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...