Heated bed not getting hot enough

We have built a Prusa i3 from 3dPrinterCzar. I have successfully printed PLA parts, and my heated bed gets to 65C with no issues.
I am ready to try ABS, but I can't seem to get my heated bed to get hot enough. After 30 minutes, I can only get up to 82C.

I have an MK2A heated bed, connected through RAMPS 1.4 and an Arduino Mega.
I am using the included 12V power supply.
I have measured the resistance of the MK2A at 1.5 Ohms.
The voltage out of the power supply, even under load, is 12.2V (adjustable, but rock solid even with both heaters running).

I'm out of ideas and not sure what else the issue could be. Any help would be appreciated.

Thank you,
Matt

I don't have a printer yet, but from my reading I think you are out of amps.

The bed resistance is a little high (1.2 ohms would be nearer the mark) but that may be your meter!
Do you have thicker wiring going to your heat bed? 18 gauge perhaps?
And, make sure your connections to the 11A input terminals are tight.
Also, some form of insulation under the PCB will help. And while I'm droning on, make sure you have a heatsink on the bed Mosfet (on Ramps board) and a fan blowing towards it and the yellow fuses, at all times.

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Waitaki 3D Printer

Thank you for the suggestions. I will try them this evening.
My meter is a very good one, so I am confident in my resistance measurement.
I'll make sure those connections are rock solid and put some cooling on the board.

Thank you,
-Matt

You don't want cooling on the board. You want insulation so that it retains heat.

Also, check your solder connections on the board itself. They should be solid, and don't be sparing with the solder.

Err, we're talking about cooling the Ramps board JB

Edited 1 time(s). Last edit at 05/07/2014 05:58PM by waitaki.

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Waitaki 3D Printer

If you're not using any glue or other stick enhancers, try throwing a magazine on top of the bed to insulate it during warmup. Also, if your heatbed is one of the dual-voltage ones, make sure that you followed the wiring directions for 12V and not 24V.....

Ok, I've checked connections at the RAMPS board. They are as rock solid as they are capable of being.
Voltage at the heated bed is reading 11.7 volts. I can increase voltage on my power supply and get that to 12V, but then I am supplying about 12.7 volts to the Arduino/RAMPS to get to 12V on the heated bed.
At 11.7V it reaches 82C. At 12V I can get it up to 88C.

I put active cooling on the RAMPS board in the form of a 12cm computer case fan about 4 inches away.
The LED on the heated bed never goes out.

I also have a bench power supply and can power the heated bed separately if need be (although not ideal of course).

What else could the issue be? I'm stumped.

Thanks,
-Matt

1.5 ohms is significantly higher than normal. At 11.7V, power dissipated is P=v^2/R, so 11.7^2/1.5=91 watts, versus a standard 12V^2/1.1=131 watts, so you are about 30% lower than most.

If you can, try upping the voltage with your lab supply to 14V to get 14^2/1.5=130W and see how things go.

Per [reprap.org] RAMPS could take 14V is you aren't using the diode to power your arduino. Maybe a nice 7805 would help, or perhaps hooking a 7812 in place of the diode would let you go higher.

Had the same problem.
Used a neoprene layer to insulated the heatbed, and it worked, problem solved.
To be clear : the insulation should be directly in contact with the heatbed.

Chris

Ok, problem now resolved.
I purchased some cork from Target on the way home and taped it to the bottom of the heatbed with Kapton tape. It now gets up to 110C with no issues.
DaveX I never even got around to testing it out with the bench power supply! That was the next step!

Thank you everyone for your help.

-Matt