Help needed: Input protection for multifunction DC input - Page 1
Forum section: electronics / projects

author: Sukasa
author_rank: Newbie
timestamp: April 05, 2023, 05:16:37 pm
content: I am working on a circuit with 9 configurable input/output pins, which can connect via some multiplexers to different interface chips in a device. Because I can't know what voltages the devices I'm connecting to might spuriously put on input pins, I need to be able to protect my circuits against harmful voltages/currents up to ±24Vdc. Protecting against anything beyond that is out of scope; past ±24vdc if something lets out the magic smoke, then it lets out the magic smoke. I'd like to protect these circuits by tripping PTC fuses to heavily limit current flow/voltage. I expect to pass maybe 5-10mA of current at most in normal operation, so I intend to size very low-trip-current PTC fuses.I have two voltage ranges I need to keep Vprotectedwithin (one or the other based on a SELECT signal): -14..14v, and -7..5v, relative to Vground. My understanding from readingthis threadis that in a case like this you would use two Crowbar circuits with diodes protecting them from reverse bias. Since I need mine adjustable, I tried to base it off an LM431 shunt regulator controlling a Triac, as shown in theCrowbar (Circuit)Wikipedia article.So what I'm asking for is a general sanity-check that my approach here with the two crowbar circuits and then trying to modify how the voltage offsets work is a workable solution that I could implement. And also any advice or help with the giant "?" in the schematic where I'm not sure the best way to connect the logic-level signal to the transistor there, if applicable. And if my approach is completely nonsensical (disclaimer: I'm at best a hobbyist, so there's a ton I don't know), then I would hugely appreciate any tips or pointers on where to learn a better approach for this

author: Sukasa
author_rank: Newbie
timestamp: April 07, 2023, 07:18:54 pm
content: My apologies for the bump. I hammered out a bit and then remembered that optocouplers exist as well as realized one of my original range limits was too tight. I've updated the image link in the OP, and also have it below. Does this look like a particularly workable adjustable double crowbar? It looks reasonable to me but there could easily be something I'm missing

author: wasedadoc
author_rank: Super Contributor
timestamp: April 07, 2023, 07:37:09 pm
content: Maybe a bidirectional TVS diode would be a much simpler yet adequate approach.

author: Sukasa
author_rank: Newbie
timestamp: April 07, 2023, 07:40:57 pm
content: That was my first thought, but the problem there is that, to simplify down I have interface chips "A" and "B" which can get multiplexed onto any of these 9 I/O pins.Chip "A" can handle signals -15..15v, and normally uses ~-10..10v signals.Chip "B" can handle signals -7..12v, and normally uses 0-5v signals.The disparity there is why I didn't want to stay with a simple TVS; I didn't see how a TVS could protect both scenarios while also allowing both scenarios to work, given that I can't know until run-time which chip may or may not need to be multiplexed to any given of the I/O pins.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 07, 2023, 07:48:04 pm
content: Sounds like you need two simple protection circuits. (Like TVS, zeners, etc)One to clamp voltages between -15..15v at the multiplexer common IO.Another to clamp voltages between -7..12v at chip "B" after the "multiplexer".

author: Sukasa
author_rank: Newbie
timestamp: April 07, 2023, 07:56:20 pm
content: That would be simpler, yeah. I had thought I'd have to worry about fault current through the multiplexer (MUX506IPWR is what I have in the current prototype) if I did that, which is why I'd gravitated towards pre-multiplexer protection. Am I over-worrying about that?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 07, 2023, 08:00:28 pm
content: The input protection is usually a 2 part job:1) some series element to limit the current (e.g. a resistor, a PTC, some active circuit like JFETs or depl. mode FETs, maybe a fuse)2) some clamping circuit to prevent the voltage going too high. This can be diodes, zener diodes or transient absorbers and for higher voltage maybe MOVs.An important parameter is how much current has to flow in the output mode and how much series resistance is acceptable.Camping with some kind of crow bar circuit is for very high currents (e.g. > 1A), like the output of a power supply. For IO outputs one usually only needs to clamp small current of a few mA of less. So the clamping part would be more like Zener diode or diodes to a common zener or similar transient absorber diode.

author: Sukasa
author_rank: Newbie
timestamp: April 07, 2023, 08:10:28 pm
content: Okay. So I think in trying to simplify down what I was trying to do / avoid spamming tons of info obfuscating what I was asking for, I did miss some important context. I apologize.This kit, at least the prototype, is connecting serial connections (Chip A is a MAX232, and B is a MAX485) to industrial gear. I wanted to be able to crowbar/PTC every input because of the chance that a wire could slip, or someone terminate to the wrong pin, and end up connecting one of these I/O pins to +24V directly from a power supply.