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I have built a RepRap Prusa i2 a while back. It worked for a while, but then I moved the printer from place to place and after a while I noticed that the printer's bed is not heating anymore. The thermistor shows the temperature (room temperature), but the bed is not heating. The wires are connected and the heated bed has a resistance.

Did anyone else confronted with this? Thanks!

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    $\begingroup$ Do some probing around with a multimeter. Is there voltage across the beds inputs, check for continuity in the wires, etc... $\endgroup$ – Tom van der Zanden Sep 21 '16 at 14:34
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diagnosing a heated bed

Switch the hot end and the heater bed wires on the board. See if there is a difference. (Note that it will not reach a thermal limit when plugged in this way! So don't leave it on too long!)

After that, if you see that your heat increases with the hot end's wires and the hot end being set on in software then you know the issue is likely with the board. At which point try updating / reflashing the board.

Alternatively, you can try a second board and see if it makes a difference.

If the bed does not heat up even using the heater cartridge connectors, then your heated build plate likely needs to be replaced. Or the wire needs to be replaced.

If you are running Ramps or a board that uses 2 hot end outputs you can likely reprogram it to use hot end heater 2 as the new output for your heated bed.

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  • $\begingroup$ using a hot end heater output on the electronics to heat a heated bed could damage the electronics. It is probably not a problem for a short test but definitely not a long time solution!. This is very dangerous. The heated bed draws a much higher current than the hot end. This could damage the electronics, melt the connectors or even start a fire! $\endgroup$ – Lars Pötter Mar 7 '17 at 17:32
  • $\begingroup$ The test is correct switching the heatbed to the nozzle heater, just be careful to set the temperature for 50° as maximum to not burn the heatbed. The electronics can support this changes. $\endgroup$ – Fernando Baltazar Dec 21 '18 at 18:20
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I have a Prusa i3 and the same case happened to me. I checked the continuity of the wires and discovered one badly soldered wire in the heated bed. After soldering the wire correctly, the heated bed turned on again.

Your problem is in all likelihood the continuity of the wires.

I hope that I helped you with your case.

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Be extremely cautious if you switch the hot end wires with the hot bed wires as StarWind0 said, only do it for a couple seconds and stop as soon as you've confirmed that temperature is going up. I did this on my FLSUN C Cube and it started smoking something fierce and just about lit on fire within seconds of my turning on the extruder with the wires swapped. I suggest you be extremely quick and also don't do this by a smoke detector if you attempt it: just in case. This is pretty much guaranteed to happen due to the amount of current draws the two components use being vastly different. I re-flashed my board and it still did not work, probably should have done that in reverse order, please learn from that mistake haha. I seem to have somehow burned out my heated bed mosfet (prior to switching the wires) and have the same problem that you do, I'm just going to buy another board because it's pretty cheap but for the sake of future people with this problem to solve I'd:

  1. check connections, especially at the screw connectors as they have a nasty habit of getting worn
  2. use a multimeter in DC voltage mode, put your black (-) probe on the DC power negative lead, turn on the hotbed and using your red (+) probe make sure you are getting about 12V through all your relevant connections including the fuses (mine are very tiny surface mount fuses, 15a and 30a located right behind the power supply, don't know how standard that is for all printers though)
  3. If everything checks out so far then cry a little bit
  4. It's probably a burned out mosfet (buggers are sensitive) but just in case, re-flash your board and hope the problem goes away
  5. test the mosfet for the heated bed with your multimeter (same way you tested everything else)
  6. I don't suggest switching wires between the extruder and the heated bed except as a last resort to make sure your bed is working, but if you have resistance readings going through the bed and wires then your bed is not likely broken and this would be an unnecessary risk. If you are going to proceed anyway then do as follows: remove extruder and heated bed wires, plug bed wires into extruder terminal, hold the power cord in your hand, turn on the extruder and as soon as the number goes up by 3 degrees (or you see/smell smoke or crackling whichever comes first) pull the plug and pray you didn't burn anything on the board. Make sure you are by a window or door and can quickly extract smoke from the room so as not to set off the smoke alarm in your apartment complex like I very nearly did.

Hope this helps somebody someday.

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It's probably a fuse or the MOSFET fried? Try checking for continuity on the fuse and voltage on the MOSFET.

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I have a Prusa i3 and the same case happened to me. However, only the power cable to the mainboard was bad.

I cut it in order to solder a new one to the 4 pin connector but inside the cable sleeving, there wasn't any copper, only grey powder. With a new connector, and cables, it is working very well now.

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