Kossel Mini strange behaviour. Defective Duet board, Dell PSU or something else?

Hi all, this is my first post, i looked for information about this subject but could not find anything.
I have a Kossel Mini from Think3DPrint3D, it's 8 months old and worked almost flawlessly until this morning.
It's connected to my computer with a LAN cable.
I was about to start a print, the heating bed and the print head were already hot when suddenly the web interface disconnected and i could not re-connect reloading the page or opening a new one so i opted to turn off the printer.
When i turned it on again nothing happened.
I've thought it could have been some (heat?) protection mechanism but instead i found it was the PSU which went in protection mode, once disconnected from the wall plug and reconnected the printer turned on again.
Unfortunately the printer is not working anymore, i'm able to connect to the web interface, see the temperatures floating, upload gcode files and also connect via usb (i updated the firmware right a few minutes ago).
What happens is that if i try to heat the bed or the print head after a few seconds the printer shuts down and the PSU is once agin in protection mode.
This also happen if i try to home the axis, after i click on "Home All" the engine starts but after a second or less the printer shuts down once again.
I'm puzzled, i have little experience in electronics, not enough to be sure about the cause of the problem.
I re-checked all the cables and connection to the board and everything looks fine, after all everything works, even if only for a moment.
I hope it's the PSU but i'm afraid it could be something in the Duet Board.
Any suggestion or test i could try to understand where's the problem?
Thank you!

It is most likely the PSU.
If it's a 12V one try to use car light bulbs or small 12V/50W halogen lamps to test the PSU.

Yes, it's a 12V 18A Dell PSU like this one: [www.ebay.it]

i don't think your power supply is powerful enough to run the whole printer hence why your havin issues i now you need more ampage if your runnig everything from one psu

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Check my rubbish blog for my prusa i3

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Check that the screw terminals on the Duet power input terminal block are still tight. If they are, then most likely the PSU is at fault and I suggest you contact T3P3 for a replacement.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Hi, thank you all for the answers.

@chris33: The PSU is powerful enough, it's 12V - 18A, it's the one supplied with the printer by Think3DPrint3D and worked without problems for 8 months.

@dc42: yes, one of the first things i checked are the screw terminals and they were tight. I guess the PSU has gone for good, i'll contact T3P3.

Thanks again for your answers!

Update: I changed the DELL psu with a bulk 12V 30A psu and it solved the problem