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Usb made simple


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I was reading the Orange Pi forum and someone in a topic conversation asked why not use a USB for power to a Orange Pi board.

I am not smart enough to know why but did find a good explanation of USB online when searchingthe internet.

Hope Armbian don't mind me sharing it here as I think it is informative for other novices like me and helps understand USB.



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Unfortunately the three most important pieces of information when (Micro) USB is misused to power devices are missing:

  1. Micro USB is rated 1.8A max. Always keep this in mind when you see a device with a powerful CPU and some USB ports (each port has to provide up to 500mA for peripherals so you can do the math how much amperage might be available for the device itself!)
  2. Cable resistance. Most cables have a resistance way too high but users don't even know that the problem is between board and PSU. In case you have to use USB to power a device always look for AWG20 ratings (most cables are 28 instead which leads to severe voltage drops!)
  3. USB encourages people to use the wrong PSUs. Neither cheap phone chargers are sufficient nor 'smart chargers' (that provide only 500mA to Single Board Computers!).

Some background info as well as calculations regarding cable ratings/length can be found here: http://goughlui.com/2014/10/01/usb-cable-resistance-why-your-phonetablet-might-be-charging-slow/


Regarding issue 3 you should be aware that 'smart chargers' try to negotiate more amperage with devices using any of the available USB power delivery protocols. A single board computer that has a Micro USB jack to be powered with isn't able to signal the need for more than 500mA and this is then the maximum it gets (causing boot and stability problems).


TL;DR: Never buy a device that comes with Micro USB for DC-IN unless you understand the 1.8A limitation, don't use host powered USB peripherals and are willing to buy also an extra short AWG20 rated USB cable. Also forget about the ratings printed on power supplies unless you measure yourself (and always keep in mind: voltage drops only happen under full load / undervoltage is the real issue).


Another good example is this here: https://www.loverpi.com/blogs/news/93532993-canakit-2-5a-vs-loverpi-2a-power-adapter-comparison

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