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I've been reading about power supply requirements for RPi and want to learn more about the practical limits in addition to the design limits. What is the power protection circuit (if any) on the micro-USB port, what is the maximum voltage it will tolerate?

I have set up a test power supply (using a bench power supply, 7.5A (18AWG) leads to a micro-USB headshell). So I can see the relatively exact supply details. I've measured the voltage at the RPi micro-USB, and noted a 0.1V cable loss at 5.5V supply, so I believe my test cable can carry sufficient power.

While exploring the under-power situations described in a lot of articles on the web, I've varied the input voltage from 5V to 5.5V (which seems stable), and briefly risked up to 6V without frying the RPi. I am interested to know at what point I can expect smoke.

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  • If you're interested in powering the Pi safely, (rather than blowing it up), then this math may be of interest.
    – Dmitry Grigoryev
    Mar 24 '21 at 9:25
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The Pi 3 is protected by the SMBJ5.0A over-voltage suppressor, which has a breakdown voltage between 6.4 and 7.1V. So above 7.1V you are guaranteed to trigger the over-voltage protection.

If the protection works as designed, you'll see no smoke: the polyfuse will limit the current to a safe value. However, 7.1V is above the rated maximum voltage of 5V components on the Pi, so there's still a good chance those ICs will blow up before the voltage suppressor has a chance to act.

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    there's still a good chance those ICs will blow up before the voltage suppressor has a chance to act... That would be my definition of a poor design.
    – Seamus
    Mar 24 '21 at 20:59
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    @Seamus It is, and the never-ending stream of questions which all boil down to "bad power supply" and "current limit of USB ports" is the proof. I really miss Banana Pi which never had power problems, allowed to connect a battery directly to the PCB, and exposed the PSU and battery voltages and currents via /sys/class/power_supply.
    – Dmitry Grigoryev
    Mar 25 '21 at 10:03
  • Somewhat off-topic, but don't they still make the BPis? I don't know much about them - are they supported by the maker?
    – Seamus
    Mar 25 '21 at 22:12
  • @Seamus They do, and they ever release new boards. Software-wise the manufacturer support was poor even for the first BPis, and I doubt that has changed. You'd be stuck with Wheezy on the Banana Pi 1 unless you learn how to install Debian. And they lost the competitive edge too: the original Banana Pi was several times faster than the original RPi (and only 1.5 times more expensive), but the latest boards are more or less on-par with Pi 4, while they still cost more. For example, Banana Pi M3 costs $150-200 where I live.
    – Dmitry Grigoryev
    Mar 26 '21 at 8:03
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    Ouch! Somewhat similar to my BeagleBone experience.
    – Seamus
    Mar 27 '21 at 5:57
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The Pi is rated at 5.0 ± 0.25V. The PMIC (on modern Pi) has a MAXIMUM input of 5.5V - exceed this at your peril.

The "protection" circuitry consists of a combination of polyfuse, ideal diode and surge limiting diode, which varies on model (consult schematics for detail). The Pi3A+ has no ideal diode.

Schematics for most models are available.

See Raspberry Pi Power Limitations

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