1
\$\begingroup\$

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.

\$\endgroup\$
6
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ Are you saying thick wires have less inductance than thin wires? \$\endgroup\$ – marcelm May 17 '18 at 21:48
  • \$\begingroup\$ @marcelm Less inductance, and less resistance. Both are helpful. \$\endgroup\$ – duskwuff May 17 '18 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 May 17 '18 at 21:55
  • 1
    \$\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 May 19 '18 at 12:29
  • 1
    \$\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 May 19 '18 at 22:54
1
\$\begingroup\$

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.

\$\endgroup\$
  • \$\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 May 19 '18 at 9:29
  • 1
    \$\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 May 19 '18 at 14:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.