This is a pic to show where it is Most ramps as far as I know come without it installed and it is usually taped to it somewhere. If it isn't there then it should be a 1N4004
![ramps_zps9eaf350f.jpg]( "ramps_zps9eaf350f.jpg")
See Also Ramps1.4 Wiki page
See Also Ramps1.4 Wiki page
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AuriuX
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dougal1957
Have you installed the Diode (D1) underneath the X and Y axis Driver boards (It is half under each of them)? It is this that allows a 5V feed to the Mega when the USB Is disconnected.
Will check the board for a D1 diode when i come home. Hope i find no diode there, just an empty space.