"Grounding" metal case without ground - Page 1
Forum section: electronics / beginners

author: daqq
author_rank: Super Contributor
timestamp: April 01, 2014, 11:21:37 am
content: Hi guys,I'm working on something that should have a metal case, isolated from the rest of the device and isolated itself (heavy coat of paint), powered by a DC 12V wall wart adapter. The electronics are inside of the case.There's a problem though - when ESD builds up on me and I touch the case, ESD discharges and in rare cases discharges into the electronics inside, which causes nasty stuff.Is there any way around this?The quickest would is to connect the metal case with the GND of the electronics inside, but that presents another problem - while the case is isolated (heavy coat of paint), I don't consider this perfect and some switched wall warts have nasty (if weak) potentials on them between their GND and the mains earth.I've wondered about connecting the case through a high value resistor/cap onto the internal GND - something that would provide a "ground" path to the pulse, but be OK for normal situations.Is there any standard quick way of doing this? Or is some serious redesign needed?Thanks,David

author: oldcrow
author_rank: Newbie
timestamp: April 01, 2014, 05:33:23 pm
content: Two solutions.The first would be to go to a plastic box but this would only be a partial solution. I have taken some good shocks off of laptop computers that were running on the wall warts despite the plastic cases.The second would be to include a power supply inside the metal box and ground the whole mess thru the mains cord. You could probably take the guts out of the wall wart then mount it inside the box. Just remember to remove the paint down to bare metal for ground connections.

author: IanB
author_rank: Super Contributor
timestamp: April 01, 2014, 06:05:31 pm
content: Try isolating the case completely from everything else, especially the wall wart input jack (in other words, don't let either the positive or negative side of the DC input be connected to the case). As long as the metal case is a complete enclosure and is fully insulated from any outside connections then no potentials on the outside of the case can affect the inside (Faraday cage effect).

author: daqq
author_rank: Super Contributor
timestamp: April 01, 2014, 06:09:42 pm
content: Thanks guys,Damn... so there's no simple solution that does not involve mechanical changes?

author: Zero999
author_rank: Super Contributor
timestamp: April 01, 2014, 06:57:32 pm
content: Try connecting a gas discharge tube between the case and negative rail and a 16V zener across the power supply rails.

author: michaelym
author_rank: Regular Contributor
timestamp: April 01, 2014, 07:00:38 pm
content: If you want to connect the PCB to the chassis I think what you are looking for is a TVS diode aka TransorbIf the PCB inside is isolated from the case and you have a large enough air gap it should be ok.If the screws are connecting the PCB to the chassis (with plated holes) you can maybe remove the plating or if you are using metal spacers for example you can maybe switch to plastic ones.I think the rule of thumb for ESD penetrating air is 1kV/mm.

author: con-f-use
author_rank: Unknown
timestamp: April 01, 2014, 07:04:39 pm
content: Static dissipative coating, available in spray cans?If the wall wart is isolated and only 12V, connecting one rail to the chassis?

author: Dave Turner
author_rank: Frequent Contributor
timestamp: April 01, 2014, 08:22:21 pm
content: I wouldn't rely on paint as an insulator.A Faraday's cage is always vulnerable at penetration points e.g. power and signal paths. This is frequency dependent.I suggest increasing the insulation thickness at penetration points by some means and ensuring a good separation of the internal components from the enclosure.See what effect that has.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2014, 12:59:41 am
content: Maybe I don't understand the OP. If the metal case is "isolated," how can that be the cause of the ESD? If the case is shocking you, I'd think it's either grounded or building up some kind of static charge?

author: IanB
author_rank: Super Contributor
timestamp: April 02, 2014, 01:33:54 am
content: I think the metal case cannot be truly isolated. It must somehow be connected to a discharge path for the static to follow. For example, the input jack for the 12 V supply may be connected to the case with a metal bezel.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2014, 02:13:48 am
content: If that's so, and the electronics are being effected by the ESD, wouldn't that imply that something electronic isn't grounded properly?

author: IanB
author_rank: Super Contributor
timestamp: April 02, 2014, 02:57:28 am
content: Grounding isn't necessarily the best idea for ESD protection. ESD damage is caused by electric currents flowing from one place to another along paths that are not designed to carry that current. If your body happens to be charged up with static electricity, then ground represents a perfect destination for that charge to flow. Suppose now your electronic circuit somehow stands between you and ground, then the charge may find a way through the circuit on its way to ground. It's a bit like standing in a giant puddle in the middle of a lightning storm. You are nicely grounded, but also rather vulnerable. Not a good place to stand.To defend against ESD the circuit should either be isolated from its surroundings so that unwanted currents can't reach it at all, or the circuit should have equipotential bonding in such a way that any static discharge currents that reach it flow round it and not through it. Isolation usually works best for small circuits and equipotential bonding usually works best for big installations.

author: daqq
author_rank: Super Contributor
timestamp: April 02, 2014, 08:10:10 am
content: Thanks for the replies!QuoteMy apologies for not being clear. The case itself is not the cause of the discharge - basically, when I come up to the device already charged up and discharge my finger into the case an arc occasionally goes onto the case and through the case it occasionally discharges into the electronics inside.

author: johansen
author_rank: Super Contributor
timestamp: April 02, 2014, 08:18:39 am
content: Quote from: daqq on April 02, 2014, 08:10:10 amyou have no choice then but to stop the build up of static charge, or discharge it slowly through say, a 1M ohm resistor.

author: VK5RC
author_rank: Unknown
timestamp: April 02, 2014, 09:36:46 am
content: Are you sure it is discharging inside the metal (Faraday) cage, or is it lifting differentially one line of the input voltage supply?

author: daqq
author_rank: Super Contributor
timestamp: April 02, 2014, 09:38:54 am
content: QuoteI'm pretty sure it discharges into a random place on the board - it has caused a reset of the system - that's how I even noticed itQuoteSo, I should connect the chassis to the GND of the electronics with a 1Meg or so resistor then?

author: IanB
author_rank: Super Contributor
timestamp: April 02, 2014, 03:48:14 pm
content: Quote from: daqq on April 02, 2014, 09:38:54 amI don't think this mechanism is possible--physics does not allow it.There is presumably some opening in the case somewhere (such as where the DC supply comes in), and the discharge to the case generates a signal that travels inside the case through that opening, probably travels along the power supply wires, and reaches the circuit that way.If the mechanism is not a differential lifting of one line of the power supply, it possibly could be an HF pulse traveling along the power supply wires instead. In that case some kind of HF filter in the DC power input may help.I'll wager that if you replaced the power supply with a battery inside the case and disconnected the DC input jack, then the problem would not occur (although other connections to the outside of the case such as case mounted potentiometers or switches could also provide a path into the circuit).

author: linux-works
author_rank: Super Contributor
timestamp: April 02, 2014, 04:05:28 pm
content: I assume you have a 3 prong ac cord going to the chassis?if so, then the chassis should be grounded. the circuit's ground can use a ground-breaker (usually .1mf X-type 250v cap and 10ohm resistor in parallel). see rod elliott's site for his view on ground breakers.there are also ESD diodes you can use to shunt static current to ground and away from controls.you could isolate the controls (if the controls are what get the sparks) via plastic shafts that go between the knob and the pot or rotary. you can have the switches be logical switches and not directly control things; then use opto couplers to isolate the front panel UI from the real circuitry.

author: Marco
author_rank: Super Contributor
timestamp: April 02, 2014, 04:41:43 pm
content: Quote from: daqq on April 01, 2014, 11:21:37 amOne more case inside the case at virtual ground ...I wonder though, why not just get a wallwart of known good construction so you can depend on the galvanic isolation and just connect the outer case to virtual ground?

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2014, 04:42:46 pm
content: Quote from: linux-works on April 02, 2014, 04:05:28 pmI think the OP said it was a 12V DC wall wart. I've never seen one with a ground, that I can recall? Do they make those?

author: edavid
author_rank: Super Contributor
timestamp: April 02, 2014, 05:06:19 pm
content: The circuit board is capacitively coupled to ground through the wall wart. You need to harden your circuit against ESD in the usual ways... clamp diodes, capacitors, RCs, LCs. You said it's resetting, so start with the reset line... try adding a zener clamp and small capacitor to it.