I'm trying to build a pi into an enclosure which is too small to accommodate a micro-usb connector, so I'm proposing to butcher a usb cable, and solder the wires to the pi. From the schematic, pins 1 and 8 of p5 would seem ideal, except that that bypasses the polyfuse. Soldering directly onto the polyfuse input would perhaps be a better idea, but it's a SMD, and I can't identify the input anyway.
Has anyone come up with a better solution?
Assuming it is a model B pi, you can power it via the normal USB ports (not micro). You'll still need to butcher a cable, unless you have a hub which supplies power on its "back" port, i.e. the one normally connected to a computer.
-- Alan Adams, from Northamptonshire alan@adamshome.org.uk http://www.nckc.org.uk/
You can also power it via the GPIO header.
Theo
I routinely power the Pi from the GPIO header. I include a polyfuse in my own supply (although I often don't bother when prototyping).
-- Andrew Gabriel [email address is not usable -- followup in the newsgroup]
Thanks to all who replied. I think this solution is optimal for my purposes.
Here are some more things to think about when you have another board connected by and powering the Pi over the GPIO.
As I said, you can include your own polyfuse. Be aware of voltage drop across polyfuses - you will need to select one with low drop.
You will need a 3.3V supply on your board in most cases to power your logic which interfaces to the Pi. For low current use, you can steal it from the Pi back over the GPIO connector. For anything more than this, you will want to build your own using a 3.3V regulator. Ideally, you want this to shutdown if the Pi's own 3.3V rail goes, so you aren't back-feeding power into an unpowered Pi's I/O pins. You can use the Pi's 3.3V output to enable your own 3.3V rail.
Another case where you may want your own separated 3.3V supply is where it's carried off your board some way, e.g. if you are stringing a load of 1-wire sensors around your house, you might not want to carry the Pi's 3.3V rail all around the house too. Include a polyfuse in that too (before the regulator) or use a current limiting regulator, just in case the cable gets crushed accidentally somewhere.
I often want a 12V rail too, e.g. for switching power relays where the choice available for 5V operation is very much reduced. In this case, I use an external 12V power supply, and then a 12V to 5V DC-DC converter. The cheapest source of these is a car cigarette lighter USB power source which can be found very easily and cheaply, and just pull out the circuit board and use it as a module on your card. (I usually desolder the USB connector and cut the board down to make it take up less space, but you could use it as-is.) For powering the Pi, you need a 1A version, but you can get them up to 2.2A if you need more for your own circuitry. These are switched-mode power supplies, so they will draw typically only half that current from your 12V rail.
-- Andrew Gabriel [email address is not usable -- followup in the newsgroup]
Be aware that these are often electrically noisy, which can be a problem if they are near RF-sensitive gear like transceivers etc, so install them in a Faraday cage (aka small metal box) and fit ferrite cores to both the
12v input and 5v output cables. FWIW I use one to power a satnav from my glider's 12v SLA batteries. Both the 5v converter and the satnav are mounted in the instrument panel, so are close to the airband transceiver; that's how I know about the radiated RF problem.-- martin@ | Martin Gregorie gregorie. | Essex, UK org |
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