BenB
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Alternative power connection

Fri Jul 20, 2012 3:30 am

So, does anyone know what would happen if I was to solder a single pin header to each of the TP1 and TP2 points (normally used to check voltage) and use this to power the RPi instead of using the USB port? The idea is to hook it up to an ATX supply that I've been using as a lab power source...

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joan
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Re: Alternative power connection

Fri Jul 20, 2012 6:19 am

Personally I'd use the gpio pins. GND and +5V are exposed. It should work. I believe the microUSB has some protection against excessive current. Don't think tp1/2 or the GPIO pins have any protection.

portets
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Re: Alternative power connection

Fri Jul 20, 2012 6:30 am

I don't know about the gpio, but tp1/tp2 are after the polyfuse.

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Lob0426
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Re: Alternative power connection

Fri Jul 20, 2012 6:39 am

The short answer is: You can power a RasPi through TP1/TP2.

The GPIO are not protected at all, but can be used to power the RasPi. But you could also go buy a cheap $5 or less 1 foot "A" to micro USB cable, cut the "A" end off, and connect that to your power supply. Then plug it into your RasPi. Then you do not have to solder onto the board at all. You would then still have the .7A poly fuse to protect your board.
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BenB
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Re: Alternative power connection

Fri Jul 20, 2012 7:04 am

Thanks guys, got it working off the GPIO pins using an LM317 to supply it with exactly 5.04v :)

I'll probably cut up my dodgy microUSB cable and connect that way as a permanent solution

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AndrewS
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Re: Alternative power connection

Fri Jul 20, 2012 2:34 pm

I power my Pi via a short USB-A to MicroUSB-B cable. So I cut an A-male - A-female extension lead (because I had more of those spare!) in half, and used that to "inject" power from a 'normal' PSU.

BenB
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Re: Alternative power connection

Tue Jul 24, 2012 9:43 pm

Nice Andrew, I was thinking of doing that, but in the end I removed the plugs from my USB A to micro USB lead and soldered them onto 1m of twisted pair from an old Cat 5 patch lead and it's now working well off an iPhone charger.

The problem I was having before turned out to be that the cable I had was dropping 0.8v over just under 1m of cable. When I opened it up the cable had less conductor over all 4 wires than a single wire from the patch lead. Testing revealed that even over 2m of cable the patch lead only dropped 0.4v

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mahjongg
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Re: Alternative power connection

Tue Jul 24, 2012 10:11 pm

There is no real difference between using the PIO pins and the test pins, they are simply interconnected! with TP1 (5V) connected to GPIO 2 and 4 and TP2 (GND) connected to various GPIO pins amongst others pin six and nine.

The difference with powering through the micro USB port is only that between pin 1 of the micro USB port and TP1 a 1,1 Ampere polyfuse F3 is placed (marked 75 because it limits the short circuit current to 750mA = 0.75A).

A ground-plane is connected to all ground points including the GND pin (pin 5) of the micro USB port. There is one "power protection device" on board, and that is D17, which general purpose is to shorten all voltages that are over the acceptable range, although exact specifications are not available).

So if you want to bypass the micro-USB port, then it makes no difference if you do it through the test ports, or through the GPIO pins, but do mind that you are bypassing the on-board fuse F3.

If you are thinking of adding a fuse of your own, you better re-think that, as all fuses do need to generate heat to "blow", so they all have a more or less significant resistance, for testing purposes
To see if a problem is PSU related) I would advice against using a fuse, unless its very tiny, and blows at a very high current (5 or 10 A) so it has a very small resistance. The F3 polyfuse can have such a really small resistance, varying between 0.05 Ohm, and 0,2 ohm 24 hours after it has blown. If you add a fuse with too much resistance you may re-introduce an unknown factor again. So if you are using a professional power supply you should rely on its current limitation circuitry, not add a fuse, at least not when you try to debug problems that might be power related.

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