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Blown Fuse

(MK2S/MK2.5/MK2.5S)

Relevant for

MK2.5
MK2.5S
MK2S
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The Original Prusa i3 MK2 series printer is equipped with fuses to protect the most important peripheries. If your printer or some of its components suddenly stopped working, it is recommended to check for blown fuses and replace them if necessary.

miniRAMBo

The printer has a miniRAMBo logic board that has three removable fuses. The fuses we use are known as ATO blade fuses for automotive applications.

Each fuse protects a different part of the miniRAMBO board:

  • F1 (5A fuse) - Extruder motor, X-axis motor, Y-axis motor, and both Z-axis motors
  • F2 (5A fuse) - Extruder heating, fans, and logic unit.
  • F3 (15A fuse) - Bed heating

Checking the miniRAMBo fuses
  1. Turn the printer off and disconnect the power cord.
  2. Open the miniRAMBo cover by removing the long M3x40 screw holding the Rambo-cover doors closed. Please refer to Electronics assembly - Finalizing Rambo Cover of the assembly manual for more details.
  3. Now you have access to the miniRAMBo. Pull the fuses out and inspect them. The photos below show what the intact fuses look like. Inspect them from below and check if the little wire inside is not broken. In case you cannot tell if the fuse is broken, replace the fuse preventively.

If you own a multimeter, you can check the components for a potential short. Multimeter usage will show you how.
Your RAMBo board can be black/brown instead of green. The color does not matter.

PSU fuse

The printers are equipped with an additional fuse in their PSUs. You can check the pictures below for how to find it.

When you are replacing the fuse, make sure that the printer is turned off and disconnected from the power source.

  • Find the PSU fuse, which is located just above the PSU's power plug.
  • Remove the cover of the PSU fuse, the best tool for that is the screwdriver that comes bundled with our printer.
  • Inspect if the wire inside of the fuse is intact. If it is damaged, then it must be replaced.
Replacing blown fuses

If you find out that any of the fuses are blown, you can easily replace them. The fuses can be found at any local car parts/hardware store or ordered online. It's a good practice to bring the blown fuse with you so that you can get the exact same one.

  1. Turn the printer off, disconnect the power and remove the blown fuse(s) from their sockets.
  2. Place the new fuse in the correct slot - Double check that you are replacing 5A for 5A or 15A for 15A.
  3. Unplug any components connected to the previously blown fuse and try to turn on your printer. Check for any kind of unwanted behavior.
  4. Once again, turn off the printer and remove the power cord. Then reconnect all the disconnected components.
 

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David H. Brown
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"ATC" fuses are an equivalent form factor to "ATO" and what I could find at the local parts store (Bussman brand). Depending on which source you read, the ATC designation is just part of a work-around Littelfuse's patent/trademark... or C=Closed vs. O=Open, referring to whether the plastic wraps around the fusable element at the bottom. As the ATC fuses I bought are open at the bottom just like the blown one, I'm leaning toward the trademark avoidance theory.
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