I think they short the tx/rx pins together, then the target device can sense this and know it can pull more current from the charger... The extra ¼ volt isn't going to make any difference.
You should aim for a wall wart that can supply at least 700mA to power a Pi. Any more is a waste, but it might indicate the PSU can actually supply 700mA should it be required.
Gordon
Quite possibly, but more likely, the extra voltage is specified to compensate for the innevitable volt drop in the cable and plug contacts resistance (and is just at the upper limit of the +/- 5% voltage tolerance normally specced for supplies used to drive +/- 10% tolerant loads).
Of course, the best way to obviate any such cable resistance induced volt drop is to use the data wires to feed back the voltage from the plug end of the cable into the sense amp inputs of the PSU's voltage regulator.
An alternative way for a PSU to compensate the volt drop in a permanently connected lead of specified resistance is to give the PSU a negative impedance characteristic that matches 90- to 95% of the lead 'impedance'.
However, such refinements don't appear to be applied in commodity wall warts (not even the more practical negative impedance trick), hence the 'slightly higher than standard' output voltage trick employed by that skypephone wall wart of yours.
-- Regards, J B Good
I was hoping somebody else would chime in with additional measurements. I'm wondering why the Pi is so much higher than the idle reading of the Beagle Bone, which is stated to be
170 mA at 5V. I'm more concerned with the long-term average on a system not doing a whole lot than the WORST_CASE load while booting up. The Bone is also a lot higher while booting Linux.Jon
I should just add my reference to the Amoi charger reveals that I didn't know the difference between a mini and a micro USB connector.
Now that my RP has arrived I see my error, and have borrowed my Blackberry charger to get it going.
-- Graham. %Profound_observation%
I made some measurements on a Raspberry B:
-Around 320 mA after booting and idling
-Around 370 mA after plugging in the LAN and having a light python server running
-Around 460 mA with LAN and WiPi
So LAN is roughly +50mA and WLAN +100mA
Pere
GREAT, thanks very much for the data!
So, if the Beagle Bone's report of 170 mA at 5 V is to be believed, that is a good deal better than the Pi. I have been using the original Beagle Board in non-battery applications, so I was not concerned with power up to now. The Beagle Board has no net, so I used some cheap
3 port USB plus Ethernet adaptors, and part of that adaptor REALLY runs hot.(really don't know if it is the USB hub or the Enet that is the power hog).Anyway, the Bone has built-in Ethernet, and it was supposedly connected and running when the 170 mA reading was taken. I would not be using wireless in this system. So, if the Bone reading can be believed, it apparently runs on half the power of the Pi! That's kind of a surprise. Does the Pi use linear regulators to produce the 3.3 and whatever other lower voltages its chips need? The Beagles use switching regulators, as far as I know. Maybe that accounts for the difference.
Thanks again!
Jon
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