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I bought a UBEC (Universal Battery Elimination Circuit), which, for those not familiar with it, is a "5 volt" dc-dc power conversion circuit (you can get them as high as 5A). It gets it's name from the function of removing the need for an additional battery, and hence weight, in some sort of "drone" like flying machine that needs both 12v and 5v. Searching for UBEC is quick and accurate, with resulting hits cheap and small.

My goal is to power other stuff and a Raspberry Pi with one 12v POE line (no, not 48v. i'm building this myself. So, if i want 12v, i'll inject 12v).

Now we get to the meat of the question.

The UBEC has a 0.10 inch spaced 2-pin female connector on the 5v output end. This is just begging to be connected to the adjacent Gnd-5v male pins on the GPIO.

Now the problem: The nominally 5v UBEC is actually producing 5.3v according to my handy multimeter, and the GPIO pins are supposed to be on the "safe" side of the on-board voltage regulator.

And the question: Does anybody have experience injecting 5.3v (or more) directly into the GPIO pins of a model B/B+? Does it work? Or does it cause the magic $35 smoke to be released from the Pi?

I would simply try this myself, but at the risk of burning an otherwise beautiful $35 Pi, i was wondering if anyone has any prior experience that i could learn from... or will my Pi become the guinea Pi-g?

Yes, i could disconnect the existing connector, micro solder a micro usb connector and go the normal route in. But i'm telling you, at night, i can hear the unprotected GPIO pins begging for that UBEC.

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    You are measuring the open-circuit voltage. It will probably drop to about 5 V when connected to the rpi. – Max Ried Nov 4 '14 at 21:27
  • @MaxRied is correct, this was an in-situ test, fearing in-vivo might have kill the patient. A 2.5-watt 10-ohm in-vitro might have made the question moot. Didn't think of it. – bmikeb Nov 5 '14 at 5:15
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I use a UBEC like that. I carefully cut off the signal part of the socket so I could fit it over the 5V pin adjacent to the ground pin.

5.3V will not harm your Pi. I have measured 5.8V on my Pi test points. The only concern would be anything connected to the USB ports.

EDITED TO ADD:

The Pi I'm currently back powering via the USB ports is showing 5.55V between TP1 and TP2.

  • Nominal USB voltage tolerances is +-0.25 V. So anything above 5.25 or below 4.75 is at least non-compliant, probably dangerous. – Max Ried Nov 4 '14 at 21:25
  • I fried my Pi today with an UBEC, not sure if it was the over voltage because it was running for hours streaming webcam. I tried to fit it into an RC car and smoke came out, i think I shoreted something but cant see how.. anyway. My WiFi is pretty damn stable though! better than the 1A HTC charger. I am wary of this 5.65V, I measured on my UBEC, I know when you apply load its supposed to drop but I think its still about 5.2v~5.4v. Maybe a resistor would be a good idea, burn of some volts, difficult to say. Check meta post... My cheap RC car ran off 4.75.. but it works fine with UBEC power. – Piotr Kula Nov 4 '14 at 22:17
  • The answer from joan is spot on! @MaxRied only states the obvious, if this was compliant there would have been no need to ask the question, now would there. – bmikeb Nov 5 '14 at 5:20
  • READ THIS: Just be careful, I fried my brand new Pi 3 doing this...I wish so bad those GPIO were protected; its the best way to power the Pi in a portable setup. See here: raspberrypi.stackexchange.com/questions/49991/… – NULL Jul 18 '16 at 20:14
  • @NULL. What did you actually do? – joan Jul 18 '16 at 20:18
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As @Max Ried has stated the voltage is likely to drop under load. It is almost impossible to build a regulator which works down to zero load.

Even if it didn't it won't harm the Pi. The +5V is only used to power the peripherals (USB Ethernet) and is regulated down for the SOC. 5±0.25V is the nominal operating range, but exceeding this slightly is unlikely to damage components.

It is OK to power the Pi through the GPIO, although this bypasses the polyfuse and the transient overvoltage protection. Strictly you should have isolation - there is a recommendation for peripherals which do this, but provided the supply is well regulated it is not necessary.

  • 5.8V as stated by @joan while being connected to the Pi hardly qualify neither as "zero load" nor "slight overvoltage". – Ghanima Nov 4 '14 at 22:18
  • For the Pi itself 6V is the danger spot at which it may be damaged. I expect for a USB device you'd have to treat everything on a device by device basis. – joan Nov 4 '14 at 22:27
  • Does USB Vcc gets passed through the entire board or is it regulated? – Max Ried Nov 5 '14 at 5:40
  • @MaxRied The Pi's micro USB power input goes through a polyswitch fuse then past a Transient-voltage-suppression diode parallel to the 5v rail. Then there is a 3.3v linear regulator that feeds the cpu. The more i look into this i'm beginning to realize my original question was kinda silly. Because, the only thing i'm bypassing on the GPIO pins is the 6v polyswitch. – bmikeb Nov 5 '14 at 6:18
  • Actually I wanted to ask wether the USB OUTPUT is balanced or not. But maybe this should be a different question. – Max Ried Nov 5 '14 at 6:19

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