I was trying to run my Arduino using an external 5V @ 1A power supply (http://www.sparkfun.com/products/8269
), but I am only able to measure 3V from the 5V pin on the board!
My Arduino is set up to run Auduino (simple FM synth), which outputs to a .5W 8 ohm speaker directly. I am currently building a small external class A amplifier to boost the volume a bit more, but am curious about the power problem.
When I run off of USB, I get a full 5V and the speaker is nice and loud. When I switch the power selection jumper to EXT and plug in my power supply (and remove the USB of course), I measure 3V at the 5V pin, the speaker is noticeably quieter and the sketch seems to be running slower in general. I thought the USB protocol only allowed for a max of 500mA to be drawn by one device, whereas my power supply is capable of 1A. Any ideas?