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I am using a Raspberry Pi Zero, with an x706 PSU hat (2 x 18650 batteries). The X706 PSU is charged by a good quality, stabilised 5V power supply unit providing up to 3A. The Pi is hooked up to many different sensors and ADC/I2C converter boards, which all run off 3V3.

I have read that the 3V3 rail is backed by a voltage regulator rated at 1.5A, with about 1A being practically delivered, and 500mA being used by the Pi itself, leaving about 500mA to be drawn from the 3V3 rail.

I have also read that drawing close to the limit may overheat the regulator and damage it in the long run. It therefore seems safer to power all the external sensors with an external 3V3. I have a module based on an AMS1117 which will convert 5V to 3V3 delivering 800mA, and is easily/cheaply replaceable.

However, it is very difficult to get clarity online on whether it is safe to power boards, sensors and ICs with an external 3V3 to be fed back to the Pi, either through GPIO or I2C. I should add that all of the sensors have some kind of circuit between the 3V3 supply and the GPIO out (no relay that would feed back the 3V3 directly into a GPIO).

I assume safety will depend on how the external regulator is wired, which leads me to my question: how should I wire this? I see 3 possibilities:

  1. wire the AMS1117 to the 5V power supply;
  2. wire the AMS1117 to the 5V pin of the Raspberry Pi. I guess this only makes sense if I can draw more off the 5V rail than I can off the 3V3 rail;
  3. do not use an external 3V3 because it risks damaging the Pi more than drawing too much off the Pi's 3V3 rail.

Any help or clarity on this issue would be greatly appreciated!

2 Answers 2

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I don't know where you found "a voltage regulator rated at 1.5A, with about 1A being practically delivered, and 500mA being used by the Pi itself".

The Pi Zero has a PAM2306 rated @ 1A; the Pi itself uses negligible current from 3.3V rail - certainly nothing like 500mA.

It is unlikely that any sensors would use significant current and you should be able to find their requirements, but if you REALLY want to use an external supply it is perfectly safe provided it does not exceed 3.3V. (NOTE under NO circumstances connect any external power to the 3.3V pin!)

The current you can draw from 5V pin is unlimited (at least by the Pi Zero) the 5V pin is directly connected to the 5V input. The Pi Zero has minimal power circuitry and no protection.

It appears that many Pi have been damaged by poorly designed regulators which provide excessive voltage.

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  • Thank you. The figures I quoted might have been from old posts about the 3B/3B+. So if I understood you correctly, I should work out the total current drawn by the full sensor array, and start worrying about a regulator only if it reaches close to 1A, which is unlikely?
    – de_a
    Aug 20 at 4:09
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Posting my own answer here in the hope that it will help others out. I was advised here and on other forums that one can draw more than 50mA and up to 800mA from the 3V3 regulator rated at 1A thar is on the Zero 2W (PAM2306) and the output inductor rated at 2A.

I ended up drawing a total of ~100-120mA from 3V3 for more than 20 sensors at the same time without issue.

In the end, the AMS1117 was not necessary but it could have been wired to both the external 5V PSU powering the Pi or the 5V pin, with a common ground.

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