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I want to power 25 adafruit NeoPixel LEDs. According to the specs they draw a maximum of 60mA at 5V each. In total that would be 1,5A at 5V.

I would like to use a 2A usb phone charger as a power supply for the LEDs. If I cut open an old usb cable, there should be 2 power lines in there. I would like to use those to power the LEDs.

Is there any reason that an usb phone charger should not be used as a power supply? Is there any reason that this is a bad idea?

Thank you very much for your advice!

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Given that you at looking to use it at 75% of it's rated capacity, you should be good to go. USB chargers are ubiquitous as basic 5V switching supplies.

It will work, given you buy a decent/reputable usb charger and not a cheap knockoff of questionable quality. A decent charger will have proper isolation between the mains side and the dc output, as well as safety features like a fuse or Over Current Protection. A cheap one may not actually put out 2 Amps.

Keep in mind that some chargers will put out slightly more than 5V, to make up for power loss over the cable. This would benefit you. You want to use a quality cable as well, with thicker wires. Cheap wires may use aluminum instead of copper (or copper coated aluminum) and of a smaller cross section, resulting in higher power loss at higher distances/current.

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I read that dirty DC works better for charging battery as it gives longer life to the battery. If so, the charger could not used as power supply unit. I think Some smoothening capacitor must be used to use the charger as power supply.

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    \$\begingroup\$ Re: "dirty DC" Can you provide a source for that claim? \$\endgroup\$ – mike65535 Nov 8 '18 at 15:43
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Not a good idea. The design requirements for a power supply are more stringent than those for a charger circuit and can't be guaranteed have sufficient smoothing or voltage regulation.

To use a charger for this is inviting problems - some subtle and not immediately obvious.

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    \$\begingroup\$ It is a good idea if you specify the limits to the OP. So many LED's per power source, etc. Do not dismiss questions without doing some thinking about what is possible-and not possible. \$\endgroup\$ – Sparky256 Mar 2 '18 at 23:05
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    \$\begingroup\$ A "USB charger" is a power supply. The real charging circuit is inside the phone. \$\endgroup\$ – immibis Mar 5 '18 at 5:08

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