What is the quality of the 5V supply on the GPIO

Question

I measured on the 5V pin to the ground and got stunned over the variance that I saw. First I had a Powered USB hub where my devices where attach and so the Raspberry. I just decided to remove my main power supply (mini usb), and noticed that the Pi where still running of the power from the USB hub (main USB). Here I noticed a voltage drop from 4.9 to 4.3 voltage.

Are there no compensating circuit on the bord that can maintain a "good" 5V?

Why I'm asking: Won't this be a problem if external circuits as ADCs and such are connected to the GPIOS?

Answer 1

No, for 5V there is nothing, for the internally used 3.3V, 2.5V and 1.8V there are voltage regulators as shown in the schematic below.

The RPi doesn't use 5V itself, only the 3 regulated voltages, and of-course the 5V is used for the USB connectors.

This voltage drop that you experienced happened when you were powering the RPi from the normal USB connectors and not through the normal way (the mini usb). So probably the power supply of your USB hub is not the best quality (like most of them), or couldn't handle the extra power requirements from the RPi.

About the GPIO pins, these are 3.3v, so all the ADCs and other stuff that you want to connect has to be 3.3v (or converted to 3.3v). Any reference voltages that are needed for external logic ICs is your own responsibility to make this as stable as possible.

So the answer on the question 'What is the quality of the 5V supply on the GPIO' is: the exact same quality as the power supply that you use to power the RPi.