Inductive probe for ignition-test? - Page 1
Forum section: electronics / beginners

author: cs.dk
author_rank: Unknown
timestamp: June 30, 2012, 04:18:53 pm
content: Hi,I have a car in my shop i want to check for misfires.Is it possible to wrap a wire around the sparkplug leads, and then hook the oscilloscope on? Or do i need some expensive HV-probe?

author: Baliszoft
author_rank: Frequent Contributor
timestamp: June 30, 2012, 09:07:46 pm
content: Just put the probe next to the coil(s) (no direct contact to the leads, just inductive). You will be able to detect dwell (charge time) and spark duration. No fancy probes are required.

author: digsys
author_rank: Unknown
timestamp: June 30, 2012, 11:53:12 pm
content: Or make a simple coil, connect between the probe and GND lead. Just a few turns of any wire, 1mm +/-Signal level will be low, but should be enough. Then you'll get a nice waveform, if useful.If you make 2 the same, you can use your 2ch scope and compare cylinders.

author: cs.dk
author_rank: Unknown
timestamp: July 01, 2012, 09:39:29 am
content: Quote from: digsys on June 30, 2012, 11:53:12 pmThanks for your answersThe coil-thing was my idea too.. Hope it will work.

author: digsys
author_rank: Unknown
timestamp: July 01, 2012, 09:47:27 am
content: QuoteYup. It's a trade-off. Low turns = low signal, low distortion. Lotsa turns = better signal, more signal distortion.You can also wrap the coil around the HT lead and look at the current waveform.Make sure the HT leads are DRY and in good condition, or you'll get a surprise from the HT fairy :-)

author: cs.dk
author_rank: Unknown
timestamp: July 01, 2012, 09:51:06 am
content: More windings = More distortion?Thought more windings = better pickup, less distortion.

author: digsys
author_rank: Unknown
timestamp: July 01, 2012, 10:16:34 am
content: QuoteUsing an inductor as a voltage pick is NOT ideal, you are basically charging / discharging an inductor andtherefore totally skewing the real signal, which has quite low energy transfer.Seeing as the OP most likely didn't need a 1% accurate waveform, this is a well good enough "quick and dirty"method, and quite safe. You don't really want to put a HV resistive divider on the HT :-)

author: cs.dk
author_rank: Unknown
timestamp: July 01, 2012, 10:50:29 am
content: Thanks for clearing up my (stupid) questionsNo, I don't need that accuracy, i just need to see if there is a spark or not. The car has some minor misfires i believe.

author: SeanB
author_rank: Super Contributor
timestamp: July 01, 2012, 11:03:53 am
content: More windings - bigger inductance, combined with cable capacitance means it rings with each edge, so distorts the signal.Try a small copper foil trip inside the jaw of a clothes peg clipped to the cable as a capacitive pick up, connected to the input of a X10 probe. Will make a voltage divider and gives a pretty good faithful signal. Clean the cable where you clip it on so that leakage does not fry the scope. Connect ground lead of probe to chassis of car. Start at 10v/div and go down to get a usable signal, there will be quite high spikes there, and you will find that it works best if you use a dual channel scope, set ch2 to do the triggering and use a probe on ch2 connected to cylinder1 and ch1 probe connected on the coil wire ( assumes you have a distributor) and enables you to see all the pulses in a line, so you can see low cylinders, misfires etc.On a coil per plug pair car you are going to need cylinders/2 probes and that many channels unless you go pair by pair.

author: Unknown
author_rank: Unknown
timestamp: July 01, 2012, 11:08:14 am
content: Engine Analysis by Jim Thurman 1974Possibly the most comprehensive source on the matter.You can find it here among many other good books.http://www.slack.com/concepts.html

author: T4P
author_rank: Super Contributor
timestamp: July 01, 2012, 12:23:14 pm
content: Sure you can get more voltage out of more windings but more windings = inductanceInductance = Some sort of filter