I have a PCB heated bed that I want to attach to my printer. I have Ramps 1.4 and use Marlin. I already checked here, as well as other sites and did a general google search but have had no luck with my question. The board heats up max 100C and when wired directly to a power source it powers on fine. When I plug it to D8 of the terminal block and turn on the bed in Repetier, it does nothing. I don't have a thermistor attached to it, I don't really want one. I was thinking the missing thermistor is the problem. When I connect at first in Repetier it does return an error for the missing thermistor, so I am assuming the firmware is looking for it. I'm not sure where or what to change in the Marlin configuration to make it just turn on. I do see several mentions but the comments in the script make it sound as it being for the extruder. Any help would be greatly appreciated.

You can turn off your thermistor around [github.com]

You can set the minimum fault temperature to below whatever Marlin is reporting the non-existent thermistor to be and avoid that error with [github.com]

You could put in a plain 100K resistor as a fake 100K thermistor and it should always measure about 25C.

Turns out I'm just a big dumb dumb...While I did have to turn off the thermistor and set the default to 0 (current is 25 so it won't power the heat bed as a security feature) when I did so, it was still not working...because I need to supply power to the extra term 11V terminal into RAMPS. The PS is 16v+ rated so I should just be able to split power to both terminals. I'll give that a try, but thanks for the help. For some reason I had trouble finding those setting in the configuration file.

Never mind. Still having issues. Made sure to provide power to the input terminal block. Still nothing. Changed those setting. Nothing. I pulled out my multimeter and tested the circuit to make sure nothing was wrong and it is good. I was a little concerned because the Ramps I have is second hand, and when I got it in, D8 was charred. I assume they tried to put something in there and the block couldn't handle it. I had to desolder the d8-10 block and resolder a 24Amp block, more than sufficient. So I was a little uneasy since I have't done some soldering in a while. I'm going to look at the settings again for Marlin, I'm wondering if Repetier is at fault though. When I get the chance, not home at the moment, I will try sending the printer the g-code to turn the bed on, see if it accepts that. As of now I have been merely trying to use the gui from Repetier.

I've been looking at the mcodes for the marlin firmware and I am not finding a simple on/off code for the bed. All I find is 140 for setting the target temp and 190 to wait until bed reaches target temp. No simple on/off. Am I mistaken on this?

Maybe an M42 P8 S1, M42 P8 S255, and M42 P8 S0 , after reconfiguring https://github.com/ErikZalm/Marlin/blob/Marlin_v1/Marlin/pins.h#L648 to not reserve pin 8 for your heater.

That worked fabulously....Thank you so much. just to clarify, does that mean the Mcode 42 means to provide power?

http://reprap.org/wiki/Gcode#M42_in_Marlin.2FSprinter does general purpose i/o. Since it looks like pin 9 does PWM, the marlin code should do an analogWrite() to turn pin 9 on full power when given an 'M42 P9 S255' while M42 P9 S0 should turn pin 9 off.

If you use intermediate values, like M42 P8 S127, it would do a 127/255=50% duty cycle, but I'm not sure what frequency it would run at, and some folks have had temperature problems with higher frequency switching of marginal MOSFETs in some hardware.