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Difference between GPIO and 3.3V pin

Posted: Wed Dec 09, 2015 7:52 pm
by linderman
Hello all,
sorry for the stupid question first of all.

I know that GPIO pins can supply 3.3V and i must try to draw max of 16mA from a single pin.

My question is about pin 1, 2, 4 (3.3V, 5V, 5V) - what i need them for? For example i can light a LED from any GPIO pin, why i need #1 - 3.3V pin since i already have all the other GPIOs that can supply 3.3V anyway?

And what about 5V pins - what should or can be used for?

Sorry again but i dont get how are they different from the rest of the general purpose pins..

Re: Difference between GPIO and 3.3V pin

Posted: Wed Dec 09, 2015 8:04 pm
by rpdom
Pins 1, 2 and 4 are "always on" power pins for supplying external circuits. They can provide much more current than the GPIOs.

For example you may have an LCD display that can be controlled by GPIO at 3.3V, but require a 5V supply to run. You connect the power input pin of the display to pin 2 or 4, ground to pin 6 (or any other ground pin), and the GPIO pins as needed.

Re: Difference between GPIO and 3.3V pin

Posted: Wed Dec 09, 2015 8:18 pm
by linderman
Thank you rpdom!

Can i use pin 1 and any other i/o pins in the same time?

Ex: I get 45 mA from pin 1. And lets say i want to get 8mA from 3 general i/o pins. This means that SUM output = 69mA? (Since i read somewhere, that max overall current from all GPIO pins must not exceed 50mA.)

Thank you again

Re: Difference between GPIO and 3.3V pin

Posted: Wed Dec 09, 2015 8:28 pm
by joan
The power rail pins are not GPIO, e.g. see https://en.wikipedia.org/wiki/General-p ... put/output

Re: Difference between GPIO and 3.3V pin

Posted: Wed Dec 09, 2015 8:30 pm
by flubbard
I would think that you would not want to do that. Generally, the I/O pins are programmable, and may be tied to resisters and other internal circuitry on the board. This is different than the general supply.

You would not want to accidentally short your board supply through the GPIO pin...that could be bad!

I should add... I believe that the GPIO are primarily for data transmission, not power supply. You would need to provide external circuitry if you wish to power something based on the signal off the GPIO. Also, you would not want to draw too much power of the board for external circuitry, simply from the standpoint of not wanting to overly tax the input power side of the Pi board.

- Barry