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I recently picked up a second hand/defective 'Renkforce RF100' printer. A pretty obscure German brand. The problem with it was: broken hotend fan cable which caused clogging that the previous user was unable to fix. I have thoroughly cleaned the hotend, rewired the fan and now it works again.

The mainboard it used is not very well known, designated as MPX.3 which seems similar to this HICTOP brand RAMPS 1.4 board. Renkforce's documentation is very sparse. Through some digging I found a GitHub repo with some customizable Marlin firmware and this diagram: RF100 Mainboard

No real circuit diagram to be found for this specific Renkforce board, however. It does look similar to the MKS Base 1.0 board. As you can see, it has support for up to two extruders, a fan and a heated bed ("EEFB"). Only one extruder (E0, labeled 5 on the picture above) is in use on this printer.

Here is where I need some advice:

I would like to add a part cooling fan to this printer. Right now, only the hotend fan (40 mm, 12 V 0.15 A) is connected to the 'Fan' (labeled 7 in the picture above) header on the board and it will run at 100 % as soon as the hotend reaches >50 °C. Now, I could of course wire the hotend and parts cooling fans together, but I would like to have separate control over the parts cooling fan, in order to only run it at specific times (e.g. ramp it up after the first few layers are printed).

My question: Could I, for instance, just screw the parts cooling fan leads into the spare extruder terminal (E1) and re-program the firmware to let it know that that pin (D7, labeled 6 on the picture above) is a FAN_PIN? Which I could then ideally address/control with the G-code M106. In a sense I would repurpose the E1 terminal for a fan - do I need to worry about (fire) safety here? Since it's an extruder terminal? Or is that terminal "simply" a 12 V connection that I can connect to whatever? Do I need to add a MOSFET next to the terminal? Or am I overthinking this and should I use another position / pin on the board? This is really the limit of my electronics knowledge right now and safety is a primary concern. Any input is appreciated!

Edit for future reference: this GitHub repo has a modern (at the time of writing) version of Marlin suitable for this printer.

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In a sense I would repurpose the E1 terminal for a fan - do I need to worry about (fire) safety here?

No, fans do not use much power, heater cartridges do.

Considering the fifth stepper driver is not installed, this board is not equipped for using the E1 heater port/terminal in its current state.

E.g, the MOSFET for the E1 heater is not installed (E0 and the bed MOSFET are installed):

enter image description here

Or is that terminal "simply" a 12 V connection that I can connect to whatever?

No, it is a PWM controlled terminal by the microprocessor, however, the VCC and GND of the MOSFET connection holes should be connected, so will deliver 12 V.

Do I need to add a MOSFET next to the terminal?

Yes, if you want the firmware to control the speed.

The chance that the microprocessor is connected to the MOSFET gate port is quite big. I don't think that this board has missing traces from the processor to the peripherals.

So, you could repurpose the pin that is dedicated to driving the E1 MOSFET, but, you should not run the fan directly from that pin. You should solder a MOSFET or add an external MOSFET module to protect the processor of over-current.

Changes to the firmware are simple, you need to add the correct port to the functioning of the fan, you may find some help in question Controlling more fans with RAMPS board or here How to use second hotend for bed heating? (this may be helpful to modify the sources, or ask another question).

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  • $\begingroup$ Good! I clarified my post a bit with which connection is which in the picture. So if I re-read your post now, something comes to mind. You state: "Yes, if you want the firmware to control the speed." Say hypothetically I don't need to control speed, can I then connect VCC and GND of the E1 (labeled 6 in the picture) MOSFET connection holes, and use the E1 terminal at 12 V without risking the microprocessor? $\endgroup$
    – Floris
    Dec 6, 2022 at 14:41
  • $\begingroup$ Correct! Connecting VCC and GND of the MOSFET pins is like connecting directly to the power supply. $\endgroup$
    – 0scar ♦
    Dec 6, 2022 at 14:44
  • $\begingroup$ In that case, I might as well connect the hotend fan to that (VCC/GND) and connect the parts cooling fan to the Fan header (label 7, which has a MOSFET installed - this printer doesn't have a heated bed btw). Thank you for your answers, they have given me a better understanding of how a board like this works. $\endgroup$
    – Floris
    Dec 6, 2022 at 14:50
  • $\begingroup$ That may work out of the box if the firmware has been configured to use terminal 7. $\endgroup$
    – 0scar ♦
    Dec 6, 2022 at 14:53
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    $\begingroup$ Correct, it is scheduled per (among some other definitions) #define EXTRUDER_AUTO_FAN_TEMPERATURE 50 in the firmware. I have found a GitHub repo with a version of Marlin 2.0.9.3 suitable for this obscure printer (will link in main post for future reference). With this firmware, I will be able to achieve my initial goal. $\endgroup$
    – Floris
    Dec 7, 2022 at 19:12

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