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Can't control heatbed

Posted by Shane_P

Shane_P

Can't control heatbed
February 09, 2015 04:51PM

Registered: 9 years ago
Posts: 10

Hi all, my Prusa i3 is having issues with the heatbed. The heatbed goes straight to 100 degrees (C) as soon as the power is turned on. I have changed the settings in Pronterface but this seems to be ignored. I have recently tried Repetier and have the same result.

The LCD shows that both the hotend and heatbed are registering room temp (they are both the same) before being powered up so I am assuming that the thermistors are OK. Has anybody got any suggestions where to start with this problem?

Since it began, I've had issues with the printer missing steps on big prints (shifting layers) and the board being considerably hotter as well. It is actively cooled but this seems to be struggling. Sorry about the lack of technical lingo but I'm still on my steep learning curve with this project. Any help would be greatly appreciated.

Printer: Prusa i3, Ramps 1.4, Marlin, Mk2B Alum heatbed, currently controlled by Repetier)

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waitaki

Re: Can't control heatbed
February 10, 2015 05:16AM

Registered: 11 years ago
Posts: 1,592

Have you got the heat bed wired correctly? It could be that the bed terminal that routes back to the mosfet on the Ramps board is, in fact, shorted to ground.

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Waitaki 3D Printer

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Shane_P

Re: Can't control heatbed
February 10, 2015 05:17PM

Registered: 9 years ago
Posts: 10

Hi Waitaki, I'm assuming the wiring is OK because it has worked well in the past and no wiring has been changed. Do you think the MOFSET could be causing the problem?

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waitaki

Re: Can't control heatbed
February 11, 2015 01:35AM

Registered: 11 years ago
Posts: 1,592

Well, it could be the mosfet - check the centre pin. This is the pin that is grounded when the Mosfet is switched by the controller, e.g., when the Mosfet is off, the centre pin is isolated from ground but when the device is enabled, the centre pin is switched to ground which provides the return path for the heat -bed. i think that pin should be at 12v when its off. Put a meter probe on the Mosfet centre pin and the other lead on PS ground input with the power off - what do you see?
Also check the enable pin (right hand one) is not getting 5v from the Arduino - if it is, and you have not switched the heater on in software, then the Arduino is sending a false signal and switching it on.