[TriEmbed] OT-ish: TVS best practices?

[Kevin McClaning]

Sorry if I'm being dense but what's a TVS element?

Generally over long lines, you want to use differential pairs to remove 
the common mode noise. In Ethernet, they actually use integrated 
transformers in the transmit and receive electronics. These are very 
linear and will remove a lot of common mode noise without saturating. 
There are active versions of the balanced/unbalanced converters that 
don't use transformers but they can be more easily knocked out by 
electrical transients.

You could also send the power as AC and transformer couple into your 
remote device. Then, you could establish your own local ground at the 
remote end that is not coupled to the local end's ground.

I'm a big fan of fiber for long runs but that doesn't help your power 
problem.

HTH,

Kevin


On 10/30/2017 1:57 PM, Brian via TriEmbed wrote:
> Hi Group,
>
> I've been working on a design for zoned HVAC which involves having 
> electronics at the ends of long cables.  As such, good transient 
> voltage suppression becomes a more important concern.
>
> My device will be connected via Cat-5 UTP across which will be 
> supplied +6V, GND, and an RS-485 differential pair.
>
> I'm wondering if anyone in the group has suggestions on the ideal 
> application of TVS elements.  I've seen reference designs for RS-485 
> that have paths to ground for each of the signal lines, but I've also 
> got the +6V line to worry about.
>
> Can I just provide paths from each signal to GND, or do I need to 
> provide paths from every signal to every other signal?  It seems to me 
> that theoretically, all four conductors could be floating at different 
> potentials relative to each other, and clamping to GND wouldn't 
> necessarily protect against a damaging potential between a 485 wire 
> and +6V.
>
> All input greatly appreciated!
> Thanks,
> -Brian
>
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