LED strips - what happens if you have a short in Data and GND pins? - Page 1
Forum section: electronics / projects

author: victorhooi
author_rank: Regular Contributor
timestamp: September 19, 2024, 07:44:32 am
content: Hi,I'm starting to get into addressable LED strips - and was hoping to get some advice on being able to diagnose issues in projects, if they do pop up.If you had a break in the strip (e.g. bad solder joint, or physical break etc) - this would just mean the LEDs don't light up after the break, correct? Is there any way to use a DMM to diagnose this sort of thing in a long run (e.g. 20-30m strip), or narrow down the exact problem area?Or second scenario - if you have a short-circuit between the Data (D0) and Ground (GND) pins somewhere in a very long run (e.g. 5-20m) - what sort of symptoms would this present with? Or are there any tools/techniques to try to pinpoint this kind of thing?Thanks,Victor

author: artag
author_rank: Super Contributor
timestamp: September 19, 2024, 09:14:10 am
content: 1. I prefer to use an oscilloscope. The DVM will only give you average voltage which might be a pointer but is a bit indirect.2. Pretty much the same as with the broken wire, except that you can measure resistance (on every D0 point, no halving) to test.The easiest is just to provide a changing colour map - like the common arduino demo code - and look for the last led that follows the pattern. The one after the fault will be stuck, perhaps at dark. It might be intermittent.Then measure resistance between the D0 of the last led and the DI of the first non-working led. Measure from the actual led pins, not the copper pads on the strip. Also from those points to ground, and also check power and ground are getting right along the strip. Breaks in those will also darken leds past the break, unless you're powering from both ends (which you should be doing for a long strip, indeed in multiple places).If all of those checks are reasonable, it can be a led - either an apparently working one that doesn't pass the data on, or one that doesn't respond, so either the last working one or the first broken one. They're hard to replace successfully and replacements from a different source won't colour-match well, so the easiest way is to cut 2 out and replace with a strip of 2 from the original coil, soldered at the copper pads.Fwiw I've never had the pcb go bad, only my joints at the end of a strip or leds themselves.In terms of problems, the most important is a good power supply distributed to the whole of the strip, perhaps at multiple points. Adafruit has a useful guide athttps://learn.adafruit.com/adafruit-neopixel-uberguide/powering-neopixels- especially the section on estimating power requirements.

author: victorhooi
author_rank: Regular Contributor
timestamp: September 21, 2024, 01:44:14 am
content: @artag - Thanks for the notes! I'm still trying to figure this out, and get a better mental model of what can go wrong, aha.Sending Test PatternsFor the changing colour map - I know that previously, I've had sections display the wrong colour, as some (but not) all of the ICs were damaged. I did recently order a Falcon F-Test, which I think can also send test patterns, and is quite portable.(Previously, I just plug in a WLED controller, and use a laptop/phone to control that - but F-Test seems like it's more portable).https://pixelcontroller.com/store/diy-controllers/85-F-Test.htmlContact Pads vs LED pinsYou mentioned measuring from the actual LED pins, not the copper pads on the strip - for the standard LED strips that could work - however, I also have COB LED strips - where all the LEDs are glued (I assume) under a white cover:So I can't actually get access to any of the LED pins - in fact, I can't even see any pads for the LEDS either? (Unless you can spot them in the photo above).For the standard strips - I can see the 4 pins for the LED - I guess I need to check the datasheet to see which pins to use?PoweringI am powering the strips from both ends - I'll have a power injection at the front of the strip - and another at the end of the strip (roughly 18m run). However, since these are addressable - I assume by default, they should all be dark until I pass in a data signal, right?Oscilloscope vs DMMI do have a oscilloscope (Analog Discovery ADP2230) - but it's not battery powered unlike my DMM - hence why I thought DMM might be easier in the field. But I'll look about getting a way to make the scope portable.But the oscilloscope is mainly used for the data lines right? You'd expect +5V (or +24V) to be pretty static over time?Scenario 1 - Break in the strip (e.g. bad solder joint, or physical break).What sort of resistance should I be seeing between DO and DI along the strip?I just tested with a new cheap 5V W2812B strip.From GND to GND pin - it was around 0.2 to 0.3 Ω. And from +5V to +5V - also between 0.2 to 0.3 ΩFor the data lines- it was around 33 kΩ from DI to DO - is that what I should expect?And can I just measure resistance between the pads at say, the start, and power pads at the end - and then try to narrow down the break?And like you said, check between say, D to +5v (or whatever voltage that strip is), and ensure there's no continuity there.You mentioned using a oscilloscope for this part, to try to find a break in the strip - is that just to measure the voltage along the data lines? Or something else?Scenario 2 - Short Circuit across pins (e.g. D shorted to GND)I'm sorry - I didn't quite understand this one:QuoteDo you mean check resistance from DI to DO, along the strip? If there's a short between a Data pin and Ground - wouldn't I be checking between D and GND? Or did I miss something here?

author: artag
author_rank: Super Contributor
timestamp: September 22, 2024, 08:04:38 pm
content: (interleaved answers but it's pretty painful doing that with this board)You mentioned measuring from the actual LED pins, not the copper pads on the strip - for the standard LED strips that could work - however, I also have COB LED strips - where all the LEDs are glued (I assume) under a white cover:So I can't actually get access to any of the LED pins - in fact, I can't even see any pads for the LEDS either? (Unless you can spot them in the photo above).Yes, that's a problem with COB. The reason I said to use the LED pins is because you can have a break at the LED solder joint. This means that if you measured to the copper pads you'd have continuity, but it wouldn't go right to the LED. So you'd think it was the LED at fault when really it's the soldering.In that case you'd have to measure to the copper pads. OTOH you probably can't repair COB strips.I am powering the strips from both ends - I'll have a power injection at the front of the strip - and another at the end of the strip (roughly 18m run). However, since these are addressable - I assume by default, they should all be dark until I pass in a data signal, right?They *usually* start dark. If the power interruption has been short they may retain their previous state. I think slow power-up may also give random states. I've seen it fairly often and te only defence I know is to write them dark as early as possible. Arduino isn't good at this as it has a long boot delay.But the oscilloscope is mainly used for the data lines right? You'd expect +5V (or +24V) to be pretty static over time?Yes. You may occasionally get problems with spikes on the power lines, or dips that are too short to see with the DMM though. Ideally, test the whole system close to your bench and don't try to find this sort of thing in the field but sometimes it's unavoidable.You probably only need about 10MHz bandwidth to see the data so an expensive portable scope may not be necessary. In the field I've usually found the tests using live data sufficient.Scenario 1 - Break in the strip (e.g. bad solder joint, or physical break).What sort of resistance should I be seeing between DO and DI along the strip?I just tested with a new cheap 5V W2812B strip.From GND to GND pin - it was around 0.2 to 0.3 Ω. And from +5V to +5V - also between 0.2 to 0.3 ΩFor the data lines- it was around 33 kΩ from DI to DO - is that what I should expect?And can I just measure resistance between the pads at say, the start, and power pads at the end - and then try to narrow down the break?Yes, measure start to halfway, then halfway to the end. Whichever isn't connected, halve again until you get down to the faultAnd like you said, check between say, D to +5v (or whatever voltage that strip is), and ensure there's no continuity there.All the good connections should be 0.3 or less. You're measuring DO of one chip tom D1 of the next, which is directly connected. All bets are off for resistance through the chip, but I'd expect it to be several k.You mentioned using a oscilloscope for this part, to try to find a break in the strip - is that just to measure the voltage along the data lines? Or something else?It's a quick way to look for valid data. You want to see a characteristic pattern of rectangular pulses going 0v to 5v. You will sometimes see signals of widely varying height - most commonly a partial short to ground or one data line to another. The latter may be impossible but I sometimes have multiple simultaneous data lines. You may want to do this if you have a large array where the refresh time is too long.Scenario 2 - Short Circuit across pins (e.g. D shorted to GND)I'm sorry - I didn't quite understand this one:QuoteDo you mean check resistance from DI to DO, along the strip? If there's a short between a Data pin and Ground - wouldn't I be checking between D and GND? Or did I miss something here?DO and D1 are connected between every pair of leds, so you can look for either D0 or D1 shorted to ground as long as you do it at every led-led junction because that won't be visible from an adjacent joint.