I have had lots of fun with LEDs on a breadboard with my Arduino Uno, and now I'm moving on to controlling an LCD that I already had on hand. It is a CrystalFontz SKD204-634 v2.2, a 4x20 character, backlighted and the characters are bright yellow. This was the best I could get before getting into the (more expensive) Vacuum Florescent Displays (VFD).
I originally had this connected to a PC through a serial port, and after soldering the appropriate pins onto the board and connecting 2 jumpers, I got it to work with the Arduino with a "Hello World!".
My questions are
1. why is the LCD much dimmer (barely lit up) when the Arduino is powered by USB than by a 9v battery?
2. what is the typical amperage supplied by a USB port? According to the LCD's data sheet, it draws a total of 531 mA (6 mA for the LCD, 525 mA for the back light). If the USB supply is less than 531 mA, then this is the problem.
3. what is the proper way to run on external power (by the pins or the barrel) and be able to keep the USB cable connected, without frying the voltage regulator? Or can this even be done?
I'm new to electronics, but not programming. I not a total numbskull in electronics though, I am aware that if you let the magic smoke out, it don't work no more!