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On my board there is a 4-pin connector as follow:

  1. +24V DC (up to 6A)
  2. RS485/A
  3. RS485/B
  4. GND

On the system there is no real earth, because there are no metallic parts (chassis, case, structure, etc...).

I'm going to protect the board as follow:

  • SM712 between I/O lines and the "virtual" earth (see below)
  • unidirectional TVS with 24V stand-off voltage between power and "virtual" earth
  • two Schottky in antiparallel between GND and "virtual" earth

The "virtual" earth is nothing but a large PCB trace that runs along the board edge (where the connectors are placed) and is connected to the actual GND at the power input connector through an inductor.

The idea is to provide an easier way for the spike's current to flow instead of enter inside board.

Here a schematic that summarize:

schematic

simulate this circuit – Schematic created using CircuitLab

What do you think about?

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  • \$\begingroup\$ So you don't have earth ground for common, but if everything is referenced to the GND input, so that is your common ground. All those inductors and diodes (L1,D1,D2) will make it worse for the protection, so remove them and replace with short circuit on the schematic. \$\endgroup\$
    – Justme
    Jan 27, 2021 at 5:25
  • \$\begingroup\$ Ok, but it makes sense to keep the trace separated along the board edge but the power input? \$\endgroup\$
    – Mark
    Jan 27, 2021 at 5:42
  • \$\begingroup\$ Are you suggesting to connect using tiny traces the GND to this large "dummy" earth trace? \$\endgroup\$
    – Mark
    Jan 27, 2021 at 5:44
  • \$\begingroup\$ No, tiny traces are also bad. As wide as possible. Ideally there should be a ground plane. \$\endgroup\$
    – Justme
    Jan 27, 2021 at 5:51
  • \$\begingroup\$ @Justme, sorry I don't get the point. If I place a ground plane (GND) any spike that comes on the GND wire will spread across the whole board, doesn't it? \$\endgroup\$
    – Mark
    Jan 27, 2021 at 8:44

1 Answer 1

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Don‘t get tricked into thinking you can control where some voltage or current spikes will flow and where not.

Usually, the ultimate goal of ESD protection is to ensure that the victim (IC) will never get more voltage than it can take.

So your design goal of ESD protection is to use protection devices to kind of clamp all signals, GND and VCC together. So if e.g. GND suddenly goes below -2000V, the other lines must follow. Think of the sensitive IC being sandwiched between protection devices.

Further advice: L is useless or even bad. Place the TVS between 24V and GND (explanation above).

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