Heatbed mystery

Hello,

I have been trying to get all the kinks worked out of my machine before I run any plastic through it, and after a second "rebuild" I am at a loss of what has happened to my heatbed. I am using an arduino mega 2560 with sprinter and pronterface.

Currently, the heatbed will not get hot. As I understand it, there is one LED on the arduino for the extruder, and one for the heatbed. When I turn on the extruder heat, it heats up to the appropriate temp and TWO LEDs turn on the board. When I turn off heat to the extruder, the LEDs go out. When I turn on heat to the bed, NO LEDs turn on and the bed does not heat up. When trying to turn on heat to the bed and the extruder, both LEDs turn on but only the extruder gets hot. The thermistors are reading correct values in the bed and the extruder, when I hold a lighter close to the therm in the bed, pronterface shows the temp increasing.

The extruder and bed were heating correctly during my first attempt to get everything working, but I had to disassemble the wiring etc to fix a few things. I have reconnected everything and now the heatbed is not heating.

Please help! I don't know what changed between my first and second assembly attempt but this is the only thing not working now. Thank you for any insight.

-Mason

My guess is that your heated bed is hooked up to the wrong pins. It's also possible that your heated bed is shorted. A DVM is your friend in cases like this. If you don't have one already, they are a $30 isn sort of Christmas gift.

Thanks uncle bob,

So if there is no voltage coming off of the board where the heatbed is connected to I can assume it is hooked up to the wrong pins? Is that something I would change in the Configuration.h file? Sorry a lot of the arduino/programming stuff is new to me. Right now the bed is connected to D10 and extruder to D8 which is how it was the first time. Thanks for your help

Sorry extruder is on D10 and bed on D8

There are two pins right by the D8 and D10 markings. One pin goes to the +12 via a PTC (yellow) fuse. The other pin goes to the MOSFET switch that does the control. The heated bed or hot end goes between the pins.

[www.nextdayreprap.co.uk]

If you have the wrong main board set in configuration.h or have messed with pins.h then things could be coming out on the wrong pins. If you have your thermistor plugged in the wrong place or the maxtemp setting wrong that also would keep things from working right.

Do you have power connected to both inputs on the ramps? the 2nd 12v input is for the heatbead.

Yes I have power connected to both 11A and 5A terminals. When I turn on heat to the bed through pronterface LED2 (red) comes on so I can assume power is getting to the board through the 11A terminal (is this right?). How would I test if my heatbed is shorted? Should I just test with a voltmeter if power is getting to the D8 terminal, and if so the bed is shorted? Thanks

Disconnect the heat bead and check for 12v at D8. Check the resistance on the heatboard leads. Should be in the 1 to 2 ohm range. If those are ok then it should work. Not sure if the led is run from the arduino or is run off the mosfet..my guess is the arduino if that is the case it would not mean you get 12V at D8, only that it is signaling the mosfet to turn on.

Edited 1 time(s). Last edit at 12/20/2013 07:48PM by cat.farmer.

The LED is connected to the output of the MOSFET on a normal Ramps board.

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In the top post in this thread - no LED's came on when the heated bed was enabled. Now we seem to have an LED coming on when the heated bed is enabled - what's changed? Does the bed work now?

Thanks for the clearifacation Uncle Bob. I knew I should have looked at the schematic.

I got my hands on a multimeter. When the heatbed is enabled, LED2 is ON (my mistake from the original post), and I'm getting about 23 volts from D8. When the extruder heat is enabled I'm also getting about 23 volts from D10. When testing the resistance on the disconnected heatbed, nothing happens. So after all that I guess I have a shorted heatbed? Is that a common occurrence? Thanks

Hmmm. My power supply is labeled 12V 20A, but seems to be outputting close to 24V.....

Well if you have 24V on the supply, the regulator on your Mega likely isn't there anymore.

Voltmeters are a good thing.....

Bad solder pads on the heatbed. After cleaning and exposing the copper a bit more then re-soldering, the heatbed is working along with the rest of the printer. Thanks for the help.