Hot bed won't heat up

Hi, I am new to this forum and RepRap. I have just finished my Prusa I3 and I am having issues with my heated bed. I am using RAMPS1.4 board and a MK2a heated bed. It was been working ok to begin with.

However now it will heat up to about 76-90 deg and then start cooling down. I am not sure why.

Any help on this issue would be great.

Ps. The led on the bed has never worked, I am not sure if this is relevant or not.

To begin with: The not working led is no problem. You could try to desolder it and resolder it in the other direction, but nevertheless, it has nothing to do with it.
Does your Heat-bed driver become really hot? I mean, the one on the RAMPS that is driving your heatbed.

Also, check in the Marlin firmware (configuration.h) to be sure the high limit is set properly...

// When temperature exceeds max temp, your heater will be switched off.
// This feature exists to protect your hotend from overheating accidentally, but *NOT* from thermistor short/failure!
// You should use MINTEMP for thermistor short/failure protection.
#define HEATER_0_MAXTEMP 275
#define HEATER_1_MAXTEMP 275
#define HEATER_2_MAXTEMP 275
#define BED_MAXTEMP 150

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-David

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Check out my FolgerTech Prusa i3 (plexi) at MindRealm.net

Ano what exactly do you mean by the driver?

MindRealm thanks for posting. I have double checked my max temp setting and they all seem to be the same as your screen grab.

Ano I took your suggestion kind of, I swapped the cables around on the Ramps instead of re-soldering the heated bed.

So the led works now. However now when I try and set the temp to 110 deg it gets up to 70 ish deg and the led goes off and the bed starts to cool down. The led also goes off and the connection on the Ramps is screaming hot.

Any suggestions.

Edited 1 time(s). Last edit at 01/25/2015 04:48PM by daithi27.

The driver is the mosfet that switches on and off the head bed. It is located on the RAMPS board. What do you mean by "connection on the Ramps is screaming hot"? Do you mean the cables between the RAMPS board and the heatbed? If so, than change the cables for larger ones, as they apparently have too high resistance, which leads to the power being dissipated in the cables instead of the heatbed.

And btw.: Swapping the cables as you did should also work just fine.

/Edit: See also this thread: http://forums.reprap.org/read.php?4,435771

Edited 4 time(s). Last edit at 01/26/2015 04:11PM by Ano.

Ano cheers for the help so far. I have used your questions as a check list and I have attached a fan to the ramps board and it has helped to get up to temperature.

I think I might have to put a heat sink on the MOSFET to help dissipate the heat better.

When I said that the connection was screaming hot I meant that the connector block on the ramps where you screw on the cables to the heated bed. I have cables rated at 15Amps so I think they are sufficient.

Thanks for the link. I came across that today.

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- David

Prusa I3

If you have a solder iron, you can exchange the mosfet that drives the heat bed for another one. Apparently the used ones are cheap components, which aren't as good (means low Rds-on) as they could be. You could replace it with a better one. It should then become significantly less hot. The IRLB8743PbF could be a good alternative. Has about 1/4 of the Rds-on of the STB55NF06L (which should be the standard RAMPS mosfet).

Thanks Ano I will give that a try.