Almost killed my replacement Rumba again. I am pretty sure at this point I'm dealing with a defective heated bed. Getting tired of this... .
I'm thinking mine might be defective as well, it's measuring almost 4x the ohms as someone else's and doesn't heat properly... The glass only gets to 65c
I tested the ohms by unhooking the bed from the rumba the just testing black and red wires, the. I unplugged the connector from the bed and tested those three sets of connectors. For amperage, I disconnected the red wire from the rumba, clamped the red wire to the black lead of the multimeter, and put the red probe from the multimeter on the screw of the positive connector on the rumba. Then turned it on. For voltage, all you need to do while the bed is heating, is put the multimeter on the red and black screw heads on the rumba, and you should get 24v
Ok bed resistance is 1.7ohms and voltage straight out of the bed terminals without the bed hooked up while heating is 24.4v, when not heating it is 0.00v.
Yeah I'm not sure, Mike Kelly has been helping me and we haven't fixed my bed so, I can't tell you what it should be..
Good news is it is 1.7ohms direct from the board and also when read from the end of the harness, so I don't think it's the harness.
Decided to try it again... This time I limited max bed power to 50% via firmware. Hopefully that will prevent further damage. So far so good with the bed holding 60C for about 30 min.
1.7 ohms is a problem and will pull more current. Might explain why you're blowing polyfuses. Triple check all your connections to ensure everything is connected correctly
Cheap multimeters can't measure low resistances very well. If its not a Fluke or similar, I wouldn't put too much stock in the resistance measurement - could be off by an ohm, certainly. Better bet is to measure the current, though most multimeters top out at 10 amps which is about what the bed draws - yours may be drawing more. If you do have a 10A multimeter, power the bed from 12 volts. It should draw 5 amps when cold.
As always, E3D customer service shines through. New bed, harness and Rumba on the way. I'll make sure to measure voltages and resistances before I do any heavy work. Catriona deserves a raise
