For those interested:
Raspberry Pi Power SystemI took a quick look at the Pi spec sheet and my theory is correct... the Pi
can be powered through the GPIO power leads; WITH PRECAUTIONS.
1) When feeding Vcc (5v) from the GPIO, use a solid power supply, preferably one listed as "regulated" (such as a computer PSU) and/or utilize a 5 volt, 2 amp 3 pin regulator and a filter capacitor rated approx. 1600uF at 6.3 volts or higher.
2) Add a 1, 1.5 or 2.0 amp 5 volt "fast blow" fuse to the circuit feeding the GPIO pin as we're bypassing the polymer fuse that normally protects the Pi with using the uUSB connection. (Shunting the onboard fuse can be a nightmare without the right soldering iron and a steady hand, so this also makes it more convenient for hardware designers in case something goes wrong, you don't have to modify the Pi board... just your PSU circuit.)
3) The Pi has a very small tolerance (up to ~0.,25 volt drop) before it runs on a threshold of becoming unstable. An external 5v regulation circuit can help prevent USB devices from draining the uUSB connector's normal way of feeding the device without the need for a powered USB hub (adding more cables to the mix and a bad power supply with a USB hub can blow the poly fuses on that circuit.)
4) Always add more amperage than you need as power supply devices themselves have a waste level. (A PSU rated at 5v 1A does not usually mean it can
sustain 1A worth of draw, but allows an intermittent "peak" of 1A for short durations.)
5) For those wishing to use a computer PSU (ATX power supply) the task is very simple. First, it's most convenient to use a PSU with a "Vacation Switch" (on/off) on the back allowing you to disconnect power without disconnecting anything from the Pi. Secondly, to "turn on" an ATX power supply, find the one GREEN wire in the 20-pin side that normally goes to the computer's motherboard (or 24 pin if it's a solid connector) and run a wire between the GREEN lead and any of the black leads directly next to it. Grounding the green wire tells the PSU to engage. Finally, using one of the D-Sub (4 pin hard drive connectors) connect the 5v rail to Pin 2 (5Vcc) of the GPIO (image here:
http://elinux.org/RPi_Low-level_peripherals) and one of the black leads to Pin 6 (Ground) or one of the other ground lines shown.
This is UN-PROTECTED as most power supplies will allow more drain on a single 5v rail than the Pi can handle, thus the suggestion for filtering and a fuse noted above.
We're going to be creating an array (8x8) of headless Pi boards and one of the goals is to minimize wiring. The power supply feeding all these boards was the first step. The second step might also be useful for those wanting to limit wasted copper and space:
Network: Since the Pi has merely a BASE-100T network interface, and we'd prefer wired over wireless, you can literally cut your cables in HALF as you only need four wires (instead of the eight within a single CAT-5 cable.) If you have (or are willing to purchase) a crimper/stripper and some RJ-45 ends to crimp, generic flat telephone cable is all you need to create your own T-100 network cables. (Or you can use standard CAT wire and share one cable for two Pi.) To do this, you need to utilize only pins 1, 2, 3 and 6 on each end (leaving the other four pins empty.) If connecting to a hub or switch, you need only connect pin 1 on one side to pin 1 on the other, then repeat for the other three listed. If connecting a Pi to a Pi you need to create what's called a cross-over cable, which is pin 1 to 3, 2 to 6 in the following fashion:
STANDARD CROSS-OVER
==============================
Pin 1 -> Pin 1 Pin 1 -> Pin 3
Pin 2 -> Pin 2 Pin 2 -> Pin 6
Pin 3 -> Pin 3 Pin 3 -> Pin 1
Pin 6 -> Pin 6 Pin 6 -> Pin 2
WARNING: For anyone over-clocking the Pi, heat-sink modules on the processor is an almost-must, but if they're enclosed (poly cases that don't breathe) the heat-sink isn't likely going to do much as there's nowhere for the heat to "go." If you implement a cooling fan, be sure the fan is connected to a power supply NOT fed from USB or the micro-USB cable. The start-up current for a fan is quite a bit AND when powering off, the fan turns into a DC generator which could lead to data problems and the likes. A diode placed on the + lead of the fan will help prevent the fan from "feeding" the Pi electricity as it spins down and a 6-16v capacitor (~2700uF) across the leads will assist with the spin-up issues.
Welcome
FAQ
Downloads
Wiki
