I have a little problem with grounding.

I realized a circuit which is controlled by the GPIO-Pins of the Raspberry. The function of my circuit is irrelevant. You just have to know that I need the 3.3 V and 5 V from the Pins for controlling some IC's. My Output needs 500mA (USB) so I need an external supply. I get this from the USB-Port of my PC.

When I connect 5V, 3.3V, GND from the Raspberry and 5V, GND from USB-Port it doesn't work. The Raspberry crashes. When I disconnect one Ground everything works fine.

I measure the two Grounds and they have a difference of 0.2 V to each other.

What should I do? Can I just disconnect the GND of the GPIO-Pins or can the Raspberry get damaged of it? What is the reason for such a different Ground?

EDIT: A little illustration:


The circuit is a sort of USB-Switch. With the GPIO-Pins you can choose which Device should work. So the MUX's are controlled by GPIO-Pins and powered by GPIO-Pin 1 and 2. USB out needs the 5V/500mA of the USB-Host. The Pins are protected with http://www.thebox.myzen.co.uk/Raspberry/Breakout.html

Do you need further information?

  • You should only power the Pi from one source. That includes the GND. Any external circuits need to be isolated if powered from another source.
    – Piotr Kula
    Commented Aug 6, 2013 at 15:44
  • 1
    If you have multiple power sources value of ground can be different (like in your case). This is when power supplies have a floating ground. You should always connect the two grounds together, to prevent problems and possible damage. That is, only the ground. So don't go and also connect the 5V lines :-)
    – Gerben
    Commented Aug 6, 2013 at 15:47

1 Answer 1


Before I go any further could you include a diagram of some sort in your question to illustrate what you are talking about? Everything below this is going on your description that may be interpreted incorrectly. I will update the answer when I have more info:

I think that you have an issue with power back feeding into the RPi. In this case you have 2 options:

  1. Isolate your circuit from the RPi. If you need to control it with the GPIO pins look into using an opto-isolator like this one, already broken out for easy soldering. this way you will have no electrical connection between the two circuits. You will need 2 power sources, 1 for your circuit, and one for the RPi.

  2. Since you are not using anything high voltage/current, you can power both through a single power supply. You will need to ensure it provides enough current, a single USB port probably won't cut it. Refer to this question as to the proper way to power your Pi from an external source and the implications of doing so.

Either of these solutions should take care of your problem and get your circuit back up on its feet.

  • agreed. electrically isolating the two devices (RPi and peripheral circuit) since the add-on does not get it's power from the RPi, sounds like the solution. Commented Aug 6, 2013 at 14:55
  • Can you explain me how I exactly isolate the circuit from the RPi?
    – Franzi
    Commented Aug 6, 2013 at 17:54
  • @franzi Check the link to your image... I get nothing when I click on it. Once that is up, I would be happy to take a look.
    – Butters
    Commented Aug 6, 2013 at 18:42
  • @franzi It was my web filter...Sorry... I need something like This... your diagram really doesn't show anything.
    – Butters
    Commented Aug 6, 2013 at 19:05
  • I thought so :D So new try I hope it is understandable. In front of the GPIO Pins is the protection from the link above. The TPS2052 is a Power Switch. When EN-0 is high Vcc is going to Out-0. Would the problem also be solved when I supply the TPS2052 only with the power from the USB-Host? (red line)
    – Franzi
    Commented Aug 6, 2013 at 20:01

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