I was wondering would it be possible to power my Arduino using a DC plug to USB wire? For an image, http://d1gsvnjtkwr6dd.cloudfront.net/large/CA-DC21-USB_LRG.jpg . I want to have the Arduino plugged into my laptop so it can power it. I know there's an USB port that I could use, but I don't want to use the USB port on the Arduino.
USB is 5v. The external power socket feeds a regulator so the voltage will drop. The regulator needs around 7v to work properly.
The USB port on the laptop is 5V? Is there a way I can modify it or add an extra component so that the DC plug to USB wire method would work?
You need ~ 7.5V into the barrel jack for the barrel jack power to be used instead of the USB 5V. I use this one. http://www.dipmicro.com/store/DCA-07510
The thing is I don't want to use a wall mounted adapter. I won't within reach of a wall outlet. I'll only have my laptop and a small 9V battery won't last long enough for me.
You could use a boost converter to bring the USB up to 7.5v to feed the jack.
Remember however that USB outlets are restricted to how much current they will provide. I think it may be about 500mA.
900mA for USB3 - but best the check specs on your device...
So hack the USB cable and connect to 5V on the power header.