Heated Bed not heating on original Prusa i3. Using a Ramps 1.4 EFB

Question

So as the title said my heated bed stopped heating mid print.

I'm pretty confident the bed itself is not the problem as I've ran continuity tests from the wires and a resistance check (which showed a total resistance of about 1.5 ohms, which is where it should be). I've done voltage checks across the input terminals which showed a successful output of 12 V.

Finally I did a voltage test across the output terminal at D8 where the heated bed should be outputted to. The result came up with nearly 0 V. I then noticed that the LED that usually lights up for D8 was turned off. The weird thing is, I unplugged the wires from D8 and this time the LED lit up like it was supposed to. Due to this, I believe the problem to be with the MOSFET attached to D8.

My proposed solution would be to move the heated bed terminal to D9 instead of D8. I wish to this because despite the firmware set up for a fan, I do not have a fan attached to D9. This leaves this terminal open and with a potentially unharmed MOSFET. Despite this I have found no information on how to go about accomplishing this.

What do you guys think? Is that even a good idea or should the component (or board be replaced)? Any help would be greatly appreciated.

Answer 1

First you should check if the bed still works when applying 12V directly. You can then also check the voltage over the 12V output terminals to measure the power supply unit while it is powering the bed. It is unclear whether you measured the voltage of the PSU while it was powering a large load.

It is also a good idea to get the external MOSFET board out of the equation (the less components you have, the less you need to troubleshoot).

From the post can be concluded that you are using the so-called RAMPS EFB setup, e.g. in Marlin firmware defined by constant IS_RAMPS_EFB in your firmware printer setup (Extruder on pin 10, fan on pin 9 and bed on pin 8).

In the pin configuration file pins_RAMPS.h you can find the following lines:

#define FAN_PIN        RAMPS_D9_PIN
#define HEATER_BED_PIN RAMPS_D8_PIN

To change the pin layout, switch the values of the constants.

Answer 2

you can swap pins in Marlin firmware. When you download it from GitHub, there is a file called pins.h (please be aware that this file is an example), and you can play with your setup to skip the broken mosfet.

Sometimes, it could be easier and faster to desolder and swap elements directly on the board - that depends on the skill set we have