HawaiianPi wrote: ↑
Sat Jun 29, 2019 2:27 am
(some cables have too much resistance)
I would argue that all
USB cables have too much resistance.
It's difficult to get datasheets for USB cables. One I've just checked uses 28AWG for signals and 24AWG for power.
24AWG copper wires have a resistance of 0.08 ohms/metre.
With a 1m cable, that would be 0.16 ohms. (1m for the power, 1m for the ground).
V = I R
If the current is 2A, then the wire will drop 2 x 0.16 = 0.32V
5V - 0.32V = 4.68V
That is out of spec before considering that the power supply itself will not supply 5V at that current. And that's a pretty good USB cable; it uses thicker wires for power, many of them don't.
The Official (Pi 1, 2 & 3) power supply uses 18AWG cable and supplies 5.1V
That's only 0.02 ohms/metre but it's 1.5m long so 0.06 ohms for the cable
With 2A of current it will drop 2 x 0.06 = 0.12V
5.1V = 0.12V = 4.98V
I too hope that USB-C will be better than the dead-reckoning that micro-USB depends on.