Heatbed shutting off PSU

Hi everyone,
I built a Prusa i3 mk2 clone, and I got it to print some stuff last week, it even printed a full Benchy. These last couple of days, prints started to fail more and more quickly, up to a point where the print would stop immediately after launching it, shutting the PWU off. I realized that the cause of this failure was the heating up of the heatbed. Indeed, the PSU shuts down directly after I click on "set temp" on the heatbed in pronterface. I took the heatbed out to check for a short circuit but nothing obvious to see, resistance reading was at 0.9Ohm, which seems to be what I should expect. The heatbed is here here, it seems to be based on the Mk3 design. I am using an ATX 300W power supply. I also tried to connect the heatbed directly to the PUS to check if the board might be at fault, but it also shut down immediately.
I read on the heatbed page on the wiki that the PSU might be a bit weak, but it does not really explain why it did work just fine until now. Any ideas?
Thanks in advance,
Baptiste

You need to put a dummy load on the 5V output of the PSU. On cheap ATX PSUs the 5V and 12V outputs are regulated as one. If there is a heavy load on 12V and little or no load on 5V, the 5V output rises too much and triggers the over voltage protection. Also check that the 12V output is rated to supply the full current needed by your printer, because the 300W rating is the total for all outputs and cannot be taken from just one of them.

Edited 1 time(s). Last edit at 10/09/2019 04:46PM by dc42.

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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!
Thanks for your reply. If this is a power issue, then wouldn't it have failed right away, instead of working for a few weeks, then fail?

if the power supply is old, the caps can die shortly after starting to re use it

Is there a way for me to check if the heatbed is not at fault?

You already have.

The resistance is correct for a 12v bed.
You tested the bed directly on the power supply and thus cut out the mosfet, so that can't be it.

That is all you can test.

It will be the power supply,

NB you need 160watts for the 12v heated bed alone (some 13.3 amps)

FYI that is nothing like a prusa mk2 bed which look like the following

Different size, different mount holes, different connectors, and has probe points for it can correct for XY skew

Edited 1 time(s). Last edit at 10/10/2019 08:09AM by Dust.

My PSU is supposed to withstand 156W on the 12V output, so theoretically speaking, that should be good enough, shouldn't it?

hell no!

160watt for heated bed alone

you need 240watt (about 20 A on 12v)

technically you need 18amp (216 watt) in a perfect world, but most power supplies lie about their capacity so you need a bit more

Edited 1 time(s). Last edit at 10/10/2019 09:52AM by Dust.