Electronics > Beginners
RS-485 protection
T3sl4co1l:
--- Quote from: Giaime on June 11, 2019, 06:54:18 am ---
--- Quote from: floobydust on June 11, 2019, 01:36:52 am ---I put the TVS on the other of the 10R (or polyfuse), to lessen the currents it experiences. Contrary to TI's circuit with SMBJ43CA TVS, at ~500pF.
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There's a reason they put the TVS at the line side, not at the transceiver side of the resistors: that's because if you do it your way, the TVS appears in parallel to the internal protection diodes of the IC.
Who knows how the current will share between them? (hint: external TVS have higher voltage drop than built-in ESD diodes...) The 10R resistors make sure the most of the energy gets dissipated in the TVS, not the IC.
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Reminder that RS-485 devices do not have ESD clamp diodes. They have a wide line voltage range: -8 to +12V or thereabouts. The question is, which diode pulls in first: the internal ESD zener, or the external one?
The internal one will most likely* have a comparable V(I) curve to any other TVS, so it's not so bad putting another in parallel as long as it's lower -- or separated by the resistor, which indeed is to isolate the current somewhat.
*Unless it's a snapback device, not uncommon in ICs as far as I know, but hard to find isolated in the wild; one of the few examples. Anyone know if transceivers use these internally, or how to tell?
For transient protection, personally, I just toss on a SMAJ6.0CA from ground to each line. Or a higher voltage (9 or 12 say) if more range is required (e.g. industrial EMC spec -- combined with using a wider-range transceiver like the ones mentioned above, to tolerate the higher Vpk).
Bidirectional is better than a unidirectional device, because under normal conditions, we can expect the signal to bounce negative, even with common grounds, and such a condition will cause bit errors.
(I didn't discuss this earlier as the subject was sustained overvoltage.)
I'm a bit "ehh" about protecting a TVS with a PPTC -- even the small chips let through a lot of energy, and even as robust as TVSs are, they can only take so much (10s, maybe 100s mJ for an SMA size part). And at the ratings needed here (~100mA fuse rating, >=32V maximum allowed drop), you need a fair size (read, slow) chip, offhand I think 1206 or so?
Another excellent use case for hybrid TVS-PPTCs -- these heat up the PTC with a TVS, giving actually reliable protection. They're hard to find stocked, though. :(
Tim
Brutte:
--- Quote ---1W 6.8V zener, and 100mA polyfuse to protect the zener.
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Anyone has access to IEC 61000-4-4 gear to watch how raw 1W zener dies when EFT is clamped to RS485 cabling? There is a reason the TVSes are there. These are rated for high instantaneous powers. Raw zener is not designed for such actions.
And even the smallest 0603 100mA polyfuse will pass 40A for 10ms without dissipating anything.
This could be a kind of protection against installers that ground the transceiver and apply 24V to data line (from 5A supply of course). But if you apply 100mA to 1W 6.8V zener (15V), it will be killed anyway after 10s so I would not bother.
--- Quote ---For transient protection, personally, I just toss on a SMAJ6.0CA from ground to each line.
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Is that enough? I think the negative absolute minimum voltage is -10V for many transceivers. I'd use 5.0CA. That would limit the common mode voltage but I cannot see any other good alternative.
T3sl4co1l:
I wouldn't worry about it much, TVSs are just big zeners, spec'd for transients instead of regulation. :)
More than you ever wanted to know, but AN784 is the particular highlight here:
http://www.icbase.com/File/News/download/ON_Reference_3.PDF
I wouldn't recommend using just anyone's zeners for such an "undocumented" purpose (i.e., no one puts peak power ratings on their zeners), but I wouldn't feel bad about it with Motorola/On Semi's. Well, I wouldn't at the time, but who knows if the appnote is still applicable?...
Tim
mikeselectricstuff:
For receive-only applications, I've found these murata 100r PTC thermistors are brilliant
https://www.digikey.co.uk/product-detail/en/murata-electronics-north-america/PRG18BB101MB1RB/490-2469-1-ND/588683
They trip so fast that you don't even need any additional clamping, even for a +48V or -48V direct short.
T3sl4co1l:
Ah yes, ceramic PTCs -- rarely seen, but interesting. Their performance is worse (relatively high Rcold) but they're available in much higher voltages (450V+?) so can be useful in just the right place. :)
And yeah, an 0603 chip is small enough, and Rcold is large enough, that something on the other side stands a good chance of survival!
Tim
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