Friday, 8 February 2013

Pi Power Dance

Do it this way, really!
Power seems to be at the root of almost all Raspberry Pi problems. I refused to believe what I read and you probably will too. The experience could be a valuable relearning of the basic high school physics of electricity or it could just be frustrating. You can avoid this learning experience by hooking power up like in the picture above. (Yes, there may also be HDMI and network cable connections to the Pi, but they shouldn't be carrying power.)

What I Learned (after much dancing about)

Most power supplies are OK: not great, but OK, meaning they can supply something close to their rated power while still providing a voltage greater than 4.75 volts at the power supply outlet. There are certainly some really crummy power supplies out there -- avoid counterfeit Apple stuff.

USB Micro B cables have thin wires: thin enough to have relatively high resistance, and thin enough that the voltage at the micro B plug on an operating Raspberry Pi can be 4.65 volts even when the voltage at the power supply is still 5.02 volts. V=IR and these cables were really not intended to be used as extension cords.

Tiny WiFi dongles use big power: Sending radio signals requires power and too low a supply voltage will cause the communication to fail. The power draw may pull the supply voltage even lower and cause erratic behaviour from low power devices like a keyboard or mouse.

CAUTION! You may be able to power the Pi from the Hub: but only Maybe! The hub will need to be one that provides charging ports capable of supplying more than the usual 500 mA of current. You will have to be lucky, and stay lucky. If the hub power supply provides 5V it will drop before it gets to the hub due to resistance in the power cord, then it will drop across the circuitry in the hub, then it will drop some more in the USB cable to the Pi. In my case that gets it down to about 4.5 to 4.6 V on the Pi, depending on what the Pi is doing at the time. That seems to be OK if there is nothing expecting any power at all from the Pi USB ports -- the hub is only getting its signals from the Pi USB and the second port on the Pi is empty. Anything hooked to the GPIO 5V pins will also see this reduced voltage and may not work as expected. Go back to two separate plugs in the wall at the first sign of trouble. CAUTION!

5.25 Volts is within spec: You could solve most of these problems by using a 5.25V like the one Adafruit sells, but they are currently out of stock and not expecting more for weeks.