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| My metal workbench keeps shocking me (and related ESD questions) |
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| electrozap:
--- Quote from: Benta on May 31, 2022, 10:48:09 pm ---OK, last input from my side, this is getting tedious: GROUND YOUR WORKTABLE TO PE! PERIOD! Second, why are you so focused on ESD? The important thing is, that your working area does not produce any false results. An ESD mat is conductive, giving problems. Cross the t's and dot the i's, please. An isolating rubber mat would make everything reliable. IF you at some point would have to handle sensitive parts, laying out an ESD mat with a connection to your table would be easy. Same with a strap. Just for info: I've never in 40 years "zapped" a CMOS part due to ESD. Last comment: the voltages you're measuring are not inductively coupled, but capacitively. --- End quote --- The ESD mat is dissipative, not conductive. If I take an ohm meter between the surface of the mat and the bench, it will show infinite resistance. I'm so focused on ESD because I work with people who wear polyester jackets and touch my boards after I finish working on them (I'm an embedded hardware engineer by trade), and we've been trying to work on better ESD practices company-wide. We've had broken microcontrollers, MOSFETs and other components. At this point, I don't want to leave anything up to chance on my end if it's at all possible. Apologies for being tedious. --- Quote from: bdunham7 on May 31, 2022, 10:49:23 pm ---You'd need tens of µA to even get a tingle and I'd expect body leakage to be nA or less. Are the secondaries of these wall warts connected to something on the bench? Anyway, yes--ground that bench! Just be aware that static electricity shocks will still happen. --- End quote --- When I short out the bench to the wall through the multimeter, it briefly shows a milliamp and then goes back down to close to zero. The capacitive part makes sense given that it will give a short burst of energy and slowly go down to zero when I short the bench through my multimeter. When I connect the 1Mohm resistor, the voltage goes up and down relatively slowly too. Next to the metal bench at the time, I had a fan, a laptop power supply, a 48V wall wart, a second 12V wall wart, a tool battery charger (It is in my garage), a soldering iron, a second fan for solder fumes, a 3D printer, a 60V benchtop power supply, and a phone charger. All that together, the phone charger and 48V wall wart were the worst offenders, and then it went down from there. I think the fact that I had a power strip sitting on the metal bench probably also made it worse. I was previously looking for physical connections until I saw that there weren't any and moving things effected the voltage. |
| bdunham7:
--- Quote from: electrozap on June 01, 2022, 12:08:24 am ---The ESD mat is dissipative, not conductive. If I take an ohm meter between the surface of the mat and the bench, it will show infinite resistance. --- End quote --- That's not really a valid distinction--ESD mats should have some resistance, but yours probably has more than your meter can measure. https://elimstat.com/how-to-use-esd-meter-to-test-mats/ --- Quote ---When I short out the bench to the wall through the multimeter, it briefly shows a milliamp and then goes back down to close to zero. The capacitive part makes sense given that it will give a short burst of energy and slowly go down to zero when I short the bench through my multimeter. When I connect the 1Mohm resistor, the voltage goes up and down relatively slowly too. Next to the metal bench at the time, I had a fan, a laptop power supply, a 48V wall wart, a second 12V wall wart, a tool battery charger (It is in my garage), a soldering iron, a second fan for solder fumes, a 3D printer, a 60V benchtop power supply, and a phone charger. All that together, the phone charger and 48V wall wart were the worst offenders, and then it went down from there. I think the fact that I had a power strip sitting on the metal bench probably also made it worse. I was previously looking for physical connections until I saw that there weren't any and moving things effected the voltage. --- End quote --- That seems weird. If it were a capacitively coupled AC leakage, I'd expect it to be more or less continuous as you can't build up an static AC charge. What meter and what setting are you using to measure this? In any case, the solution will still be the same--ground bonding! |
| antenna:
This reminds me of something that happened to me... I have a Paco G-30 signal generator (one of those tube era kits). There was a 3-lead split capacitor (like two capacitors in series) between the two transformer connections to ground. Unfortunately, the schematic showed a two-prong AC cord and the previous owner clearly followed the schematic right down to a lack of grounding. Anyhow, what happened was the set of capacitors there acted as a voltage divider putting 60v on the case of the signal generator, and if I would set it on my metal desk, that 60v coupled into the desk where I would be consistently reminded of the problem. For that reason, I would suspect any metal-cased equipment that is running on a 2-prong cable! |
| m98:
Just to chime in on the ESD setup, at work in the lab we use solidly grounded (direct connection to PE) metal sheets as the table surface, and just lay the ESD mats on top of that. Not to say that's ideal, but it works well enough. Don't see why you'd want a separate insulator below the ESD mat, just to then again connect it to a grounding point. |
| electrozap:
--- Quote from: m98 on June 01, 2022, 07:22:52 pm ---Just to chime in on the ESD setup, at work in the lab we use solidly grounded (direct connection to PE) metal sheets as the table surface, and just lay the ESD mats on top of that. Not to say that's ideal, but it works well enough. Don't see why you'd want a separate insulator below the ESD mat, just to then again connect it to a grounding point. --- End quote --- That makes sense. I understand that putting another barrier may help with avoiding dead shorts and static with the board, but I didn't think that the mat itself would need to be separated from ground and then connected to it again. As an update to the surface, I used a drill and stepped up a small hole through the benchtop until I could drill in a flat self-tapping screw with a ring terminal through to the wood underneath, and I attached the other end to another ring terminal that I attached to PE/Earth Ground. I'm not seeing any AC voltage on the multimeter anymore, and the entire surface is grounded now. Thank you so much for the help everyone! |
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