Additional power supply for circuits controlled by GPIO

[RaspbJan]

Good day guys,

I'm a total noob and I believe this question treats about very common practice, but I can't find satisfying info
Could you tell me pls some general rules about connecting external current sources to circuits controlled by raspberry GPIO and NOT burn RPI?
Assuming I would like to experiment with this circuit: http://arduino.cc/en/tutorial/ShiftOut (using RPI ofc) and connect much more LEDs... RPI could not support so much current, so there's some external power needed.

Pls give some advices
greetings

[FLYFISH TECHNOLOGIES]

Hi,

Could you tell me pls some general rules about connecting external current sources to circuits controlled by raspberry GPIO and NOT burn RPI?

Supply the external circuit with additional power supply and connect negative poles (GNDs) together. DO NOT connect positive output from this additional power supply to GPIO's power pin(s).

Take care that voltage levels are proper (they are if you supply this external circuit with 3.3V, they are probably not if you supply it with 5V or any other voltage).
Be aware that some protocols-related elements can create significantly higher (or even negative) voltages on their outputs - for example you must not connect circuit with RS232 output directly to GPIO even if this circuit is supplied with 3.3V.


Best wishes, Ivan Zilic.

[RaspbJan]

Thanks for your reply Ivan,

So, if I understand well - I should connect GNDs together, but Vcc goes ONLY from external? And GPIO 3.3v (+) pin (meaning pin nb 1 on raspberry gpio board) I just left unused? Sounds interesting, I thought when connecting some chip, there always must be its Vcc and GND connected from the same power source (to enclose current circulation)

Thx again, I'll try it!

[FLYFISH TECHNOLOGIES]

Hi,

So, if I understand well - I should connect GNDs together, but Vcc goes ONLY from external?

Correct.

And GPIO 3.3v (+) pin (meaning pin nb 1 on raspberry gpio board) I just left unused?

Yes. This means that your circuit is supplied from external power supply and not from RasPi.

I thought when connecting some chip, there always must be its Vcc and GND connected from the same power source (to enclose current circulation)

It is. As said, you have to have GNDs connected together. Consequently, the Vcc and GND of the chip are connected to the same power source - external power supply.
So, the supply power has circulation via external power supply, where the inputs into this circuit (= outputs from GPIO) have current circulation throught GND back into RasPi.


Best wishes, Ivan Zilic.

[RaspbJan]

All clear
good night!

[InvisibleStrike]

Sorry for poking this topic again, but I have a small question, just to clarify. I have a Breadboard with its own power supply. Now if I want to power an LED without any IC (so just led+resistor) using the PI's power I would just take some free GPIO pin and ground, connect those up and I can control the LED. But how do I make that setup use the breadboard's power supply and not draw power from the pi?

[rzusman]

Use a transistor or FET.