I think I blew the fuse that powers my Arduino from the 12V input on the RAMPS board. Does anyone know which fuse/diode is responsible for this, and where I can get a replacement?

Do you have a multimeter?
It would help if you could test your wiring / psu / controller / arduino etc for faults, then someone will talk you through the process of finding your problem.

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A wise man once told me "Never trust a man who doesn't own a shed!"
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Gen 6 daughter board
[forums.reprap.org]
Mendel PCB heatbed
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The fuses are supposed to automatically reset, although I suppose f you gave it too much voltage/current you could fry them.

The fuses protect different circuits. The larger of the two basically protects the heated bed circuit. The smaller fuse protects the rest. What is not working on your system.

I also agree with slymike. A simple voltmeter is indispensable for a hobby like this.

From what I can tell there is no protection in the 12v coming from ramps to feed arduino. You may have fried regulator on arduuino (or worse) see if it can stii be powers from USB without ramps mounted. Hoook ramps back up and power all on. Make sure bottom of arduuno isn't getting hot. If it is you got permanent issues.

I think ramps 1.5 needs to include protection on the power running from ramps to arduino.

according to the RAMPS 1.4 page D1 feeds the Arduino. I am going to test it now.

Yup, no continuity across D1 in either direction(diode should limit the current to one). Is it just a generic rectifier diode? I think I have a bunch of those around here somewhere.

I noticed on my ramps that the arduino wasn't getting power via the ramps, but I never suspected the diode. I thought I saw a gap in the PCB trace and tried,to no avail,to repair it. I ended up by tapping off of the power connector connector coming into the ramps and adding a DC barrel connector that goes into the arduino jack.