How to install a MOSFET to prevent burned plugs on the RAMBO board

There have been many issues with burned plugs of the heat bed power supply on the RAMBO board. In the easiest cases just the plugs of the heat bed melt and burn. The next level is that the connectors on the RAMBO board get damaged and finally the RAMBO board gets damaged. There are mainly two solutions for it:

  • solder cables direct to the RAMBO board
  • use a MOSFET to keep high away from the Rambo board

The first solution requires some soldering skills and will void the warranty of the RAMBO board. In this tutorial, I will only consider option 2. I will compare both solutions in a different blog.

The aim of using a MOSFET (a special transistor) is to keep high currents away from the Rambo board and use it only as a switch for turning the heat bed on / and off and take the power from somewhere else. It’s not that the board is not capable handling these currents but the plugs are the weak points and especially the one for the heat bed tends to burn in some cases. There are different plugs on the market and they capable of handling currents up to 15A, which is on the edge of what is needed. Besides the vibrations during the printing process let the plug come loose a bit. And that is exactly the time when the melting and burning starts

.First lets start with tools and material:

    • screw driver
    • write stripper
    • crimping tool
  • wire cutter

For the wiring we use AWG14 silicone cables. They are highly flexible and are also well suited as cables to moving parts.

  • about 1m red and black silicone cables AWG 14 and AWG 16
  • 4 WAGO terminals (3 pins)
  • 6 gray ferrules
  • MOSFET
  • 4 terminal connectors

Our strategy is as follows: one of the cable pairs coming from the PSU will serve as power supply for the heat bed and will be switched on/off through RAMBO and MOSFET. The other cable pair will serve as power supply for the RAMBO board and hence the hot end.

We will no go through the wiring step by step:

There are two plugs on the Rambo board. The upper on serves as power supply for the board itself, the motors, and hot end, and the sensors. The lower on serves as power input for the heated bed. We will not use it as such but we need to power this line as well since we want to switch on/off the MOSFET through the board.

We split one of the power lines coming from the PSU into two and reconnect to Rambo. On the left of the picture you can see the two cables coming from the PSU. These are connected to WAGO terminals. On the right there are two red and black cables. Put a terminal connector on each red/black pair and connect to the RAMBO board like in the above picture. Note that the red cable is in the right side of the terminal connector. As for cables, silicone AWG 16 are used.

The second cable pair from the PSU is connected directly to the power input of the MOSFET. Since in this tutorial we avoid soldering, we connect the head bed  via two additional WAGO terminals to the MOSFET.

As a switch for the MOSFET, we use the output of the RAMBO for the heat bed. Every time the Rambo board turns on the heat bed the power is supplied by the power line that runs through the MOSFET. Here AWG14 cables are sufficient since only a current smaller 0.5A runs through.  As a MOSFET we use a MKS MOSFET board which is available from Amazon or aliexpress.

The following picture shows the full wiring diagram:

As final step the circuit including the MOSFET should go in some box to avoid contacting the board. A box for the MOSFET is available on thingiverse.

In this way the keep high currents completely away from the Rambo and hence from the plugs. Actually, the current to heat the bed is about 13A, which is close to what the plugs can handle (which is 15A). Instead of the Molex plugs, we use WAGO terminals that are capable of handling much higher currents.

DISCLAIMER: Don’t do the replacement if you do not feel comfortable working with electricity. A wrong connection may damage your Rambo board. I will take no responsibility for that.

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