Power questions

Hello. I've assembled a Prusa Mendel with RAMPS 1.4 but when I run this test code [reprap.org] nothing responds except the LED on the board. The board is currently only powered by USB. My main question is can the printer be powered through USB alone or is a separate 12 V source essential for the printer to operate (hence why my test failed). I can't seem to find an explicit answer for this, although from what I can tell USB can't supply the power that a 12 V / 5+ A source would. On a related note, I don't know what kind of connection is on the board for power, so I'm not sure what kind of adapter I would need to connect the board to a power supply.
I'm new to 3d printing and only have limited experience with electronics so by all means talk to me like I don't know anything. Thanks in advance for any help.

You definitely need a 12V source to power most of the RAMPS board. Specifically, it runs 1/2 of the stepper driver boards (the part that actually drive the stepper motors). It also runs the hot end, heated bed and fan connectors.

For a power supply, you need to figure out if you're going to run a heated bed or not. If you're not going to run a heated bed, you can get away with a 5A power supply. With a heated bed, I'd recommend a power supply that does at least 17-20A total to run both the heated bed and the hot end/fan/motors. Many people say you can get away with a 16A, but I've found that in many cases that's not always true, as the amount a heated bed can draw varies a lot from the average value of 11A that it's rated at.

The connectors on most RAMPS boards for power are screw connectors. You need to remove the insulation from the wires, insert each wire into the connector and do up the screw for each wire. It's always advisable to twist the wires so that the strands stay together and all go into the connector (a single strand may cause a short that could cause problems).

In the case of RAMPS 1.3 and 1.4 boards, there are two screw connectors. One is for the 12V for the Hot end/fan/motors (usually marked 5A) and the other is for the heated bed (usually marked 11A). Be careful to get the polarity correct, as getting it backwards could lead to all sorts of disastrous issues.

Things you can test with only USB power are:
Thermistors (temp readings)
End stops

These all work off the 5V from the Arduino, which in turn is supplied by the USB connector if you've got it connected into a PC.

Thanks for the information, it is very helpful. One thing though, my board does not have screw connectors for power. It has four pins recessed within a plastic housing like this [www.3dhacker.com]. How would I connect a power supply to this? Thank you again.

You can use a 360 psu 16.5 amps as ramps draws 11st from one input and 5amp from other

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You should have a second part to that connector on the right. It has 4 screw terminals and it snaps into the part you have soldered to the board.

Ultimachine has them here.