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Marlin help for heated bed

Posted by jobzombi 
Marlin help for heated bed
May 25, 2014 06:10AM
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.
Re: Marlin help for heated bed
May 25, 2014 10:20AM
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.
Re: Marlin help for heated bed
May 25, 2014 11:39PM
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.
Re: Marlin help for heated bed
May 26, 2014 08:31PM
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.
Re: Marlin help for heated bed
May 26, 2014 11:26PM
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?
Re: Marlin help for heated bed
May 27, 2014 11:35PM
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.
Re: Marlin help for heated bed
May 28, 2014 01:16AM
That worked fabulously....Thank you so much. just to clarify, does that mean the Mcode 42 means to provide power?
Re: Marlin help for heated bed
May 28, 2014 09:52AM
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.
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