Electronics > Beginners
Negative Voltage spikes on scope. Automotive application
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:
--- Quote from: james_s on August 22, 2018, 10:39:51 pm ---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.
--- End quote ---
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:
--- Quote from: Joe Dillman on August 23, 2018, 01:47:27 am ---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.
--- End quote ---
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.
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