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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.

Circuit

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

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  • \$\begingroup\$ Zeners are usually used in reverse breakdown rather than the ordinary forward conduction. So the figure you show would have the zeners conducting when either D+ or D- go "far enough" above GND. This would clamp the levels on the data wires relative to ground, and would be a form of protection for the inside of the box assuming the zeners were properly selected. \$\endgroup\$
    – RBerteig
    Commented Nov 2, 2011 at 8:28
  • \$\begingroup\$ Thanks, I understand now. I'm trying to get the BOM so I can see what actual part is being used. \$\endgroup\$ Commented Nov 2, 2011 at 15:59

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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)

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  • \$\begingroup\$ I added a link to the section in the schematic. Perhaps they are special diodes, the TVS diodes you mention sound ideal. I don't have the BOM yet so don't have their part numbers. \$\endgroup\$ Commented Nov 2, 2011 at 6:46
  • \$\begingroup\$ @fredbasset - Thanks for the schematic - it's a bit hard to see the junctions but the diodes would clamp excess voltage to ground when reverse breakdown is exceeded as RBereig says. When you find the part numbers out you can confirm whether they will be suitable. \$\endgroup\$
    – Oli Glaser
    Commented Nov 2, 2011 at 16:17

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