RAMPS 1.4 Won't Power On with 12V PSU

Hi guys I'm super new to the world of 3D printing.

I purchased a RAMPS 1.4 Board kit with MEGA. Anyways I got everything wired right but for some reason as soon as I plug in the green 5A 12V power clip(I'm not using the 10A as it's on a external MOSFET) The power supply shuts off and nothing on RAMPS works. I took all the wiring off and stripped it down to just the RAMPS board and the MEGA and plugged it in and still nothing. The power supplies green light shuts off. So then I thought let's try turning the power supply on without the green connector connected to the board. I did that and the power supply works. I'm getting a 12V reading on the PSU as it's brand new, when I plug a external 12V power adoater or the USB into MEGA, the mega works just fine and the RAMPS board flashes its lights. Maybe I'm suspecting the RAMPS board is no good? I mean it's pretty impossible to wire incorrectly with just two wires running from the PSU to the + and - on the green clip going to RAMPS. Any thoughts?

Anything look afoul on it?

Have you checked with an Ohm-meter that there is no short circuit between 12V and GND rails? This might happen sometimes due to solder blobs or badly edged PCBs.

Would it be possible to provide more information on the power supplies?
And how you wire them.
Also, you said you do not connect the 10A rails. I might be wrong, but it is my understanding that even if you use external Mosfet, the board still needs power on the 10A rail to control that external Mosfet.
Does the described behavior happen with all the components connected (stepper drivers, endstops, inductive sensors, etc) or it is the same with just RAMPS+Mega?

The 11amp rail is just and exclusively for the D8 mosfet. If your not using D8 you do not need it.

An external mosfet can be a logic level one and plugged into any digital IO line. But its if a 12v driven mosfet attached to D8, then yes you still need to power up the 11amp rail.

Edited 1 time(s). Last edit at 03/28/2018 08:49PM by Dust.

Didn't think of that. I kinda auto assumed the bed being connected to D8 with RAMPS1.4, D8 being dedicated and all. Just learned a new thing!
I believe the only board which doesn't have actual dedicated direct heatbed connection is the MKS Mini v1.2.
"External logic level mosfet" kinda sounds like an automotive 12V 20/30A Relay, on or off, no PID for heatbed. An external mosfet with a logic controller to accept digital commands must cost a pennie.

Anyway, back to subject. Reading first post again I have more and more questions. Some photos of the whole assembly would greatly help identifying the issue. Then again, I remember while watching Tom3D live streams where he was making his Dolly (Prusa i3 MK2 clone) printer he got himself both defective Mega2560 board and RAMPS 1.4 board. So it could be as simple as defective hardware. The worst could be that the Mega2560 is already fried.
Are you able to upload a firmware to Mega2560 board at this point?

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Proud owner and maker of build from scratch FDM 3D printer [forums.reprap.org]

Did you look at these wiki pages for troubleshooting?

[reprap.org]

[reprap.org]

1. Can you turn on the power supply by connecting a dummy load to the Green (PS_ON) wire to ground?
[reprap.org]

2, Have you checked to see if you need a dummy load on the +5V rail?
[reprap.org]

Disconnect everything from the RAMPS and go through the procedure and check your connections. It should be as simple as putting a load on the +5V side to ground and grounding the PWR_ON to ground to get it to come on.