Unexpected waveform behaviour from current clamps - Page 1
Forum section: electronics / testgear

author: martinrub
author_rank: Newbie
timestamp: January 31, 2017, 04:54:07 pm
content: I'm seeing some unexpected behaviour when I use a current clamp.I'm using a Fluke i30s AC-DC current clamp, but I've seen the same thing with a cheap Hantek CC65 clamp.I'm scoping the current flow in an automobile high-tension, ignition secondary lead, (spark plug and coil leads), and it DOES NOT matter which way round I clamp the current clamp, the waveform looks the same - I had expected the positive part of the waveform to show negative and vice versa when the clamp was reversed.Any idea why the waveforms look the same instead of mirror images about the time axis when the current clamp is seated 180 degrees about?Hopefully, the 2 attachments will show almost identical-looking waveforms independent of which way round the current clamp was placed.

author: artag
author_rank: Super Contributor
timestamp: January 31, 2017, 04:58:36 pm
content: Perhaps you're not really seeing the current, but the voltage spike from the source picked up by the wiring to the current probe.Connect the earth of a voltage probe to the tip of the probe, minimising the size of the loop of earth wire. Then put the probe or its lead near the high voltage wire .. see how easily it picks up a signal.

author: ADT123
author_rank: Regular Contributor
timestamp: February 01, 2017, 05:11:54 am
content: The secondary ignition signal is very high voltage (>10,000V) but very low current. The clamp is not picking up any current, just interference - hence it does not matter which way around you use it.Suggest you use the clamp to measure the current feeding the supply to the ignition coil (primary current). You can used the scope to measure the primary voltage (typically about 300V so an attenuator is required). For secondary voltage you need a ignition pickup.Try following the guides herehttps://www.picoauto.com/library/automotive-guided-tests#t4Select the type of ignition system you have in the middle column then the signal you want to measure on the right.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 01, 2017, 10:23:29 am
content: This is my test jig. Just a small pressure chamber with the 4 plugs, various coils, power supply, the ignition and a board with a micro and FPGA to simulate signals to the ignition. I have different harnesses to test brands of ignitions.To look at current, I use a Pearson wide band current transformer, model 410. Scope shots showing the secondary current and voltage. For this setup the current's rise time is about 18us. It decays in about 400us.An arc from little too high of pressure, no boot and too wide of a gap. In the last three you can see the line where the boot stops. Tracks damage the ceramic.The newer LeCroy is effected by these tests, not to mention the USB will normally fail. The old LeCroy has no problems and if something were to go wrong, I have spare parts to repair it.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 01, 2017, 10:31:29 am
content: joesmith is correct.The best way to measure the ignition current is to use a fast reacting pearson current probe, either model 410 or model 110 will do well.The Fluke i30s will not work at all.You will see typically about 50 mA peak current and a linear decay to 0 mA over the time of the spark duration.

author: martinrub
author_rank: Newbie
timestamp: February 01, 2017, 06:36:36 pm
content: Thank you, everyone, for such clear and detailed replies.Earlier, I went back to basics and looked at a simple sinusoidal current of around 5A using 2 current clamps side by side. On the first run, the 2 waveforms were in phase. And when I reversed one of the clamps the waveforms were 180 degrees out of phase, exactly as expected.Many thanks for pointing out the error of my ways. It all makes perfect sense now. I'm most grateful.