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I need to power a Raspberry Pi 3 via its GPIO header: I have a beefy regulated PSU capable of providing stable 4.98V, which I planned to route to the 5V pins of the Raspberry as stated in this answer: How do I supply power through the GPIO?

The problem is I need to use a lot of 3.3V devices (3-4 ICs and various outputs) and I've read that I can only take ~80mA from the GPIO header reliably. I am planning to add an LM317 biased to provide the necessary voltage, but I am not sure if I can connect it back to the board or if I should leave it separated from the Raspberry and use it only for my other devices.

What is the safety margin I have to respect to be able to connect an external 3.3V to the Raspberry header?

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Francesco Bertolaccini
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Never connect a power source to a Pi 3V3 pin. You'll likely damage the Pi.

Power via one of the 5V pins - preferably with a 5V supply (you can use lower voltages but that will stop things which need 5V from working).

The 5V pin feeds in to the power supply chain which will then supply the 3V3 rail. You can draw say 750 mA from the Pi3's 3V3 rail. The often quoted 50 mA figure is for what may be supplied by the GPIO. The 3V3 rail is not a GPIO.

joan
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  • So this source is misinformed? https://pinout.xyz/pinout/pin1_3v3_power – Francesco Bertolaccini Jul 26 '16 at 22:14
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    @FrancescoBertolaccini Yes, it is wrong. The 50 mA limit is for the power which may be sourced or sinked through the GPIO (although I've used more than 50 mA through the GPIO without damaging my Pis). The 3V3 rail is completely different. On a PiB+, Pi2, Pi3 the 3V3 rail can supply far more, perhaps up to an amp in total (although that would cause the regulator to get hot and perhaps die). – joan Jul 26 '16 at 22:23
  • I believe the source of the 50mA was Gert's comment "The raspberry-Pi 3V3 supply was designed with a maximum current of ~3mA per GPIO pin. If you load each pin with 16mA the total current is 272mA. The 3V3 supply will collapse under that!" **3*17 => 51 mA**. – Milliways Jul 27 '16 at 05:15
  • @joan do you know if there is a place with reliable information about what may be expected to work vs. fail on which models if the Pi is supplied with 3.3V on a 5V supply pin as you mention? I have found many contradicting sources / stackexchange answers / etc. – Zorglub29 Feb 28 '20 at 09:02
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See Raspberry Pi Power Limitations

In particular NOTE the Foundation warning:- "Under no circumstances should a power source be connected to the 3.3V pins".

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Milliways
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I am by no means an electronics guru and have nothing to back up my findings except experience, but I have run a Raspberry Pi3 for a minimum of 30 minutes with a 3.7v 500mAh Li-Po battery connected +to Pin1 and -to pin9. I have done this to record video while running a Lighttpd server to serve the video over the Internet. It seemed to function fine.

What will cause the Pi to fail is a mystery to me, but I own about 7 Pi's, so I am not concerned with a hardware failure.

My camera is hooked up to a servo to "look" higher and lower, I have NOT tested the servo while running the PI on the 3.7v Li-Po battery. This seems like it could pose a power supply problem. With my servo drawing 5v power from Pin4 (grounding to Pin6 and control at Pin12 [GPIO18]) then I would be relying on the Raspberry Pi3 to convert the voltage from 3.7v to 5.0v - that seemed like TOO MUCH to ask of it. I don't know why, but since I don't use 3.3v, I just think it's an "internal" voltage that they give us access to for reasons I have yet to explore.

I think the RAM, Processor, camera, and other "internals" can handle 3.3v or 3.7v just fine. I think the other "stuff" on the "board" itself is NOT designed to jump voltage UP so as to power 5v peripherals. Even low-power 5v LED's seem dangerous to me and and I am not willing to sacrifice a Pi to find out if I can "blink an LED" or move the Camera.

All I'm saying is that I HAVE had success running the Raspberry Pi3 model B V1.2 at 3.3 volts (I have NOT tried this on a Pi2 or any other version), but I don't think it is wise to try to use the GPIO pins if you do this.

Johnny Catt
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  • Umm. 3.3V on the extension header is connected to the XR77004 power regulator's 3.3V output. This IC is not designed for powering the rest of the Pi that way, at minimum it's overloading at least two internal protective diodes, and I'm not going to try to reproduce this feat (if true). – Matthias Urlichs Dec 07 '20 at 09:36
  • NB, even worse, the buck converter fights your LiPo battery. It wants 3.3V on that output and will burn off power to achieve that. Frankly I can't believe it'll produce a Power Good signal under these conditions. – Matthias Urlichs Dec 07 '20 at 09:38
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I've just tested the setup on my Zero W board: fed it with 3.28V directly through 3V3 GPIO pin, and left 5V pin unconnected. Looks like it somehow works quite well: under CPU stress test it draws 0.5A from the external power supply, and temperature of the voltage regulator stabilizes at 33°C (with core temp reaching 57°C).

From the schematics it looks like the voltage regulator, seeing the voltage being less than 3.3V on its output, short-circuits 3V3 and 5V rails, therefore having some voltage on 5V rail to have something to make 1.8V from. Of course, anything requiring 5V to operate (HDMI or USB) will not work, but Wi-Fi works just fine.

I have this feeling, however, that going even a little bit above 3.3V will likely fry the voltage regulator (as it'll try to PWM-regulate the voltage on 3V3 rail and short-circuit it to the ground). Might make sense to feed the board through a Schottky diode to drop the supply voltage to something closer to 3.0V, just in case (have not tested it yet, though).

Goes without saying, other boards may have different voltage regulating schematics. I assume no responsibility for burned boards.

Dmitry Prokoptsev
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