Transient-voltage-suppression (TVS) diodes to protect IO Pins - Page 1
Forum section: electronics / beginners

author: mribble
author_rank: Regular Contributor
timestamp: May 24, 2014, 12:56:29 pm
content: I'm working on a project which will be in an enclosure, but will have many connectors that expose either pins from the Atmel SAM3x8 (http://www.mouser.com/ProductDetail/Atmel/ATSAM3X8EA-AU/?qs=sGAEpiMZZMtQuSbTnHsVtm6dgCW59dFS) or an LTV827 optoisolator (http://www.digikey.com/product-detail/en/LTV-827S-TA1/160-2040-2-ND/388468).I'm wondering if people with more experience than me have any opinions on using transient-voltage-suppression (TVS) diodes on pins exposed to the user via plugs. Most projects I've looked at don't have these (such as the Arduino boards), and my past projects have not used them. I had few few unexplained failures (1%-ish), but I'm not sure if it was this or something else. However, I've had a person say I should really consider adding them to be safe. The problem is to protect all my external pins would require about 80 of these diodes which is a considerable expense to the project. Do you think the SAM3x8 or optoisolator pins should really have this sort of protection?My gut is that there is already enough protection in the microcontroller and optoisoltor either via dedicated protection or just naturally protect via the silicon geometries. I know certain types of transistors like mosfets are sensitive to ESD, but none of my external pins connect to mosfets. So basically I'm looking for people with experience to say whether they think the added protection of TVS diodes is worth it to them in this kind of case.Thanks!

author: Simon
author_rank: Unknown
timestamp: May 24, 2014, 01:57:38 pm
content: The thing with a TVS is that it will have a cut in voltage and a clamping voltage which is usually twice the cut in making it hard to actually protect fully in such a restricted range

author: olsenn
author_rank: Frequent Contributor
timestamp: May 24, 2014, 02:07:28 pm
content: Transient voltage supressing diodes, as far as I am aware, typically only supress voltages in the kilovolt range. Perhaps even as low as a few hundred volts, but not 6V. A zener diode is most likely what you want... these are diodes intended to be operated in the breakdown region and are trimmed to a user-specified breakdown voltage.Take a look at the attached image for example. Note that the resistor may be replaced by a PTC

author: Simon
author_rank: Unknown
timestamp: May 24, 2014, 02:19:19 pm
content: I think you can buy TVS diodes specifically for IO pins a zener i was told is not as fast as a TVS

author: sleemanj
author_rank: Super Contributor
timestamp: May 24, 2014, 03:48:15 pm
content: Quote from: olsenn on May 24, 2014, 02:07:28 pmTVS are available in low clamping voltages no problem.Sent from my HTC Dream using Tapatalk 2

author: madires
author_rank: Super Contributor
timestamp: May 24, 2014, 04:08:02 pm
content: TVS diodes got a capacitance in the low nF range which will cause problems with high speed signals. But there are special ICs like the SRV05-4 with diode bridges (hiding the capacitance of the TVS) which got just a few pF.

author: mribble
author_rank: Regular Contributor
timestamp: May 24, 2014, 04:43:17 pm
content: Interesting discussions. Here is an example of the TVS I might use (though I'm not sure if my assembler would like parts this small...).http://www.mouser.com/ProductDetail/ON-Semiconductor/ESD9B33ST5G/?qs=sGAEpiMZZMuNo3spt1BaV70npiNHLXn2u6c0F5xYot0%3dThey have a capacitance of 15 pF, breakdown of 5V, and Camping of 10.5V.I'll note Wikipedia says TVSs are often used for this type of a case.http://en.wikipedia.org/wiki/Transient-voltage-suppression_diode

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 24, 2014, 08:24:27 pm
content: Read up on the standards used for transient testing.They all generally start with a capacitor charged to a specified voltage, then discharge that into your circuit through a specified network, either a straight resistance, or an RLC shunt network to slow the risetime and control peak current.HBM (human body model; casual static shock) uses 100pF and 1500 ohms. So a 2kV HBM test reaches a peak of 1.33A and discharges (99%) in about half a microsecond. Others such as CDM (charged device model), system discharge or lightning have different combinations of R, C and rise/fall time pulse shaping.Even for lower voltage tests (like 2kV HBM or 500V CDM), the voltage supply is so far in excess of your circuit's capacity that it can be assumed to be a current source. A rather large, very fast one. Take this for example:http://www.onsemi.com/pub_link/Collateral/TND410-D.PDFComparing HBM to IEC 61000-4-2 system level tests.Even after the clamping device, as mentioned, expect a clamping voltage easily twice the rated breakdown voltage. This is worse for devices with high ESR (MOVs in particular, but worth mentioning schottky diodes too), and can be dramatically worse for slow devices during the turn-on transient. Two level clamping can be very useful.Clearly, whatever it is you're doing needs to:1. Filter the sharp bits if possible (this makes high bandwidth connections to the outside world, like USB or HDMI, difficult to construct and protect!)2. Clamp the current with a fast device with unlimited* current capacity3. Series resistors to distance the clamp node (which might have peaks up to, say, +/-50V) from your low level circuitry4. Additional protection at the device if deemed necessary (e.g., schottky clamp diodes, analog or digital buffers, etc.).*For example, diode forward bias, or reverse (avalanche) breakdown, but not like, a zener-strapped BJT or MOSFET (both of which have significant equivalent resistance or go into current limiting).Only schottky diodes and breakdown are instantaneous; junction diodes, and latching devices like GDTs or SIDACs, take some time to respond, which lets through more of the leading edge of that spark. This is on the order of a few ns for junction diodes and GDTs, to 100s of ns for SIDACs I think.For a low bandwidth (i.e., under 1MHz or so) analog or digital signal, I would consider "gilding the lily" as:1. Nice hearty ferrite bead in series with the pin (also helps EMC)1a. If this is an input, or an output that doesn't need to source much current, then a series resistor as well (say 22-10k)** (above 500 ohms or so, the ferrite bead can be dropped, as the resistor is doing more on its own)2. Capacitor to GND (100p-0.1u)3. Clamp device. Can absorb it directly (breakdown), or divert to supply/GND (the supply then needs a TVS to prevent the voltage from rising too high; local bypass capacitors will help too).4. RCR filter to further reduce peak current and attenuate spikiness (say, 10-5k and 100p-0.1u)5. Clamp diodes at the device6. One more series resistor (10-5k)7. TVS on relevant supply/iesYou can take or leave any of these except #3, and I would recommend at least #6 with it.** Note that an SMT chip resistor probably won't save you from ESD strikes. Even if the material it's made of does not break down, the voltage will simply arc over it. (There are HV rated chip resistors that must be sealed in potting, if you have a situation where that's applicable.) If you have the space to spare, a longer resistor (1812 or 2512 chip, or axial THT) will help with this.The general plan is:- Clamp that massive current so we don't see it in the rest of the circuit,- The resulting voltage now looks like a constant voltage source, so add current-limiting series resistance,- and assist the device's internal clamp diodes (if present, and if desired)Note that #5 can use the device's internal clamp diodes (if present), but be careful, and expect less robustness as a result. You should calculate the series resistor so it delivers less than fault current (compare to the device's HBM spec, for example) at the clamp's peak voltage. That way, you aren't spending excess capacity on the clamp and all, trying to protect something that's damned anyway (at a given level of transient).There are lots of options for the clamp -- choose based on your circuit requirements and protection level.- Junction diodes (clamp to supply): low capacitance, low leakage, higher voltage drop, slow response (~ns).- Schottky diodes (clamp to supply): medium to low capacitance (generally unsuitable for 100Mbit+ signals), medium leakage (unsuitable for sensitive analog inputs)- TVS (avalanche / zener diode): medium to low leakage, high capacitance (generally unsuitable for >1Mbit signals, though often usable on medium speed, low impedance buses, e.g. RS-485); good for protecting low voltage supplies, where the high peak current capacity helps- MOV: medium leakage, very high capacitance, fairly high ESR: good for lightning protection on higher voltage power lines (over 50V or so), where the energy capacity is required. May need additional protection, due to the relatively high peak voltage.- GDT (gas discharge tube): response depends on voltage rate-of-change; always breaks down at a high voltage, then clamps to a low voltage. Extremely low leakage and capacitance, high current handling. Good for high impedance wide range inputs (e.g. scope probes!) or lightning protection (e.g. telecom).- SIDAC: Initially turns on like a zener, but keeps turning on (like a GDT does, but at SCR speeds). Good for lightning protection on slow circuits, but probably needs additional protection following it due to the response time.As for the present case, I would never, ever put naked logic pins on the outside world. That's just asking for trouble. At the very least, clamp diodes (e.g. BAT54S) and a series resistor (since we're talking CMOS GPIOs for the most part, maybe 22-100 ohms). The supply will need good bypass and a TVS, hopefully a low voltage one (however, they don't usually make them under 6V because the zener effect is too sloppy; I don't think there is a solution for very low voltages).If you can define inputs and outputs, and signal types, you can improve things further. Logic outputs can be buffered (e.g., 74HC245s), and inputs can be filtered (say, 1k + 100p). The buffer chip might be a socketed DIP if you're really concerned about reliability and support, but you should probably have some protection on it anyway (your worst enemy will be the buffer turning into an all-way short under transient conditions, putting some moderate fraction of the transient on all pins -- reducing but not eliminating the hazard to your MCU).As for what you're protecting against, examples of faults include not just casual static electricity, but residual voltage on cables -- and wires within the cable -- due to handling, atmospheric pickup, previous connections and so on. Isolated equipment can pick up a charge. CRT monitors aren't so common anymore these days, but I remember when you could gather quite a lovely charge off a TV and discharge that into whatever you please.There are more esoteric approaches as well. Consider Ethernet: they avoid transients -- and ground loop hazards -- altogether by isolating the signal with a transformer. (Not perfectly of course; a differential mode transient will still be coupled by the transformer, but the transformer will saturate quickly and limit the amount of energy delivered, so it's still helpful even in the worst case.) This can only be done with AC signals, and requires careful design for high speeds (controlled impedances). Okay, so you can do RS-422/485 this way too, or anything else, but it requires an isolated transceiver and logic isolators; big hassle. (The hassle with Ethernet is all rolled into the PHY, which comes on a single chip, so you don't have to worry about it externally.)Tim

author: mribble
author_rank: Regular Contributor
timestamp: May 25, 2014, 01:37:06 am
content: T3sl4co1l, wow that was an extensive summary. Much of went well beyond the scope of what I'm looking to do, but none the less it is good to see all that in one place.My goal is to evaluate a minimal solution that gives some protection, but is inexpensive for a large number of channels. Other requirements are 2 MHZ signals on some pins and analog on others. So with that in mind it seems your suggestion is a series resistor of 22-100 ohms on the signal line and then clamping it with something like the BAT54S diode. Then the supply needs a good bypass, but since my voltage is 3.3V I'm not sure what to use there. Any suggestions?Thanks again!

author: Smokey
author_rank: Super Contributor
timestamp: May 28, 2023, 02:08:28 am
content: Quote from: T3sl4co1l on May 24, 2014, 08:24:27 pmBump for a killer post...

author: David Hess
author_rank: Super Contributor
timestamp: May 28, 2023, 07:46:39 am
content: Quote from: T3sl4co1l on May 24, 2014, 08:24:27 pmDiode bridges can clamp accurately at low voltages, and can be fast, but are relatively complex compared to single devices and cost power. They can also be used to protect outputs.QuoteI like adding socketed buffers at exposed interfaces for exactly that reason. Optocouplers are also suitable where even more protection is required.

author: MrAl
author_rank: Super Contributor
timestamp: May 28, 2023, 10:37:41 am
content: Hi,Most uC chips have two clamp diodes on all but possibly one pin (high voltage programming pin on some).To take advantage of that, you can use a single resistor in series with the pin. Size it as high in value as the application permits.This won't be enough for every app just some, but you didn't make it clear what you actually need yet.

author: Psi
author_rank: Super Contributor
timestamp: May 28, 2023, 10:59:43 am
content: Use a series resistor to limit the current to a higher voltage first.eg, lets say your MCU IO can handle 30mA source/sink.If you put 10k in series then to get above 30mA you'd need V=IR 0.03A * 10,000R = 300V before your MCUs IO drivers cannot overcome the surge. Now you can use a TVS that will clamp at like 100V and you're all good. It relaxes your requirement to have a TVS with clamping voltage anywhere near the MCU's IO voltage.Obviously depending on the application 10k might be too much series resistance.But you get the idea.Just remember, the higher voltage your TVS can kick in the better, because ideally you want someone else's TVS in some other product on the same supply rail to kick in first. Let them take the brunt of the surge so your device doesn't have to deal with it.Oh and don't use 0201 series resistors for high voltageObviously the series resistors need to be able to handle the high voltage.And you should follow standard pcb track design rules for those input traces carrying voltage that high, creepage etcYou should also throw some caps in there to GND, just for RF/noise immunity.

author: Simon
author_rank: Unknown
timestamp: May 28, 2023, 11:17:47 am
content: Quote from: MrAl on May 28, 2023, 10:37:41 amI guess that after nine years the OP figured it out.

author: Psi
author_rank: Super Contributor
timestamp: May 28, 2023, 12:28:46 pm
content: It would be nice if SMF made the background color different on all posts older than a year.

author: David Hess
author_rank: Super Contributor
timestamp: May 28, 2023, 12:36:24 pm
content: Quote from: Psi on May 28, 2023, 12:28:46 pmIt would be nice if SMF made the background color different on all posts that you already read.

author: Miha
author_rank: Contributor
timestamp: May 29, 2023, 08:34:54 am
content: Quote from: mribble on May 25, 2014, 01:37:06 amSomething like that:https://www.mouser.co.uk/ProductDetail/onsemi/ESD5Z3.3T1G?qs=OSf9jACorvaH3Y8oRWEcuQ%3D%3DBut exist chinese analogs, that costs 1..2 cents:https://www.lcsc.com/product-detail/span-style-background-color-ff0-ESD-span-Protection-Devices_UMW-Youtai-Semiconductor-Co-Ltd-ESD5Z3-3T1G_C5252448.html

author: Simon
author_rank: Unknown
timestamp: May 29, 2023, 09:24:24 am
content: Quote from: Psi on May 28, 2023, 12:28:46 pmit would be nice if people did not just bump threads. No idea what they were thinking. The threads will always remain for future reference and to be picked up by search engines. I have even managed to google a question in the past and find my own thread I had forgotten about in the google results.

author: MrAl
author_rank: Super Contributor
timestamp: May 29, 2023, 10:47:14 am
content: Quote from: Simon on May 28, 2023, 11:17:47 amHi,Yes, nine years, time flies, but still relevant today.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 29, 2023, 12:46:41 pm
content: Quote from: Simon on May 29, 2023, 09:24:24 amIt's a technology problem, you can't really blame the users (except the one who gets the warning message). The age of a thread is totally non-obvious, just a small number in the middle of all visual noise. Only the original bumper gets the warning message. If they choose to ignore it because they disagree with your idea, then there's nothing you can do.Adding a modification in the forum code to highlight dates older than say 6 months inbold redwould solve the issue much better than the current warning message, but it seems no one wants to modify the forum code so expect to see this same pattern happening again and again.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 29, 2023, 03:34:18 pm
content: Could also just disable necromancy entirely, with a big notice on screen to write a new post with backlink to the question in reference.Whether that would make things more convoluted or not, I don't know. It doesn't seem to happen often enough here that it would be a problem either way.Tim

author: Simon
author_rank: Unknown
timestamp: May 29, 2023, 06:36:19 pm
content: Quote from: Siwastaja on May 29, 2023, 12:46:41 pmsomeone who was not the OP and who knew it was old posted simply to bring it back to the top. He had no relevant question, just decided tat it should be discussed further apparently.

author: Smokey
author_rank: Super Contributor
timestamp: May 29, 2023, 10:44:22 pm
content: Quote from: Simon on May 29, 2023, 06:36:19 pmI'm the bumper in this situation.I was researching TVS diodes and input protection, and found this thread with an excellent post by TT3sl4co1l. Bumping this thread is a way to bring this back to the top so more people can be exposed to good content that they otherwise might not have seen. I know I'm not the only one here that frequents the "show unread posts since last visit" page partly to get exposed to stuff that I may not have even known about.The intent was not to continue answering that specific person's old question, but to boost a thread with really good content to maybe a new audience and see if anyone has anything else to add. Continue the discussion if you want. Or not. That's up to you. I don't understand how the age of a thread is even really relevant to there being good information in it and an ongoing discussion. It's not like this is a political thread that's only maybe relevant for a couple days anyway. We are talking about TVS diodes here. Who cares if the last time people talked about TVS diodes here was in 2014... I thought this was an engineering forum, and you guys are complaining about engineering topics?

author: thm_w
author_rank: Super Contributor
timestamp: May 29, 2023, 11:06:32 pm
content: Quote from: Simon on May 29, 2023, 09:24:24 amI agree, although not enough people here reference old threads/posts when someone has already taken a lot of time to write a detailed response out. The beginner might not know what to search for, or the post could be hidden in a 10 page thread.Quote from: Smokey on May 29, 2023, 10:44:22 pmI see your point, but, I wouldn't reply to a thread unless there was something additional you wanted to add or discuss, regardless of its age.If you think the content is useful, the next time you see a TVS question you can link them to this post or quote it instead.

author: Simon
author_rank: Unknown
timestamp: May 30, 2023, 07:57:49 am
content: Basically in an ideal world the best of the forum would be curated into a wiki. I think Dave started one, I am not sure what happened.

author: MrAl
author_rank: Super Contributor
timestamp: May 30, 2023, 09:33:43 am
content: Hi,Yes it's another case of the forum etiquette police (ha ha).More talk about reviving an old thread issues than talk about what the main subject matter is about.I see this on other forums too. OH MY GOD, SOMEONE POSTED IN AN OLD THREAD!! OH NO! WE ARE ALL DOOMED!! GOD HELP US ALL !! THIS HAS BEEN THE 10th OCCURANCE NOW OVER 10000 THEADS, UP FROM 9 OVER THE PAST 2 YEARS, THAT'S 0.1 PERCENT !! WHAT IS THIS WORLD COMING TOO??? OH NOOOOOOOOO...WE HAVE TO FIX THIS BEFORE IT'S TOO LATE!!ha ha ha ha ha ... etc., etc., i could go on and on with thisI see some forums just posting a note. I suppose that's ok. They just tell the user to think about it.I do suppose it may sound odd when someone says in year 2014 that they needed a transistor replacement, then in year 2024 someone replies and says, "Oh yeah you can use 2N2222A instead". That's a little late i guess. It might be different if they said, "Well transistor substitutions can be found on the manufacturers website", which is for the more general audience. Or maybe, "Anyone that needs this replacement can try the 2N2222A". So, a reply directly to the ancient user post person seems odd, but not if it is in the general sense.I should note that in this particular case i was interested to read about the different methods for input protection, which is something i am always interested in no matter what year it happens to be in. I like to read about different, hopefully new methods.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 30, 2023, 01:15:16 pm
content: The main cause is, because the thread has left peoples' memories, no one remembers that it's all more-or-less settled, that the OP or respondents likely aren't even around anymore, etc.In other words, people are annoyed by getting their ignorance shown up. Or negligence at not checking the dates.No one would ever admit that that's the reason, rather blaming it on the causative agent. But you have to admit that's an annoying experience.Tim

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2023, 04:10:26 pm
content: Quote from: Simon on May 30, 2023, 07:57:49 amThat would be an awful lot of work. Before that I would implement threading.

author: Simon
author_rank: Unknown
timestamp: May 30, 2023, 05:13:27 pm
content: Quote from: David Hess on May 30, 2023, 04:10:26 pmThe essence of the person bumping it was to keep the good information available. Well if that was simply put on a page under TVS diodes then it would be there forever. Google does a good job of finding forum pages even a decade later.

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2023, 01:43:39 pm
content: Quote from: Simon on May 30, 2023, 05:13:27 pmGoogle does a worse job as time goes on and politically incorrect pages get lost deliberately.

author: MrAl
author_rank: Super Contributor
timestamp: May 31, 2023, 02:00:03 pm
content: Quote from: T3sl4co1l on May 30, 2023, 01:15:16 pmHi Tim,Pros and Cons,Cons and Pros,Post it again, thenTouch your toes.(chuckle)I haven't written a poem in some 15 years or more.

author: aabill
author_rank: Contributor
timestamp: September 20, 2023, 06:43:56 am
content: SmokeyI (amongst others I am sure) benefited from your bumping without which I would not have seen the excellent educational content from Tim which is relevant to my current project.Thank you.

author: Smokey
author_rank: Super Contributor
timestamp: September 23, 2023, 07:33:27 pm
content: Quote from: aabill on September 20, 2023, 06:43:56 amOMG!! WTF!! Why are you bumping this old thread!!!! It's making the "post a picture of your cat!" thread not be on the first page!!ha! ... I kid... I kid... everyone relax

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2023, 11:58:24 pm
content: Quote from: T3sl4co1l on May 30, 2023, 01:15:16 pmThis year I started work on repairing a Tektronix DM501A multimeter. The high impedance multiplexor and buffer in the ADC chipset has high input leakage current, which is a common failure. So I did an internet search and it ends up that someone designed a similar repair many years ago ... me. I did the design work on a repair to replace the input multiplexor many years ago while helping someone on the Tektronix email list and had forgotten about it.