what do you think is reasonable input protection for a DIN connector probe? - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 11, 2014, 05:31:35 am
content: WHat do you think is reasonable input protection for a exposed port that a non-contact sensor (thermometer, light measurement, something isolated) plugs into?I am using a female DIN type connector on the box, so its not really possible to contact the terminals with a finger while moving the equipment around.Would you bother putting some kind of input protection on that? The probe is pretty much entirely made of isolated plastic, but it is meant to be handled by a person. I guess its possible to discharge through the plastic if you are charged enough.I was thinking :http://www.littelfuse.com/products/pulseguard-esd-suppressors.aspx1 part solution, nano-amp leakage current, perfectly acceptable.They even advertize it should be used when the threats are transient, like unplugging or plugging in a probe.I think that a TVS diode might be more appropriate, as Idon't need the super fast specs of those pulseguards, but I don't know much about selecting an appropriate TVS diode... if it is appropriate in this situation.Is it over kill to protect that connector at all?Is that enough? Everything is hooked into OP-amps on a +-15V supply.I don't really know what kind of harmful things to protect my input against.. what could get through the plastic? what voltages should I be looking at?I thought GDT + TVS, but it seems like really over kill for a plastic probe connected to a DIN.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 11, 2014, 06:03:57 am
content: Shottky diode clamps to the rails plus current limiting resistors if that won't mess up the sensors. From your description you could probably get away with nothing, but as long as it doesn't hurt your measurement, it will add a bit of reliability, including if someone starts plugging things in you didn't expect.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 11, 2014, 06:09:23 am
content: Quote from: ejeffrey on April 11, 2014, 06:03:57 amwould you advise that in addition with the ESD protector? What should I put the diodes at, same as the power rails?are there any anomalous effects from the diodes? They will just leak and add capacitance right, just like the ESD protector chip.*edit ima read this, dont bother answering that question.http://www.analog.com/library/analogdialogue/archives/41-05/input_protection.htmlbut I will still likely blindly implement whatever is suggested in this thread.how about if there is inductors and ceramic capacitors (passive HF filter)? should the diodes be put before them, are those parts in danger?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 11, 2014, 06:56:32 am
content: Don't forget the act of plugging in a charged connector, or of charged objects coming near the open connector.I have no idea how many times I've shocked myself on the USB, HDMI or other connectors on my laptop, due to the static of sitting up and dragging my butt off the chair. Half inch spark easy. Most of these connectors have a prominent ground shield (for good reason!), but something like a (regular size, not mini) DIN could easily end up with an unlucky finger in the middle, and zap goes the pins.I put some combination of diodes, RCs or TVSs on every off-board pin I can.Note that:- Junction diodes are low leakage and low capacitance, but slow- Schottky diodes are high leakage and high capacitance, but fast and relatively low drop- Zeners and avalanche diodes are fast, but high capacitance, and the clamping stinks sometimes- MOVs are good for beefy, noncritical things, like power lines, but have a lot of capacitance and internal resistanceBecause junction diodes are finite speed, the residual couple-nanosecond spike in a 61000-4-2 system level test is quite tall: from what I've seen, usually upwards of 20V (in an 8kV test), and that's for the single-device solutions (and if you want to do it discrete, stray inductance will kill you fast).Due to the leakage and capacitance, schottky are generally unsuitable for sensitive analog inputs, or high speed, low voltage data lines (like LVDS, USB, HDMI, etc., where junction diodes are usually used -- the remaining spike is just tolerated, I guess). They are generally good for digital logic (inputs and outputs of the lower speed or higher voltage variety) or stronger or less accurate analog inputs, or buffered outputs.TVSs are nice, especially at lower voltages where they can dump a lot of current without melting (which still isn't all that much, because you're dumping energy into silicon, which there isn't much of). One downside: they don't really make TVSs for less than 5V, the zener effect is just too crummy down there. Low voltage supplies or logic don't really have much choice but to use clamp diodes.MOVs aren't usually suitable for signals, but since they handle so much energy, they are good where you need heavy 61000-4-5 lightning transient tolerance.There are other latching style devices, like gas discharge tubes (GDTs), polymer things, thyristors (SIDACs, SCRs for crowbars, etc.) and so on, which are good for taking the brunt of it, but act too slowly (or turn on at too high a voltage) to be quite as useful as the others, on their own.Stacking protection by one extra stage would be a good way to better guarantee performance in the presence of massive abuse. Consider that >20V poorly clamped nanosecond spike; if you can afford some series resistance after the protection diode, then you only have 20V interacting with the target device (say, a digital logic buffer or something like that). 200 ohms is enough to reduce the resulting current to the 100mA level, which is in the same ballpark as typical CMOS "latch-free" specs. That is to say, around 100mA, you probably won't be getting sparks to trigger nasty failure modes like CMOS latchup.Tim