I'm designing a measurement and control board to interface with an old (70s) PLC. The board consists mainly of I/O ports, both analog and digital in- and outputs. To have my boards not fried by the first thunderstorm, I'm trying to ESD protect these inputs.

A few remarks:

  • wires of up to 200 m length connect to the ports
  • these wires are not especially shielded. It's not bell wire, but close enough.
  • I cannot change the wires.
  • There's no high speed signals going over these wires. The fastest signal is a (up to) 115 kbaud RS-485 bus, everything else is below 50 Hz.
  • I have voltage levels of ~1 V (RTD excitation), 10 V (0..10 V analog) and 24 V (digital "power" outputs)
  • every input consists of a signal- and a ground line

My ideas so far:

  • directly connect the ground wire to the respective board ground (PWRGND, AGND, DGND)
  • connect signal to its ground with a 100nF capacitor to short HF
  • connect signal to PWRGND with a single bidirectional TVS diode to divert high-voltage peaks e.g. induced by lightning close by
  • maybe connect a SMD ferrite bead (or other small inductance) in series

After looking for parts, however, I'm a bit confused. Mainly, there's lots of high-speed data line ESD protection ICs, but no dedicated (and cheap) low-speed ESD protection ICs. This made me wonder if I'm doing anything wrong.

What's the suitable approach to protect a circuit board in these conditions from overvoltage?

  • \$\begingroup\$ Lightning strike will also induce current into large loops of cable (it acts like an air core transformer, the lightning bolt is the primary conductor generating a magnetic field, and whatever cable loop which happens to pick up the flux is the secondary). This is why lightning fries cable/DSL modems, which are at the intersection of potentially very long cables. If your cables are earthed on both ends and create a ground loop, this may occur. \$\endgroup\$ – peufeu Oct 9 '17 at 19:07
  • \$\begingroup\$ I've thought about that. Nothing but the topmost star point is earthed. (Does that suffice?) \$\endgroup\$ – Finwood Oct 9 '17 at 19:15
  • \$\begingroup\$ It should be OK, if there are no loops. If the sensor at the end of the cable is mounted on metallic stuff though, make sure the insulator is rated for high enough voltage (several kV). Nothing fancy or expensive, but 0.5mm creepage on your PCB isn't what you want... \$\endgroup\$ – peufeu Oct 9 '17 at 19:17

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