I'm about to build myself a new BrewPi and I have some Arduino power/USB questions.

Something smiliar to a combination of these projects:



The device will have a Raspberry Pi Zero W, an Arduino UNO (clone?), a LCD screen with backlight, and some relays and probes. The Pi must be connected to the Arduino via USB. The Arduino will drive the screen and some relays/probes. For simplicity's sake, I would like to power everything from the same regulated 5V power supply, and for ease of wiring I would prefer to power the Pi only and have it power the Arduino via USB, with the Arduino then supplying power to the rest.

My preference: 5V ----> Pi Zero --(USB)--> Arduino --(5V)--> Screen/etc.

However, according to this datasheet the max draw of the screen might exceed 1000mA: https://www.vishay.com/docs/37314/lcd020n004l.pdf

This obviously exceeds the 500mA limit from the Arduino's 5V pin while being powered via USB. However, a Pi Zero has few safety features and does not limit outgoing current from it's USB port: https://www.reddit.com/r/raspberry_pi/comments/3zyhuq/pi_zero_usb_power_outputmilliamps/

Given this, is it feasible to short/bypass the Arduino's polyfuse and then draw more than 500ma from the 5V pin of the Arduino without too many problems?

If not, can I 'add' the power supply's 5V to the Arduino's 5V (either from the Pi's 5V pin, or direct from the power supply) at the same time as maintaining the USB connection (which uses the same 5V source)? I'm stretching the limit of my circuit understanding here. If not how should I power the other components?

Would it instead be better to have a 12V supply and power the Arduino directly, then attempt to power everything else from its 5V pin (remembering that there is still a USB connection from the Pi back to the Arduino). What I'd really like to avoid in this situation is having to provide a second 12V->5V conversion.

Any help or advice would be greatly appreciated.

  • 3
    neither the RPi, nor the Arduino are meant to be used as a power supply ... using them as a power supply makes them act as an expensive fuse
    – jsotola
    Jun 14, 2020 at 23:55
  • just run everything off the 5v, don't use USB connectors.
    – dandavis
    Jun 15, 2020 at 3:54
  • @dandavis My concern with that option is I must also have the USB connected from the Pi, and I haven't managed to find a source that says that's going to be OK
    – DanielC
    Jun 15, 2020 at 5:06
  • When you power the Zero and the Uno with the same 5V power supply, the additional USB connection between them should be OK. But the power has to run outside of both boards.
    – chrisl
    Jun 15, 2020 at 7:30
  • 1
    I'm not sure where you got the 1000mA from. The LCD itself only has a max of 10mA. You might be talking about the ratings of the background LED with has a max of 1080mA. I very much doubt you can run the LED at that current. It would probably be very bright for a short while, and then die. That value is probably the max pulsed current (in short bursts).
    – Gerben
    Jun 15, 2020 at 14:55

1 Answer 1


Do not bypass the fuse. Drive your external components directly from the external 5V power supply. Do not pass everything through the Arduino.

You say you have a regulated 5V power supply. Assuming it provides enough current you should use it to drive your external components directly.

You might need to add a filter capacitor near the feed for the Pi/Arduino, to filter out fluctuations in the 5V supply as your high current components switch on and off. You should be able to google "Power supply filter capacitor" to learn more.

  • Yeah, this. They didn't put that fuse there just to stifle you while the thing is actually capable of more. They put that fuse there so you don't burn up the boards you paid good money for.
    – Delta_G
    Jul 16, 2020 at 0:52

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