're talking coil-on-plug style ignitions...

[Joe Dillman]

Good advise guys.  Yes I'm well aware of the dangers of ignition voltage.  My Pico has built in 250V protection plus I use a 10:1 on primary.  Never seen higher than 400v. Would depend on turns ratio, but still a faulty coil could send HV to the wrong place.

Most stuff I deal with is coil on plug.  There are still two wire cop coils that primary voltage measurement can be taken but many of them are three wire setups with built in IC so only power ground and command signals present.  No large spike on the ground of that kind.

[drussell]

Most stuff I deal with is coil on plug.  There are still two wire cop coils that primary voltage measurement can be taken but many of them are three wire setups with built in IC so only power ground and command signals present.  No large spike on the ground of that kind.

Yup...  No weird spikes but also no useful information on how the plug is actually firing. 

It's either getting the logic signal to tell it to fire, or it isn't.  That, unfortunately, doesn't tell you diddily squat about what is actually going on at the plug (unless you measure the supply current), but it seems that you understand all of that already. 

[james_s]

The other thing you have to watch out for is flashover, it could happen inside the coil but I've also had it occur externally, once to my hand when I was fiddling with something. Caused by a worn boot on the cable or some moisture on the tower, I'm not really sure which but it hurt.