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Guides for connecting RGB led strips like WS2812B, which can be addressed individually, often suggest to add a capacitor in front. For example, the NeoPixel Guide states that

Before connecting NeoPixels to any large power source (DC “wall wart” or even a large battery), add a capacitor (1000 µF, 6.3V or higher) across the + and – terminals […] The capacitor buffers sudden changes in the current drawn by the strip.

But why would I want to buffer that? What happens if I don’t add a capacitor?

fyi – I do not have a background in electronics, and I probably lack fundamentas here.

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2 Answers 2

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The wire between your power supply isn't just a wire – it has a resistance and an inductivity. People tend to use thin wires, much too thin, and then wonder why their stuff fails in mysterious ways.

schematic

simulate this circuit – Schematic created using CircuitLab

If you switch on many of your NeoPixels at the same time, they demand a change in current which cannot be supplied in an instant by a thin wire – because that wire has too much inductivity. In result, the voltage dips, which may be visible but may also lead to spurious brownout resets.

The advice given in the guide however is imperfect, because common 1000 µF aluminium caps also have a high parasitic inductivity in series. You had to use an array of ceramic caps instead. Low ESL Al caps is an alternative in between.

But forget that idea completely, you should simply use thick wires between your power supply and your NeoPixels.

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  • \$\begingroup\$ Are you saying thick wires have less inductance than thin wires? \$\endgroup\$
    – marcelm
    Commented May 17, 2018 at 21:48
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    \$\begingroup\$ @marcelm Less inductance, and less resistance. Both are helpful. \$\endgroup\$
    – user39382
    Commented May 17, 2018 at 21:55
  • \$\begingroup\$ Yes, and I don't talk about a factor in diameter of 2, but of 10 or more. Then the difference becomes noticeable. People today hook up 10A circuits via 0.1mm² cables, then wonder why this fails. \$\endgroup\$
    – Janka
    Commented May 17, 2018 at 21:55
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    \$\begingroup\$ If your strip is fairly long, you should think about paralleling it by a thick power wire, and tap that one inbetween. \$\endgroup\$
    – Janka
    Commented May 19, 2018 at 12:29
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    \$\begingroup\$ An aluminium cap also has a series inductance, ESL. It's not intended but depends on the thickness of the foils used, same as series resistance ESR. \$\endgroup\$
    – Janka
    Commented May 19, 2018 at 22:54
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You need a capacitor because while the led color change can cause a large voltage drop due to resistance, inductance, power supply quality, etc, the problem lies in that these smart leds have a small microcontroller in them, that is sensitive to brown outs (dips in the input voltage). Once they brown out, they reset in unpredictable ways. The capacitor is to help prevent that from happening, by buffering the voltage drops seen close to the capacitor, and therefore the first led.

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  • \$\begingroup\$ So, in this case, a brownout may cause temporary malfunction of a LED microcontroller, but no permanent damage is taken, right? And for WS2812B, this may happen below a voltage of VDD of 3.5 V according to the WS2812B data sheet? Or at which voltage? \$\endgroup\$
    – Simon A. Eugster
    Commented May 19, 2018 at 9:29
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    \$\begingroup\$ Sometimes. The rapid change in voltage can be the issue just as much as the actual voltage it drops to. But no, brown outs typically wouldn't cause damage. \$\endgroup\$
    – Passerby
    Commented May 19, 2018 at 14:47
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    \$\begingroup\$ Indeed. I don't know the specifics of Neopixel, but addressable LED strips typically use something like a Serial Peripheral Interface in a daisy-chain configuration. If one microcontroller resets, it will likely result in garbled data for the rest of the chain. \$\endgroup\$
    – Fax
    Commented May 27, 2020 at 14:50

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