How to protect rs485 line from high voltage? 1

[JimboJones21]

Hello All,
I wanted to know if there is a way to protect RS485 IC (75176) from high voltage such as 50V because if I get a spike on my lines my 75176 chip blows.
Can you simply hang a 15Volt zener on each line to ground to sink the 50V or put a fuse in line with the lines? It has to be PCB mountable also.
Thanks for the help!

 

[Skogsgurra]

Fuses don't work. They need hundreds of miliseconds to blow and your input is dead in a few microseconds.

Best way is to find out from where the spikes come. You have measured them, then you should be able to locate the source.

In most cases the source is load transients that couple into ground of one of the systems and not the other system's ground. That creates a common-mode spike. The standard solution to that is to connect a common ground wire between the systems. Then both systems will have the same transient with no common mode voltage between them.

There are also special devices like TranZorb and others that may help. But they are usually for higher voltages.

Gunnar Englund

http://www.gke.org

 

[JimboJones21]

Thanks Gunnar, what was happening was the body of a capacitor was broken open and touching the chassis of the device. The cap has 48V across it. When this happends the RS485 chip blew. The cap problem has been fixed by repositioning it so it is not cut open by the chassis.

The devices all have the same ground, how is it that the 48V on the ground causes only the RS485 to blow and nothing else? Are the internals of the RS485 transmit and receive lines referenced to the ground making them suseptable to a large voltage difference like this? I also have a micro which is still operating fine.

I've seen one method of putting current limiting resistors in series with the DO/RI lines, but this limited the length of the cable because of signal attenuation. Would the idea with the zener work? Thanks again for the help.

 

[Skogsgurra]

Jimbo,

The RS485 is not referenced to system ground. At least as long as your common mode voltage is less than about +/- 7 volts. After that, there will be currents flowing that can destroy your Tx and Rx circuitry. A resistor can work for low data rates and short cables. But I wouldn't recommend it. Not at all.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

Optocoupler. Protect the optocoupler input with a transorb, plus a MOV or gas discharge tube if you're in a really hostile environment.

Companies like BB Electronics make a living from selling little modules which do this sort of thing for the retrofit market where the original designer didn't harden his design for the real world environment:

http://www.bb-elec.com/product_family.asp?FamilyId=1&TrailType=Sub&Trail=19

Obviously if you want to make it board-mountable then you need to design your own circuit, but you get the idea of the principle.




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