donkey
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Joined: Thu Feb 11, 2016 8:28 am

Need a power supply recommendation

Thu Feb 11, 2016 8:51 am

I've purchased an addressable LED (neopixel-ish) strip from Amazon which doesn't come with a power supply and I'm trying to hunt down something suitable. The voltage requirement is 3.5V-5.3V and the seller told me the LEDs combined will require 2.4A (120 LEDs @ max. 20mA / channel).

I have a couple of adaptors already but they don't seem suitable. I have a 12V 2A adaptor - voltage too high, I assume it may damage the LED strip, and I have a 5V 1A adaptor, amps too low - I assume the adaptor will get very hot and the LEDs will be dim, but I wonder if is safe for me to use this with the LEDs on a low brightness setting for a test.

I read that a device will only draw the amps that it requires, but what about the voltage? What are the rules of tolerance for safely powering something when I can't find an adaptor which provides the exact requirements? Currently the closest match I think I can find is a 5V 4A adaptor on Amazon. Will that be okay?

Any recommendations or advice are greatfuly appreciated!

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rurwin
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Re: Need a power supply recommendation

Thu Feb 11, 2016 9:29 am

This is the one I purchased for the same purpose: http://www.amazon.co.uk/gp/product/B005 ... ge_o05_s00 although I haven't got around to using it yet I am happy that it will do the job.

Note that it needs installing inside some case by someone who knows what they are doing -- the live connection is open and it is right next to the 5V terminal.

Also, with 5V around, make sure that you protect the GPIOs against ever being connected to it. That will fry your Pi. Connect the ground on the power supply to the ground on your Pi. I'm intending to power them both from the same supply by feeding the power to the Pi on the GPIO connector but if you have separate supplies do not connect the 5V together, just the grounds.

The rules are that the volts must be correct, but the amps only needs to be high enough. 4A is fine, even if you are only taking 2.4A from it. It is better in fact as you will be stressing the supply less by running it at less than its maximum current.

HOWEVER Your supplier's maths is wrong:

120 LEDs ecah with three (R,G and B) channels is 360 channels. 20mA x 360 = 7.2A
See https://learn.adafruit.com/adafruit-neo ... uide/power for a more nuanced discussion.

donkey
Posts: 2
Joined: Thu Feb 11, 2016 8:28 am

Re: Need a power supply recommendation

Sat Feb 13, 2016 8:28 pm

Thanks, I appreciate the reply! I've ordered a 5V 8A supply with 5.5m connector, I already have some jacks that will allow me to convert that to volts and earth.

I'm looking at following these instructions when it arrives. Exciting times ahead. http://popoklopsi.github.io/RaspberryPi ... p/#/ws2812

Rnd(100)
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Joined: Mon Feb 08, 2016 9:19 pm

Re: Need a power supply recommendation

Sun Feb 14, 2016 9:31 am

With regards to the current required :

If you run them all (I.e. Red, green and blue together to produce white) at full brightness then it's 20 mA per LED so yes a MAXIMUM 7.2A current draw but if you're buying addressable led strips then you're probably paying the premium because you want not just white.

The neopixel library drives the LEDs with PWM, so by setting the maximum brightness parameter much lower than 255, (to test it - if you can't wait) with a 2A psu, you could set the max brightness to 50 I.e. About a fifth (50/255). This should mean a current of around 1.5A (7.2/5).

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