Problems with heat bed

[aps]

Hi there, I need some help. I assembled my new i3X, and at the first test the heat bed heated up to 65°C in short time.

But after 30 Min. while the first testprint, I registered the temp was dropping down to about 55°C.
Now the heat bed is warming up to about 50°C, then it takes very long time for every °C more, can't reach the 65°C after 20 minutes.

I am measuring

• 1 - 1.5 Ohm resistance on MK2A head bed (incl. the LED)
  Heatbed current is about 4-5 Amps when powering
  PSU voltage drops down from 13V to 11V when heatbed powers on
  the power MOSFET on the GT2560 Board is getting warm approx. 60°C (estimated)

Any hints what is going wrong there?

[Mark]

the resistance is in normal range, how's the PSU? you select 220V or 110V?

[aps]

220V, the first thing I checked before PSU mounting.
Is this normal that voltage is dropping down when heating the bed?

[aps]

220V, the first thing I checked before mounting PSU.
Is this normal that voltage on PSU is dropping down when bed is heating?

[Mark]

it will drop a little, but not that much like yours, you can use a 12V computer power to test it.

[aps]

Thank you Mark, I connected a 750W 12V/62A ATX PSU
Started with heat bed temperature of 21°C, target 65°, after 20 minutes I was stuck on 60°C

I have still the original firmware on board. Is the Repetier-Host able to change the PID-values?
Because as said, at the first run it was heating up much faster.

[Mark]

can't remember the Gcode command, you can check the reprap wiki gcode page for the details. If that's not gonna worm, then you might need a new heatbed...

[aps]

I tried PID autotune for heat bed with

Code: Select all

M303 E-1 C8 S90    //this will run autotune at 90 degrees, for 8 cycles

and used S60 for 60 degrees, but also did not reach this in time.
There was also the information that 70°C should be reached in less then 3 minutes.

Next step will be to replace the heat bed, this will take some days.

Thank you for your help to locate the problem.

[elocm]

Hi aps,

on my Prusa I3 M201 the heatbed goes quite quickly to 80°C. Anyhow this model has a 24V power supply and quite a different board. Did you check that all your connectors on the board are ok and not "melted". This could create additional resistance and reduce the effectiveness of the heatbed.

[aps]

Thank you elocm, but the heatbed was broken in the first 30 Minutes. Replacing the heatbed solved the problem.