Solved Bed heater output stuck "on''

The heater output on my (Kickstarter Robo version) Arduino board no longer de-energize so I had to disconnected the heater. Does anyone know if this a common board failure, or is it worth reloading the firmware before I condemn the board?

 

Sounds like you blew the mosfet.

 

Man, I hate it when that happens.
Any recommendations on which version of board replacement I should get?

 

You can either get a new ramps 1.4 ($20) or unsolder the mosfet and solder a new one on: http://www.mouser.com/Search/Produc...06Lvirtualkey51120000virtualkey511-STP55NF06L ($1.54)

 

As I wait for my new Ramps board to arrive (on a slow boat from China) I went ahead and replaced the suspect mosfet. Unfortunately, the problem appears to be deeper. With the wires disconnected the heater output only measures 6vdc, regardless if the heater output is triggered or not. So, before I pop in the new ramps board can someone confirm what the nominal resistance of the heater should be? Mine is around 1.5 ohms.

 

1.5 ohms is correct. You might have blew the polyfuse as well.

The circuit for the bed heater is incredibly simple if you can follow the 12v line: http://reprap.org/mediawiki/images/f/f6/RAMPS1.4schematic.png

 

Nope. My 14A polyfuse is good and I have 12v at LED2 at all times. All the resistors in that circuit are good too. Geeze, I hope it's not the Arduino board because didn't order a replacement (yet).

 

Well that's an easy test. Unplug the arduino and see if it heats up. You can also measure the pin power coming off D8 and see if you're getting 5v.

LED 2 would to me indicate that the mosfet is still bad, though 6v (maybe 7?) would indicate that 12v is jumped to 5v to give a logic of 7v between them.

 

It's possible I overheated the new mosfet while soldering. In hind sight I wish I'd taken voltage readings before reconnecting the heater. That's why I asked about nominal heater resistance. I just want to feel confident that the heater itself wasn't the roof cause of the mosfet failure.

Today when I fired it up cold I measured 8.7v (D8 to +). I do have 3 extra mosfets, so... but I think for now I'll just wait for my replacement RAMPS and put my manual toggle switch back in series with the heater.

Thanks Mike

 

So, my cheapo replacement Ramps board is junk and I tried another new mosfet in the original board but no luck. I see a lot of Arduino/RAMPS combinations out there. Can anyone recommend a reliable and affordable replacement set?

 

Thanks Mark. Did you bump up the size of your F2 polyfuse (to 14A) and/or the Q3 mosfet? I've only printed PLA so far but like to run the bed around 60c, which I was doing when the board(s) failed. Someday I'd like to give ABS a shot someday...