Core Electronics Forum

Pi, Fadecandy, LED's and capacitors

New user here, but going big. I am planning a project running 4 x 7 metre WS2812 B 60/m LED strips, entire project run by a RPi, each strip controlled by a separate power supply and fadecandy. There will be power injection every 2 metres or so. Data will be fed to each 64 IC section.

My question is: In the Adafruit NeoPixel Überguide, a capacitor (1000 µF, 6.3V or higher) across the power connection is recommended at the start of an LED strip. Should I put a capacitor in wherever there is power injection?

I’ve done a number of searches which bring up the use of a capacitor, but haven’t found anything that talks about capacitors and power injection.

As the UberGuide says

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 as shown above. The capacitor buffers sudden changes in the current drawn by the strip.

The current draw of the LED strips can vary dramatically, depending on things like how many LEDS illuminated, brightness, and such. The large cap is to help prevent the power supply sagging when all the LEDs are on blind the watcher bright.

So yes, add the cap to each power supply.

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Ah of course. I think I had tunnel vision. I need to add the capacitor to the feed in line before any branches that take the power injection further along the strip. For some reason I was focused on placing it as close as possible to the start of the first section of LED strip, which is some way along a branch in the power circuit. In reality, it is only the first section with respect to my final data layout, which is mainly defined in software.

Thanks for your reply, it flicked a switch in my brain somewhere.

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You should put a capacitor at each power injection point closest to the LED strips. The longer the leads without capacitors, the more inductance and more radio hash. Might pay to put a small ceramic in parallel with the electrolytic, electrolytics don’t bypass high frequencies as effectively as ceramics.

Also you should wire each injection point with a separate power and earth back to the power supply, don’t daisy chain. This minimises voltage drop, and also minimises interference between one LED bank and another. If you daisy chain, the devices at the end of the chain will be affected by the voltage drops caused by devices at the start of the chain. This may be small but why take the chance?

I bought a 12V 5A supply from bright_future_usa (ebay) for $17 delivered. Works fine. Size power supply to double the expected draw or 2/3 maximum draw, whichever is greater. Switching power supplies are usually most efficient around 50% load. Not a recommendation, just a FYI.

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Thanks. I was wondering if multiple capacitors would improve things in a large installation, or on the other hand, cause any problems. I hadn’t seen the idea of adding a small ceramic at all, I’ll look into that as well.

With regard to power supply, I have over-sized the feed-in power lines (almost double the calculated value) to minimize voltage drop, but I’ll work out a way to increase the number of feed-in lines. The install is built-in so next to impossible to change wiring later, but I intend to do a mock up beforehand to try out the cable lengths I will be using. And I will be throttling back the maximum brightness in software.

Thanks again.

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Do you have a reading recommendation for your comment on caps above? Or a recommendation on an appropriate cap to use? Thanks.

I subscribe to Silicon Chip magazine and look at their circuits. Almost every chip in a circuit has its own ceramic bypass, usually 100nF. This was stated to reduce radio interference and unwanted interaction between chips.

Regarding electrolytic capacitors, I revert to basics. A one Farad capacitor drops one Volt per second if the current draw is one Amp. So if you have a switchmode power supply delivering say 5A and you want to reduce ripple to 100mV and the switching is at 100kHz then you need 5 [for the Amps]/(0.1 [Voltage drop]*100,000 [frequency] Farad = 1/2000 Farad = 500uF. I don’t know how tolerant your LEDs are of ripple in the supply, but reading the specs implies they just need something in the region 4 to 5V. I think 1000uF is overkill but if they are cheap and fit the space then go for it. I don’t know if the people who recommend 1000uF have done their sums or decided it was ‘big enough’. Only worth refining the values if you were making a few thousand and the cost of spending time doing the research is offset by reducing the cost of production.

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