Timeline for Overvoltage protection for RS485 inputs
Current License: CC BY-SA 3.0
10 events
| when toggle format | what | by | license | comment | |
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| Feb 1, 2016 at 20:06 | comment | added | Anthony May | But yeah, the beauty of a PTC + TVS/Zener approach is that the PTC provides significantly more series resistance (in its cold state) (between outside world & TVS/Zener, before getting to the RS485 chip, for the TVS/Zener to work with in clamping both transients & longer term fault currents. | |
| Feb 1, 2016 at 17:36 | comment | added | Anthony May | There are double-back-to-back TVS diodes available (i.e. in a single package), like Littlefuse's SMAJ11CA (as 1 example, tho 400+ came up in my first pass search on Digikey, but I can't paste in the long search URL). Getting the hold-off & breakdown voltages right is a tradeoff, & governed by the specs of which ever RS485 chip you're trying to protect; perhaps you can drop the hold-off well below 12V (which is normally RS485's max differential) to bring the break-down lower, if your installation has a good common ground, for example. | |
| Feb 1, 2016 at 12:36 | vote | accept | avl_sweden | ||
| Feb 1, 2016 at 12:36 | comment | added | avl_sweden | I've now realized that I could easily use zener's back-to-back, as per Techydudes response. I was just a little slow in realizing how it would work :-). With the right PTC the current could be held below 50mA. This would require a 2W Zener or so for the Zener to survive, a component which is readily available. | |
| May 19, 2015 at 17:34 | comment | added | avl_sweden | Of course the fuse by itself will not suffice. My problem is that a fuse won't trip fast enough to save a transient suppression diode, at least not any 2A fuse I've found, not according to the datasheet. And I worry that PTC before TVS will just slow cook the TVS. The PTC will ensure that the fuse doesn't trip. So it'd need to have a very low hold current to not fry the TVS. Maybe such a component does exist? I'll check it out! | |
| May 19, 2015 at 12:49 | comment | added | Anthony May | It's not up to the datasheet to tell you whether your specific application needs biasing resistors. but I was asking in case they could form part of the protection mechanism. So you're doubtful about the response time of PTCs, but want to depend on fuse protection? Then you've already lost :) Perhaps: PTCs in series with the differential pair inputs, and then a pair of ~12V zener diodes back-to-back to ground on each of the pairs? | |
| May 18, 2015 at 6:42 | comment | added | avl_sweden | I have no resistors. According to the datasheet for the ADM483E, these aren't needed, and I like to save the components. I still doubt that the PTC will respond fast enough to save the gates of the ADM483E. | |
| May 17, 2015 at 23:34 | comment | added | Anthony May | Do you have line biasing resistors (as well as terminators)? | |
| May 17, 2015 at 19:10 | comment | added | avl_sweden | What if the 24V is applied to GND and one of the RS485 inputs? And are the PTCs really fast enough to save the RS485 chip? | |
| May 17, 2015 at 14:38 | history | answered | Anthony May | CC BY-SA 3.0 |