Neopixel Strip DC Resistance in Power Line - Page 1
Forum section: electronics / projects

author: Wobbegong
author_rank: Newbie
timestamp: June 02, 2016, 09:37:48 pm
content: Hi, does anyone know the DC resistance per unit length (such as ohms per meter) in the 5V power trace of an Adafruit Neopixel strip?Don't see this specified anywhere and was wondering if anyone had done the measurement.Thanks,- S

author: ajb
author_rank: Super Contributor
timestamp: June 04, 2016, 04:55:08 am
content: I just measured two specimens I have here, they're 4m long WS2812B strips, 60 LEDS/meter From Ali Express. These are four-wire measurements sensing at the top of the solder joints at each end.Strip 1:+5V: 0.495RGnd: 0.483RStrip 2:+5V: 0.498RGnd: 0.492RSo that works out to roughly 125mOhms per meter, or 2.1mOhm per LED.

author: Fungus
author_rank: Super Contributor
timestamp: June 04, 2016, 05:36:07 pm
content: Quote from: Wobbegong on June 02, 2016, 09:37:48 pmI don't know the exact resistance of the strips but you should definitely plan for having to inject power at various points along the strip if you have more than about 100 LEDs. Those things suck uphugeamounts of current.eg. 100 LEDs is 6 amps if you set them to 100% white (ie. 60mA per LED).I've got strips that are 4 meters long and 144 LEDs per meter...that's34 ampsif I turn all the LEDs on simultaneously. There's no way in hell that will work if I just connect the power at one end of the strip.

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 04, 2016, 05:55:37 pm
content: Quote from: Fungus on June 04, 2016, 05:36:07 pmYeah but in a lot of use cases you wont have them all on full white. I have 80 running in an LED music visualiser and it never peaks over 750 mA.

author: Fungus
author_rank: Super Contributor
timestamp: June 04, 2016, 06:17:49 pm
content: Quote from: ajb on June 04, 2016, 04:55:08 amAnd themore interestingnumbers:Power consumption when you turn on all the LEDs: 14.4 amps (60*4*60mA)Average current trhough the strip in the first meter: 12.6 amps (14.4+10.7)/2Average voltage drop between LEDs in the first meter: 0.0265V (12.6*0.0021)Voltage loss across the first meter: 1.59V (60*0.0265V)(ie. your 5V supply is now a 3.4V supply and there's still 3 meters of strip left to go...)Power dissipated in the first meter of strip (nb. just the strip,notthe LEDs): 20WBottom line: You cannotrely on connecting one end of those strips to a power supply and crossing your fingers.Plan to run some thick cable in parallel with the strip and tap off 5V into the strip at regular intervals. All those numbers get a lot smaller if you do that (apart from the first one, that one stays the same).

author: Fungus
author_rank: Super Contributor
timestamp: June 04, 2016, 06:19:36 pm
content: Quote from: Aodhan145 on June 04, 2016, 05:55:37 pmLet's hope your software is perfectandthat there's never a power glitch(or at least let's hope that you bothered to put in a fuse)

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 04, 2016, 06:27:25 pm
content: Quote from: Fungus on June 04, 2016, 06:19:36 pmOhh shit... It runs un fused off standard USB. I have only ever used it with good power supplies though they would just turn off output if it did.

author: Fungus
author_rank: Super Contributor
timestamp: June 04, 2016, 06:33:32 pm
content: Quote from: Aodhan145 on June 04, 2016, 06:27:25 pmAll USB ports are fused, assuming they obey the USB spec. If itissucking 750mA then you've been lucky so far (and haven't tried it on many USB ports).Whatever: Just don't write "engineer" on your CV and don't ever show it to somebody as part of a job interview.(...and don't enter any competitions with me as judge)

author: Aodhan145
author_rank: Frequent Contributor
timestamp: June 04, 2016, 07:04:18 pm
content: Quote from: Fungus on June 04, 2016, 06:33:32 pmI made it when I just started electronics I have stepped up a great deal now. I am only 16 though, Hopefully engineer will be on my CV in the future.

author: Fungus
author_rank: Super Contributor
timestamp: June 04, 2016, 07:16:11 pm
content: Quote from: Aodhan145 on June 04, 2016, 07:04:18 pmDon't worry, we've all made stuff like that when we started.If you want to impress at a job interview then explain the problems to them before they have a chance to say anything ("Yes, I know it uses too much current, I made it when I was starting out. I know better now, I even added a fuse!").

author: ajb
author_rank: Super Contributor
timestamp: June 05, 2016, 02:49:59 pm
content: Quote from: Fungus on June 04, 2016, 06:17:49 pmWhile you're right that you can't just drop power at one end of a for meter strip and expect great results, it's notquiteas bad as these numbers suggest. One of the problems with the drivers used in the WS2812 is that the output current drops off pretty quickly as the supply voltage drops below 5V, so in practice the voltage along the strip doesn't drop off nearly so quickly. If you wanted to accurately model the power consumption of the strip you'd need to model the V/I curve and incorporate it into your calculations. In many applications where brightness and color consistency isn't highly important you can go farther between power injection points without adverse effect.

author: Wobbegong
author_rank: Newbie
timestamp: June 06, 2016, 07:56:20 pm
content: I only realized after my original post that I asked the wrong question. There isn't a constant current drawn across the entire string since obv the first 10% of the strip is carrying a lot more current (ie all of current to subsequent LED pixels) than the last 10%, so the overall dc resistance of the strip isn't too useful a figure. Really the best way to model is as parallel loads separated by a few mohm each, or just to follow the rule of thumb that others have posed to inject a fresh 5V tap every couple hundred LEDs.Thanks for the input!