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?