Externally powered devices and USB connection

Hi there!

I know, a similiar kind of question has been asked several times: If you can connect the USB and the DC power jack at the same time. This is kinda not the thing I want to know, but close. I want to learn on a general basis how to go about having two power sources (external and PC)

I have two upcoming projects, which need to be connected to a PC by USB (not just for debugging). Because of current limits, they need to be powered by another power source.

Project 1: I have a small breadboard Arduino (without FTDI chip), a device to be controlled (assume it is e.g. a large LED) and a cable with a USB plug (a FTDI chip built in and ends in 4 wires (TTL: VCC, GND, RX, TX)). The LED and the Arduino will be powered by a corresponding wallwart with 5V.

Project 2: Pretty much the same as Project 1 above, but the device to be controlled has a TTL connection, so I don't need an Arduino in between.

So my question: How do I go about such constellations? I think it is about whether to connect the VCC and GND connectors "after" the FTDI chip to the Arduino/device. My first guess would be to connect GND but leave VCC out, but I am really not sure.

Thank you!

Well, I just ordered some of these


to power my breadboard, and they are working perfectly. If this is what you think you need, then I would suggest buying some. They power the breadboard whithout having to use the Arduino. I used them for a bunch of servos and motors on a school project.

On the other hand, I could have completely misunderstood your question.

Yeah this wasmt quite my question. The question is not how to power the arduino. I have regulated 5V to power it. But I need to connect the thing to the PC, which also provides 5V. I want to avoid damaging the PC by any means, so I wonder how I connect the FTDI adaptor to the arduino/device.

If your device has it's own power source, you can always just not connect Vcc on FTDI adapter to Vcc on board. Connect gnd, tx, rx, and if programming over serial, DTR as well.

However, you don't need to do this with most Arduino boards, because they have taken measures to not backpower the USB +5v line. You can see what they do with the opamp (used as a comparator) and that p-channel fet in the schematic - that's their power switching circuit. If you're building the board, you can simpler protection against back-powering the USB by putting a schottky diode on the Vcc line from the FTDI adapter.