Heated bed cannot reach 65°C

Hi,

I've got a Prusa i3 and I recently bought a MK2B Aluminium heated bed. It was printing fine and getting to temperature fine but now it can't seem to get up to temperature? I'm Printing PLA and I want my first layer to print at 65°C but it gets to about 62°C and just won't climb any higher? I don't understand what has changed. Has anyone got any clues as to what is going on or how to fix it? I'm using Cura to slice and loading the g-code into Repetier Host. Below is a screen shot of the temperature curve as seen by repetier host:



Thanks,

Chris

Insulate under heatbed.
I use a piece of cork sheet. Was less that $5 from a flooring store.

Measure voltage in the heated bed, where the wires are connected. This sould be more than your supply voltage -0.5V. If not, you likely need thicker wires from PSU to controller and from controller to heated bed.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

The mosfet in your board might be getting too hot and shutting down, are you cooling them with a fan? To check for that, just plug your heater bed straight to your power supply and see if it get to temperature, if it does, the your problem is lack of cooling.

Thanks for the feedback guys. Really appreciate it. I have a fan running but it is not very powerful so I think I need to get something else hooked up and maybe print a proper fan bracket to hold it in place. The wires should be fine as I have had the bed heating up fine before and the wire is 20A rated automotive cable. I will try insulating under the bed as well but I can't seem to understand why it was working and is now having problems?

Could the MOSFET be permanently damaged if I haven't been cooling it properly? Could I replace it with a more robust MOSFET?

Thanks again guys. Much appreciated.

Chris

Sure you can replace it with a better one, and yes they will decay and stop working properly if they have suffered overheating (the amount of damage will depend on how much and how long they were overheated) .

So I fired up the printer again last night and turned on the heated bed. I touched the MOSFET heatsink after the bed had heated up to about 62ºC and it was cold to the touch? I'm going to order some more MOSFETS, replace the one on the board and print a fan mount to improve cooling. Will hopefully solve the problem. Will update when it is done to let you know if it worked.

Thanks again for all the help guys.

Chris

Before you do that, go to your configuration h file and change the power settings to your hot bed to "full" 255 and chech again.

Hi ggherbaz,

Thanks for all your help so far. Some more information for you.

I checked the power settings in the Config file and it is already set to 255. I figured out that bed PID is off (which I think is probably best until I sort the problem). I checked the resistance of the Heated bed and I am reading about 2.8Ω without the wires and about 3.1Ω with the wires. Measuring the voltage across the D8 terminal I'm reading about 11.8V. Doing some rough calculations (I'm not an electrical engineer so may be getting this completely wrong) I get that the current draw through the heated bed and the wires based on my 12V supply is about 3.8A. Given this that would suggest a maximum power of about 45W. From searching around I see that the ideal power consumption for a heated bed is about 120W. Could this be my problem?

As a possible solution I did some more rudimentary calculations and came to the conclusion that if I used a 24V supply I would get a current of about 7.7A and that would give me a power of 185W. Is this correct? I was thinking that I could up rate my power supply to a beefy 24V supply and put a 24V to 12V step down module to power the rest of the board like this one. On this basis I would need a 15A 24V power supply to provided me with 10A for the heated bed and 5A for the 12V circuit like this one. I think would also have to replace the resistor inline with LED 4 on the ramps board to protect it from the raised voltage but I'm not sure about the Polyswitch. It says max voltage 16V so I guess I would have to replace it but would it be better to short the link and put a blade fuse in the supply wire before it even gets to the board instead?

Apologies for all the questions but as I said I'm not an electrical engineer and trying to get my head around all this is pretty confusing so I might not have it right?

Thanks,

Chris

The MK2B has to ways for heating if I'm correct. One way is 24V and the other is 12V. Maybe you wired it wrong?!?

Resistance (12V): 0.9 - 1.6 Ohms
Resistance (24V): 3.5 - 5.9 Ohms

At 12V you need to wire (2) and (3) to ground.

Edited 1 time(s). Last edit at 11/03/2014 09:47AM by Wurstnase.

Hi Wurstnase,

That makes sense. I thought it was a wire to one or the other not both! I have it wired to 1 and 2 only at the moment. I will rewire tonight and let you know. Fingers crossed.

Thanks,

Chris

Sorry, been busy. I didn't thought about the wiring, Wurstnase is right, on 12 volts your amperage consumption should be around 7~8amps.

Wurtnase, That was the problem! thank you for the solution. Heating up quickly now and just had it up to 70°C in about 5 minutes (if that). With the ground wire soldered across 2 and 3 the resistance dropped to 1.8Ω with the wires on.

ggherbaz, Thank you for all your help, I greatly appreciate it.

Yet again the reprap forum comes to my rescue!

Thanks,

Chris

No worries, just read it and thought that the MK2 has ~1.6 Ohm and lower. But 3 and more should be that worse I didn't believe.
Have fun with it...