I'm building a word clock and its a grid of 20x20 letters, each with its own LED (I know, its absurd for just a clock). I've recently got the LED's on a strip. Its the WS2812B type, just like the neopixels. There are 3 rolls of 5m strips and a single 1m strip. When I tried to power the 1m strip, everything worked perfectly and all looked quite lovely. But when I tried the 5m strip (just one of them), this happened: https://www.dropbox.com/s/5w1lok6wa7dfvzd/IMG_0046.MOV.mov?dl=0

If you didn't see the video I recorded of it, for a second or two after powering the arduino (uno) on, it works as expected but not long after it all stops the colors and fades to red, sometimes it just instantly jumps to it, like the second time in the video. I don't know what happened but all I can guess is that the arduino is not able to power the strip properly. Any ideas on what I should do to be able to power a total of 16m of these strips?

  • Each LED needs 60mA. Do the math. Apr 29, 2016 at 7:57
  • So that would be 24 amps right?
    – mr-matt
    Apr 29, 2016 at 7:59
  • And now you know why they don't use 2812Bs for it. Apr 29, 2016 at 8:00
  • So how in the world would I go about carrying that out?!
    – mr-matt
    Apr 29, 2016 at 8:00
  • 2
    I'd still run them lower than full brightness. Most people can't tell the difference between 230 and 255 unless they're right next to each other, and even 100 is still decently bright. Apr 29, 2016 at 8:14

2 Answers 2


WS2812 use around 1mA when they are off. So that's already 400mA with no LED lighted. USB can supply around 500mA. So you only have 100mA available.

Each led package has three colored leds inside; Red, Green and Blue. Every color will use around 20mA when fully turned on.

So you can only turn on 5 colors at full brightness at 500mA.

Using a 2.5A supply you could go up to 105 colors, or 35 leds full white.

This would be more than enough for a word-clock, as most leds will be off.

  • Thats not enough for our word clock, as I said in the description our clock is 20x20, which is 400 LED's
    – mr-matt
    Apr 29, 2016 at 18:47
  • The point is that you aren't lighting all leds at the same time. The word clock is quite useless if all leds are on all.
    – Gerben
    Apr 30, 2016 at 14:15

You're trying to power a consumer which draws more current than the arduino could provide.

To fix this, you need an transistor and a ~5V battery (e.g. 4.8V NiMh, or a stronger batteries plus suitable resistors, to not burn out the leds)

Look at this (awesome paint) shematic to understand the principle. The arduino will switch the transistor through S2 which then closes the circuit. Through S1, the arduino can set the LEDs values.

Note that for savety, you should also set a diode between S1 and the LEDs, so the battery cannot burn out your arduino, if the LED strip is cheap made.

The reason why the behavior at your setup differs might be because of a capacitor on the arduino which works similar to a batterie (With extreme little capacity). The capacitor might provide enough power to run the LEDs for a very short periode. After discharging, the capacitor needs some time to recharge and thus it may vary in effect.

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