bed heater issues

I am having an issue not sure how to correct it, have a 900w 12 ps, a heater that works (tested it by hooking it up direct) the board reports the temp rise, pronterface shows it, but still get no voltage on d8, have metered both sides of the fuse and both read 12.3 v, but the board just will not turn on the heater, no led comes on either, i see the led but it does not come on like the one for the extruder, the printer works fine for pla, but i have 12 kg of abs i cant use because the heatbed issue, any ideas..

what firmware are you using?
sounds like the mosfet for D8 may be bad.

Marlin firmware, how do you test a mosfet? and is the one for d9 that i wont be using the same

?

If it is the mosfet, and would it be possible to drive a relay off the D9 outputs to actually carry the load for the heated bed since that tiny fuse wont take that kind of load, or, if i put a fuse there different fuse there instead of the small one would it melt the rest of the board?

i just want the bed heater to go on and off by it self so i dont have to stand there and manually switch it on and off

Only safe way would be a relay. Unless you want to burn your house down.

Hi,
I had problems with my bed as well. I am printig since weeks with the MK2B and Ramps 1.4. Yesterday I came in the printing room and it was smellig really toxic. I immediately stopped printing. I could not find any damage on the arduino/Ramps board. But my printbed stopped heating. I thought I unvisible burned my Mosfet Q3. Soldered ist out, but it was OK. Then I checked the 12V 11A Plug - completely burned at one side! Repared it and now I use a relais. I recommend it as well! IMO this part of Ramps is bad designed. Now the PCB Fuse and Mosfet stay cool! Important: A mechanical Relais should only be used in bang-bang mode.

By the way, this is my first posting ;-)

Manu

Edited 2 time(s). Last edit at 01/20/2014 03:29AM by Protoprinter.

the relay i have ordered is a solid state one

hope this fixes the issue

Brian
ps we can skip the whole house burning down part, too much hassle dealing with the insurance people