Head-Bed: Will not turn off (12V const. at Ramps D8 connector)

Hi,

I have a Ramps board (v1.4?) and I am using Repetier (v. 1.5.6?).

My heat-bed is connected directly to a 12V DC power-supply. In serial I have a MOSFET.
My idea was to control the MOSFET-Gate through the Heat-Bed power-supply-Connector of the Ramps-Board, which is the D8-connector (see attached picture).

But I am measureing constantly +12V at the D8-connector - also when the heat-bed-temperature is exceeding the set temperature.
Hence, it never stops heating.

Any ideas, what I am doing wrong?
Thanks!

That is how it should work because all these board are switching the ground.



why are you putting this in the Repetier thread

Edited 1 time(s). Last edit at 04/23/2016 10:03AM by amigob.

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P3steel DXL, with Due/RADDS/Raps128 dual Wade's extruder

Quote
amigob
why are you putting this in the Repetier thread

First: Thanks for the schematics. This explains, why I have always +12V at D8/P$1.
I placed this in the Repetier thread, because may assumption was I have to change something in the configuration file from the Repetier software (e.g., I was in doubt that D8 is for the heat-bed).

But with your schematic, know I understand, that D8/P$2's potential will be changed, depending if Repetier turns the heatbed on or off. Hence, the gate from the MOSFET, which is on the heat-bed has to be connected with D8/P$2.

BTW:
I have seen, on default (when the heat-bed is turned off in the repetier software), I have +12V at D8/P$2.
Vice versa, when the heat-bed is turned on, D8/P$2 is 0 V.

Makes sense, but is there a way to "switch" this to: heat-bed turned off = 0V on D8/P$2; heat bed turned on = +12V at D8/P$2

Edited 3 time(s). Last edit at 04/24/2016 04:20AM by BigMan200.

that would mean that on the digital side you have to invert the signal, I would not do that because then
the heater is on when you have 12V power supply and no 5 volt power supply. that is a dangers situation.
What you could do is us a digital pin out ( AUX 4 connector ) to directly switch your FET, but you need to have a FET that is "on" at 5V gate voltage.

Edited 1 time(s). Last edit at 04/24/2016 04:37AM by amigob.

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P3steel DXL, with Due/RADDS/Raps128 dual Wade's extruder