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I have recently read that using Raspberry GPIO with 5V is risky as it may damage the GPIO pins. I did used my Raspberry with 5V without knowing this fact and I was lucky that sensors I have used were not strong enough to damage my GPIO.
However if this is the case, how does it make sense to have a 5V output from a device that is not compatible with 5V logic? Isn't it a bit misleading?
There are two types of pins on the GPIO header. The predominant type is for digital signal processing. These are I/O pins and operate at 3.3V.
The second type of pin is a power pin. There are power pins for 5V and 3.3V. These pins typically supply external devices or sensors requiring an analog power source, but they do not support 5V digital signal processing.
It might have been a cleaner design to designate all the GPIO pins exclusively to digital signal processing and provide separate terminal headers on the Pi board for the 5V and 3.3V power taps. Perhaps this would require board space they didn't have to work with.
The Pi's power network has multiple levels. The input power to the Pi is 5V so that explains why a 5V power pin is provided. The SoC and signal processing operates on the 3.3V power rail. And there is a third power rail at a lower voltage of 1.8V that supplies the Ethernet controller chip.
I can't imagine a scenario where you would damage the GPIO by using the 5V power pin. The only limitation is the current draw and that will vary by Pi model and any other current consumers (e.g., connected USB devices) running on the Pi at the same time.
I wrote a blog article about the supply of power from the 3.3V rail if you're interested in a deeper dive on that topic.
It has a 5V power rail because it is powered from 5V. The availability of the rail allows powering from the expansion header as well as the microUSB socket.
A 5V power rail is also convenient to power the hundreds of thousands of components which can be connected to the Pi and require 5V.
Similarly a 3V3 rail is provided for those components which require 3V3.
It is clearly documented that the Pi's GPIO are 3V3.
