I'm examining an RS-485 driver circuit that uses what look like Zener diodes between D+ and gnd and D- and gnd. Anode is connected to gnd, cathode to the D+ or D-. So in this arrangement the idea is for the diodes to shunt current if gnd goes above D+ or D-? Circuit is at http://resplendid.com/rs485connector_withdiodes.png.
How effective in general are Zeners at this type of overvoltage protection? On an RS-485 transmission line I would think some pretty large voltage spikes could be induced, wouldn't the diodes blow up pretty easily with a large spike? When blown, to they fail open or closed circuit? Maybe putting some MOVs on the bus would be a better idea? In general what's a good protection scheme for an RS-485 network that's going to be used outside?
Thanks,
Fred
Best Answer
I wouldn't trust a standard zener to protect a line running outside. Can you show the circuit? Are you sure it doesn't mean a TVS diode anyway?
You could use a single (say 6-10V) TVS on the lines as you mention.
A more expensive but more robust solution would be to use a three stage protector like this (or make your own)
It combines a gas discharge tube with a series impedance and a TVS. You get the benefits of the gas discharge (high current capability, high breakdown voltage, slow) with the TVS (fast, low clamping voltage)