problem with ramps 1.2/arduino

hello there guys i need some major help!

i have purchased all the pars i need to build a reprap and i really want to start getting into the world of 3d printing. I assembled my machine from scratch outsourcing all the parts by myself however i am having issues with my electronics setup. I have an arduino mega 2560 hooked up to my ramps board connected to my pc via usb without any thing else plugged in. I was able to start up pronterface and the arduino worked okay up to this point. After this i decided it was time to test by connecting only the 12v power supply to the ramps board. consequently once the power supply was connected, it immediately cut out my pc and the built in short circuit protection in the pc shut it down almost instantaneously. So i immediately cut power and disassembled the setup to check for any short circuits of potential problems, but there appeared to be nothing. i tried again and again and still the same problem occured. i haven't got to the stage of actually connecting anything to test just the usb power supply from the pc and the 12v psu connected to the reprap. note i have also managed to blow an arduino in the process so i am now having to buy another one, the only issue is if i plug it in to the new one ans the same thing happens another blown arduino will set me back another few quid. has anyone else had this problem/issue before and is there any way to solve it? i don't know whether the ramps board is faulty, whether its the psu supply or the arduino board.

also with my vault meter the pc psu supply connected to the 12v input on the ramps is measuring 12.35v steady supply.
the arduino functioned perfectly before ramps was connected
also it appears on my ramps board the diode is solider in if that is significant?
i have also tried various cables and none seem to make a difference on the usb.
the pc psu which has been used to power the reprap is rated at 500w

i am know electronics expert but any help would be appreciated.
many thanks,
05beckettt

Obviously something is wrong with the RAMPS or parts on it. So

1. Only connect the printer over a powered usb hub. This may save your computer usb port a live. Better the hub gets dead then your computer, also there never a 100% safety.
2. It could be a stepper driver if you have them already on the board. Check if you have them the right way around. If they are, they could still be the source. Try also without them and add them one by one for step 3.
3. Grab your multimeter and power the ramps without arduino.
- First check the current flowing to see if it is a simple short. Should need nearly nothing without hardware attached.
- Check the voltage on the arduino connection pins, especially the gnd/power pins should have what they need. From your description I think somewhere is a bridge between 5v and gnd.

Hopefully you have now found the defect part. If you are lucky it was only a stepper driver, which is easier to replace.

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If the problem is related to a 5v and ground shortage would that effect the boarx without the drivers connected and also what am i looking for specifically? will it be a shortcircuit or what am i looking for, as i say i have a limited knowledge and any help is appreciated

I'm not an expert myself, but have some experience for simple testing.

What you are looking for is a short as you said "consequently once the power supply was connected, it immediately cut out my pc and the built in short circuit protection in the pc shut it down almost instantaneously." That should only happen if the arduino draws more then 500mA current and that doesn't happen with a full functioning Arduino+RAMPS. Only reason is a defect or a wrong placed/defect stepper driver as the arduino was working well without.

If you put power to the ramps board without arduino, there should be no current flowing, except if you have a short on the part powered. That is what the first test should be. The empty board should draw nearly no current, so if you put your current meter between power and board it show 0.0x or so. If not the board has a short you need to find. If it is ok, you can start adding the stepper driver. Check after each driver inserted. If your board is completed and you still have no current you can connect your power normally and start measuring the voltage on the pins to the arduino. With diode it should power the arduino so the power pins should have 5v and gnd pins should have 0v.

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Repetier-Software - the home of Repetier-Host (Windows, Linux and Mac OS X) and Repetier-Firmware.
Repetier-Server - the solution to control your printer from everywhere.
Visit us on Facebook and Twitter!