giuino wrote: ↑
Tue Nov 13, 2018 4:05 pm
It's simpler to find a 12v power supply than a 5.2V one (5.2v is odd, because standard USB is generally 4.85V, so any generic "5v" adapter will result in an undervolt Pi).
"standard USB is generally 4.85V", is nonsense the standard is 5.00V, not 4.85V. But yes, all power supplies do have an percentage that voltage can deviate, but nominally
they should deliver 5.00V
The problem is not that, actually, but when you use such
thin USB cables that the voltage drop over them is more than [5.00 (nominal USB output voltage) - 4.65V (low voltage threshold) = 0.35V
], then you get in trouble!
That means that any (USB) cable, used with a 2A current running trough it, and a resistance larger than R=V/I = 0.35V/2A = 0.175 Ohm, (=175 milli Ohm) will cause a problem.
As the currents runs trough twice through the length of the cable, then, for say a cable of typically a meter long, the copper length is 2 meter, and so we need a copper wire gauge where two meters of it has less than 175 mOhm, or one meter (the typical length wire gauges are expressed in) must be better than half that or 87.5 mOhm. so according to wikipedia (https://en.wikipedia.org/wiki/American_wire_gauge
) we need a copper wire gauge equal to or better than 24AWG (for 80 mΩ/m) , that means we need wire with a copper diameter of half a millimeter.
Half a millimeter copper wire (for both power wires) you will find only in really thick cables, some cheaper cables use much thinner cables, some don't even use copper wires.
I have opened up some cheap Chinese USB cables to find that the "copper wires" consisted of small strips of electro plated (with aluminium) plastic, to keep the cost down, and increase flexibility of the cable.
To keep the copper thickness of the USB power cable low, one solution we can choose is to compensate the voltage drop a bit by starting with a slightly higher voltage to begin with, so with 5.2 Volt, the cable may drop 5.2 - 4.65 = 0.55V before the under-voltage detector is triggered.
The official supply, not only can provide a stable output voltage of 5.2V at 2.5A, it also uses a wire gauge that is sufficient for the task.