I am trying to power my arduino uno using a USB cable plugged into a PC. However, I am not using the built-in USB jack on the board. I have isolated and verified that I am getting 5V out of a USB port, but this changes to 2.7V when I connect the power to the '5V' and 'GND' jacks in the 'Power' area of the arduino.
I have noticed that connecting resistors between the USB port and the arduino pins increases the voltage measured if the resistance is less than about 500 ohms (with 400 ohms resistance I get about 4V).
Any ideas? Please ask if I have left out important information.
Thanks for your help.
Ok, lets see if I can decipher what your are doing and why it doesn't work as you would like. You are not plugging your usb cable from the pc to the arduino usb connector, but rather you have taken a usb cable and somehow broke out the +5vdc and ground wire coming from the PC when you plug the cable only into the PC. So now you have taken the two usb power wires and directly wired them to the arduino +5vdc and ground pins on the shield power connector. However you have found that the voltage drops to +2.7 volts when wired like that.
OK, if that is all correctly stating what you have done, here is what most likely is the problem. You haven't clearly stated if you have a shield of some kind attached to your arduino board, or what other additional external components might be wired to the arduino, here is my theory:
If you don't attached a active USB device to a PC usb connector, but just trying to draw some power from the jack, you are only allowed up to 100ma of current. When you attach a real USB device (like an arduino board) the PC usb driver software and the usb device negotiate for more current allowance, up to a 500ma max. So because of this it sounds like your arduino board and set-up requires more then 100ma and the PC is just cutting back the voltage to keep it at that 100ma limit and that is why you are seeing the low voltage.
If I'm correct then your only choices are to cut your total arduino board and external components current requirements to 100ma or less, or your going to have to power your arduino board by some other method such as using an external DC power module (8-12vdc) to the arduino's external power connector, or get a regulated +5vdc power supply and wire up to the arduino +5vdc and ground pins, or string some battery cells together ( 3 AA in series) and wire them to the +5vdc and ground pins.
So your choice, but what you are trying to use won't work unless you can keep total current consumption under 100ma.