Help with TVS selection? - Page 1
Forum section: electronics / projects

author: Things
author_rank: Regular Contributor
timestamp: November 07, 2013, 01:33:52 pm
content: Hi all, I bought some addressable LED strip a couple weeks ago and have been messing around with it a bit. Unfortunately I've found out the hard way a few times they're extremely sensitive to ESD/transients. Their nominal voltage is 5V, and below about 4.6V they start acting up. Their absolute max rated voltage is 5.3V - not a great deal of headroom. I'm looking for a TVS device, preferably SMD, that I could physically solder to a few places along the strip, to hopefully prevent me killing so many LED's!I think clamping at 5.3V or so would be ideal, however element14 don't seem to have any that clamp below 6V, though there are a few with breakdown voltages around 5.3V. Not sure how I should be picking the best one for this? I'm a bit worried as they do have quite a beefy PSU behind them (there are a lot of them, about 36A worth).Cheers

author: dr_p
author_rank: Frequent Contributor
timestamp: November 07, 2013, 02:04:55 pm
content: 5V LDO?

author: minime72706
author_rank: Frequent Contributor
timestamp: November 07, 2013, 03:43:04 pm
content: You may be able to get away with 5.1V zener diodes. Protection only works if you have a good ground return, though.

author: calexanian
author_rank: Super Contributor
timestamp: November 07, 2013, 05:42:45 pm
content: I would go with a different approach. I would monitor the supply voltage and if it ventures out of range cut power to the entire supply via a latching relay or something of that sort. Other than that it sounds like a commercial power supply if it is of that kind of amperage. They are the 3 color strip kind right? I find it hard to believe they are ALL in parallel. Typically they series many up to get the amperage down. Also generally speaking unless they are being fed by a constant current source, in which case they would most certainly be series connected to a certain degree LED's are tolerant of higher supply voltage because they are only going to have their bias voltage across them and everything else would be dropped across the external regulation means IE, resistor, current regulator, lump of coal, etc. Is this an actual problem of them failing you have encountered or just a concern. As for TVS's they are a blessing and a curse. Without proper design when they clamp they put they basically become a low ohm heating element and can destroy whatever they are connected to unless power is interrupted or limited by something else. Additionally they just don't make them in those low voltages. Zeners are a good option, however with such a large current capable power supply I feel they would be nothing more than a little pop on the way to the big bang. Also the way they matrix those led strips may make implementing them difficult.

author: SeanB
author_rank: Super Contributor
timestamp: November 07, 2013, 07:19:00 pm
content: You probably would be better off with a power supply supervisor IC driving a beefy SCR to guarantee the tripping of either a power supply current limit, a Polyfuse or a circuit breaker. At least those will have the ability to have close setting limits. You can even get some Dallas Semiconductor ones in a 3 pin package, though there may be others now.

author: JoeyP
author_rank: Frequent Contributor
timestamp: November 07, 2013, 07:41:58 pm
content: Neither TVS nor Zeners are going to help you with this issue. You have a very narrow operating range, and they simply don't turn on that abruptly. By the time they're conducting enough to clamp something like a 36a supply, they'll be well above their nominal rated voltage. If you drop the value low enough to clamp at the required voltage, they'll be dissipating a lot of power at normal supply voltage.You could use a solid state relay in series with the supply, with a comparator which will turn it off quickly as the voltage exceeds the critical limit.

author: RossK
author_rank: Contributor
timestamp: November 07, 2013, 08:20:13 pm
content: The OP is suspecting ESD is killing the LED's not sustained over voltage from the power supply. I would recommend protecting against both.My 2-cents:As for ESD, chances are the LED's will survive a pulse well above 5.3v. Look at (and link) the datasheet if you have one. Anyway, the TVS's you found that start clamping at 5.3v would provide a lot more protection than using nothing at all. Additionally you may need to protect the TVS's, and your LED's, with a PTC (restable fuse) to protect against longer duration over-voltages which are not caused by human ESD. Use bi-directional TVS's if reverse power hookup is not a concern and if you can find them, if you must use uni-directional TVS's the PTC is a must to protect against reverse power hookup.

author: Corporate666
author_rank: Unknown
timestamp: November 08, 2013, 12:15:18 am
content: Quote from: RossK on November 07, 2013, 08:20:13 pm"Addressable LED", I am guessing the OP is talking about maybe WS2811/WS2812 chips which have a constant current driver chip inside the LED package? Those could certainly be susceptible to short term transients more so than the LED die.I like the idea of a FET being turned on/off by a transistor using a zener as a voltage reference.No idea how fast it would be in dealing with transients, though.

author: calexanian
author_rank: Super Contributor
timestamp: November 08, 2013, 12:19:46 am
content: OIC. an ESD prob with the drivers.nevermind

author: Things
author_rank: Regular Contributor
timestamp: November 08, 2013, 09:36:19 am
content: Yes, sorry if it wasn't that clear, I think my issue here is brief overvoltage spikes, such as from the power supplies dropping into regulation, ESD and the like. The LED's themselves are the WS2812B's (integrated chip in LED). What I've found so far is that usually the first LED in the string is always the first to fail. However, since each LED has to pass all the serial data for the remaining LED's, if one dies, all the others after it won't work until you remove it either.I had one fail right in the middle of the strip the other night, it still partially worked and passed data through, but as it got warm, it started corrupting the data to the following LED's and they'd flicker like mad. Luckily said LED was flickering pretty noticeably itself, so I was able to find it pretty easily along the strip and cut it out. But so far I've had about 3 LED's fail on me, and being in waterproof silicone tube, fixing them is annoying, so I was thinking of some sort of protection that'd make them a little more lenient to brief overvoltage spikes, both on the power and data lines.