Connecting via USB and external DC trips house power...

Hi all,

Maybe I am being monumentally stupid, but I have had a google and looked through the troubleshooting guide, to no avail.

I am using an Arduino Uno r3 as a replacement "brain" for a commercial breadmaker (Swan brand).

The breadmaker has its own power board, including a fairly variable 12V line which I have used successfully to power the Arduino. I am using the digital outputs (5V) of the arduino to send signals back to the power board where various relays live, and they share a common ground.

So far so good.

I am using USB to upload new code from my laptop, and have noticed that when I connect my laptop to mains power (not battery), and the arduino to both the breadmaker and my laptop, that whole circuit in my house trips (ie. I have to reset the circuit breaker and reprogram alarm clocks etc.).

I'm a little concerned I am doing something dangerous to draw so much current (I'm not sure if it's the breadmaker or the laptop which causes the trip).

How does the arduino handle both USB power and External power? Why when my laptop power cable is connected does it cause a current surge, where does this come from, and how can I fix it?

Any help to help me pin down what the cause is, and to eradicate, would be most appreciated!

And I need to power the breadmaker while having a serial connection from the Arduino for debugging...



Does the circuit breaker have a Ground Fault Circuit Interrupter (GFCI)? That's fairly common for kitchen and bathroom circuits. A very small current from the AC line to Ground will trip one of those.

Hi John,

Thanks for your reply. I had a look and the circuit breaker does indeed have a Residual Current Device which I think is the same as Ground Fault Circuit Interrupter (GFCI).

I'm still a little perplexed why current is flowing only when the laptop power is connected, but as a short fix I can work on the Arduino while the laptop is battery powered.



Hi Amadeus,

If it is an rcd, I expect it will trip on an unbalanced current of 20 or 30mA. 20mA is a bit sensitive, and will often 'nuisance trip' when vacuum cleaners or electric cookers are used. The rcd operates when the phase and neutral currents differ by more than 20mA, say, generally this happens with an earth fault of phase to earth. In your case, I suspect the fault current is flowing through your usb cable. If you are in the UK, I can probably explain how you can check it out, but I am not famiiar with domestic wiring installations in other parts of the world.

Best wishes,


Hi Ray,

Thanks for your help. I am in the UK and would be interested in tracking this down!

I’m also away for 10 days, so I will be able to reply then.

Thank you!


Just to update, I have extensively tested the setup, and the problem only occurs when both the Breadmaker supply and the Laptop Mains supply are on.

I'd still be curious to see how the USB/laptop power interface causes this!