I have revisited this and found that the USB cable I used for my test (cut from a defunct optical mouse!) has quite a high resistance. I repeated the test with another USB cable and got very good results:domble wrote:framil said:
I bought a single USB socket Masterplug model USBPLGW from Sainsburys last week, £2.99, reduced from £3.99. Great for charging most devices, but not for powering a Raspberry Pi. Label states: 5V at 1A. I tested it under load:
No load (open circuit): voltage 4.88V
10 ohm load: voltage 4.57V (gives current = 457mA)
5 ohm load: voltage 4.17V (gives current = 834mA)
Raspberry Pi data seems to say 4.75V to 5.25V supply is required, must be capable of providing at least 700mA at 5V.
Admittedly I only bought and tested the one, but I doubt that others of the same type are adequate under raspberry Pi load.
I bought a USBPLGB (same, but black version) from Sainsbury's. In the absense of my Pi (two weeks to go...) I just measured the output.
No load (open circuit) 4.92v
10ohm load 4.98v
5ohm load 4.78v
Data lines at 2.0v and 2.5v (according to wikipedia, apple chargers have one line at 2.0v, the other at 2.8v, to indicate a 1A capable charger).
That was measured via a normal metre-ish length USB A-B cable (not a micro one) plugged into a PCB mount socket (not fitted to a PCB, no device).
And noise wise just a few mV of ac according to the Fluke (didn't look on a scope).
So looks good to me. Pi-tastic!
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