How to protect floating device from ESD ? - Page 1
Forum section: electronics / projects

author: nutron
author_rank: Newbie
timestamp: September 28, 2025, 07:51:48 pm
content: Hello.Could somebody please help me to understand something about the fundamentals of ESD protection in "floating" devices?I would like to know how to protect a device that is made of a PCB inside of a plastic case with a metal bodied push button and mounted to a wall.One would think that because of the plastic and the battery power there would not be a path for the electricity to flow.In this picture i put a metal can on top of a plastic box and even there i could observe static discharge when i touched the metal can after being statically charged.

author: TimFox
author_rank: Super Contributor
timestamp: September 28, 2025, 08:10:32 pm
content: The IC that needs protection doesn't know where "ground" is.Normal protection circuits, of which many can be found in the usual places, limit the voltages on the IC pins with respect to the power pins of the IC itself.Similarly, one can protect an input connection from excessive voltage with respect to the circuit-board power rails, and protect the power rails from excessive voltage from each other.You will still get "zapped" when walking across carpet in low atmospheric humidity to touch something.

author: PGPG
author_rank: Frequent Contributor
timestamp: September 29, 2025, 10:42:38 am
content: 1. You should have continuous GND plane.2. You should have limit all input/output wires by using Zener diodes (there are special destined to ESD) connected to this GND plane.3. You should add some impedance between these protections and rest of your circuit to allow the most current be going through your external protectors and not through your circuit.About my point 3:Long time ago in one of 'Tidbits' at these homepage:https://emcesd.com/I have read that he has measured voltage at SMB small voltage transil during ESD and got 200V pulse. It is due to its parasitic inductance (SMB is a big case) and very high dI/dt of ESD pulse.At special ESD protection Zeners you should expect smaller pulses (smaller inductance) but they will be much bigger (assume for example 50V, but very short) than nominal Zener voltage.My point 3 is about isolating this pulse from your IC pins.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 29, 2025, 11:35:06 am
content: Quote from: nutron on September 28, 2025, 07:51:48 pmA return path like a shared earth or ground is not required for ESD to occur. This demonstration shows ESD between two bodies with no obvious return path:If this is counterintuitive from a circuit point of view, then good, it should be.

author: tszaboo
author_rank: Super Contributor
timestamp: September 29, 2025, 12:09:37 pm
content: If you inspect the image below, it's a test setup for ESD compliance testing.You will have some capacitive coupling between the large GND plane inside your device and the metal sheet on the desk.If you don't have a large ground plane, than you have some capacitive coupling between every single line in your device and the desk. One of these choices makes your life simple.

author: ledtester
author_rank: Super Contributor
timestamp: September 29, 2025, 12:26:18 pm
content: Quote from: tszaboo on September 29, 2025, 12:09:37 pmFor some reason the image is not showing in my browser, so here is the URL:https://www.researchgate.net/publication/350209279/figure/fig2/AS:11431281415247740@1746037853527/ESD-testing-configuration-according-to-IEC-61000-4-2.tif

author: Feynman
author_rank: Regular Contributor
timestamp: October 02, 2025, 04:37:58 pm
content: If two objects on a different potential come close to each other they try to exchange charges to even out the potential. When the exchange is happening very rapidly due to high conductivity of the objects, you have an ESD event. It has nothing to do with one object being grounded/earthed or not. The exchange of charges happens no matter what.Actually, it's the other way round: If both objects were connected to ground/earth, there wouldn't be a potential difference to cause an ESD event in the first place.That's what's done in electrostatic protected areas: Everything is connected to one common potential (protective earth in most cases). No potential differences, no ESD events.

author: helius
author_rank: Super Contributor
timestamp: October 02, 2025, 04:44:18 pm
content: Quote from: Feynman on October 02, 2025, 04:37:58 pmOf course the requirement to be connected to a common potential at all times is impractical for any serious task (just consider how you get a PWB into or out of the protected area, or how you apply adhesive tape). If ionized air of both polarities is directed at the work bench, charge differences will be neutralized in a gradual, safe way before they have a chance to cause ESD events.

author: nutron
author_rank: Newbie
timestamp: October 06, 2025, 04:10:10 am
content: Let's say we use theleftcircuit where there mounting holes are attached to the ground plane (left circuit on the PCB).I assume the that the most likely path for the flow of electricity would be through the wire , through the PCB , through the plastic case , to the wall.Let's say we use therightcircuit where there mounting holes arenotattached to the ground plane (rightcircuit on the PCB).I assume the that the most likely path for the flow of electricity would be through the case , to the wall.The other possibility could be that the negative and positive charges accumulate at the switch and the finger of the human and the destructive forcesmanifest in the circled "destructive zone". anything in that zone could be damaged by the discharge.Would ESD protection be even required in that case ?