Earthed groundplane on test bench? - Page 1
Forum section: electronics / testgear

author: Circlotron
author_rank: Super Contributor
timestamp: September 20, 2013, 12:47:26 pm
content: Okay, so it's no so much a test equipment question so much as test environment.Been developing an ignition coil driver board and with about 8 amps being switched it produces a sea of noise. Placing the scope probe around the board looking for places conducted noise might be sneaking in. Then I find that if I touch the probe on the same place the probe alligator clip is grounded I can still see some residual noise even though the scope input is now shorted.Have tried to make all ground leads short and direct and single point.I was wondering if it would help to have a large earthed sheet of metal across the top of the bench, top surface suitably insulated of course.

author: Psi
author_rank: Super Contributor
timestamp: September 20, 2013, 12:51:18 pm
content: Are you testing this while the ignition coil is producing arcs?Because the arc will behave like a radio transmitter, it will throw out lots of RF crap which may couple to anything nearby including parts inside the scope.ie, merely having test equipment near the arc may result in noise on its inputs.

author: Circlotron
author_rank: Super Contributor
timestamp: September 20, 2013, 12:56:38 pm
content: Yeah, I am making arcs. Action is only about 500mm away. I'll get some longer (thick) wires and put it several metres away, maybe in an earthed tin box. But this thing with the scope leads shorted, I have seen exactly the same thing when working on SMPSs. Somehow got to get everything at equipotential at HF I think.

author: Psi
author_rank: Super Contributor
timestamp: September 20, 2013, 01:05:49 pm
content: yep, if you're getting large voltage spikes on the power wiring the will be a potential difference on even a short bit of wire.Is your circuit isolated from the mains?If not then it's possible your getting some voltage spikes flowing down your scopes earth clip to mains earth and back to your circuit.

author: macboy
author_rank: Super Contributor
timestamp: September 20, 2013, 01:08:10 pm
content: Even though you touch the probe tip to the end of the ground lead, "shorting" it out, the loop of wire (ground lead) still creates a nice effective little antenna to pick up RF garbage.

author: Circlotron
author_rank: Super Contributor
timestamp: September 20, 2013, 01:26:08 pm
content: Quote from: Psi on September 20, 2013, 01:05:49 pmIt is earthed to the mains via some other stuff it is connected to. Everything is plugged into same power outlet though. Maybe I should run a lead from the circuit to the main earth stud on the front of the scope. Would be difficult to get it shorter than 300mm or so, so with the inductance of that length I don't know how effective it would be. Still worth a try.

author: Circlotron
author_rank: Super Contributor
timestamp: September 20, 2013, 01:36:49 pm
content: Quote from: Psi on September 20, 2013, 12:51:18 pmWith the probe connections shorted and touching nothing else the scope is blank.When they are shorted and touching the earth on the board the scope shows noise.

author: jahonen
author_rank: Super Contributor
timestamp: September 20, 2013, 02:12:23 pm
content: It looks like a common mode noise problem. Those things are quite nasty and difficult to get rid of. You might try putting a ferrite clamp on the probe coax, that might help a little.Regards,Janne

author: Whuffo
author_rank: Regular Contributor
timestamp: September 20, 2013, 05:23:58 pm
content: Conductive topped workbenches are a tragedy waiting to happen. It's just a matter of time before you get between some voltage and the bench top.If you're working with solid state electronics, an anti-static mat on the bench top is a good idea. Otherwise, stick to non-conductive tops. You'll be glad you did.

author: Circlotron
author_rank: Super Contributor
timestamp: September 21, 2013, 02:22:14 am
content: Quote from: Whuffo on September 20, 2013, 05:23:58 pmIndeed.Quote from: Circlotron on September 20, 2013, 12:47:26 pm

author: Circlotron
author_rank: Super Contributor
timestamp: September 21, 2013, 02:25:33 am
content: Quote from: jahonen on September 20, 2013, 02:12:23 pmAh, yes! I'd totally forgotten about that approach.

author: Whuffo
author_rank: Regular Contributor
timestamp: September 21, 2013, 02:23:33 pm
content: Quote from: Circlotron on September 21, 2013, 02:22:14 amSuitably insulated from what? Earth ground, chassis ground, you? Once you get past 20 volts or so, you're entering a dangerous area. I lost a friend when he was leaning on a chassis and stuck a hand in the wrong spot. For what it's worth, you are more conductive than you think - and 40 MA flowing from hand to hand is enough to stop your heart and kill you instantly.My workbenches are topped with 5/16" tempered glass. No accidental shorts, no weird capacitive effects, nice and safe. And the solder blobs and other smut are easily scraped off with a paint scraper - leaving an undamaged surface. If I couldn't afford this, I"d go with plywood - maybe with some Formica on top.And always remember: the power strips go on the BACK of the bench, not along the front of it. That loose end of your solder roll will find a way to give you a very unpleasant surprise otherwise.Do the math; determine the breakdown voltage of the "suitable insulation". If you're working with ignition systems, the voltages get pretty darned high. In free air, 40 KV will easily arc over a 1 inch gap.

author: Circlotron
author_rank: Super Contributor
timestamp: September 22, 2013, 05:12:54 am
content: Quote from: Whuffo on September 21, 2013, 02:23:33 pmEarthed metal groundplane insulated from everything else by a suitable covering. Slightly conductive static mat maybe.