Meanwell LRS-350 RSP-320 PSU cover for Prusa Mk3 with rear AC/Mains receptacle

Important note:

This model is intended to be a direct replacement for the silver PSU on Mk3 and early Mk3S - it is not 100% compatible with the black PSU on later Mk3S/Mk3S+ printers. The key difference between the black and silver PSU is the switch and AC/Mains receptacle. On the silver PSU, the switch and AC/Mains are two separate parts which can be used with other generic power supplies. The black Prusa PSU is non-standard and has the switch and AC/Mains integrated into the chassis. It may be possible to disassemble the black PSU and re-use its switch and AC/Mains receptacle, but that is beyond the scope of what is presented here. For a plug and play solution you'll need two additional parts listed below.

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Intro:

This is a remix of https://www.thingiverse.com/thing:3082178 which was designed for the discontinued Meanwell NES-350-24. I like the design for its simplicity, rear-facing AC/Mains receptacle, and integrated cable-tie slot but the model was not compatible with the newer slim Meanwell units as is.

Why upgrade the PSU?

Prusa includes a 240W PSU. When heating up the heatbed and extruder from room temperature or below, the actual load on the PSU exceeds 300W and often approaches 325W. You can test this for yourself with an AC/mains power meter.

The PSU can run reliably at this wattage for a short period of time as it has a generous over-current (OCP) and over-power (OPP) limit. But the internal components age far more quickly under these conditions reducing reliability, and the overall PSU efficiency plummets below 80%.

By upgrading to a 320-350W PSU, not only does the PSU efficiency improve, but the PSU will probably last longer since the components won't heat up as much. The only drawback with these higher-rated power supplies is that they include cooling fans which can be noisy under certain conditions. Most models have built-in fan-speed controllers which switch off the fan at idle and vary the fan speed according to the PSU load/temperature. If the fan is too noisy, you can replace with a Noctua fan. Adding a sound-baffle to the fan outlet or inlet is not recommended as it will create too much back-pressure and the fan wont flow well.

Compared to the original cover design by sneaks, I've changed the following:

• thickened upper PSU mounting area to accommodate 30mm “slim” PSUs such as the Meanwell LRS-350-24 and RSP-320-24 (RSP is similar to LRS models but with quieter PWM fan speed control, active PFC for improved efficiency, and full range AC/Mains input instead of 120V/240V switch)
• moved the power switch closer to the AC/mains receptacle for simpler wire routing
• added relief lines near the AC/Mains socket so you can bend and snap out a piece of the bottom cover to ease power-panic PCB fitment
• simplified filets/chamfers for quicker printing
• added stiffening strut
• added cutout for z-axis motor wires

Required hardware:

Most nuts and screws can be reused from the original PSU cover or from the spare parts kit.

• 3x M4x8mm or M4x6mm Flat screws (Mount PSU to Cover) (M4x10mm is too long)
• 1x M4x10mm Button screw (Mount PSU to MK3 Frame upright - can use one additional screw if an additional hole in drilled into the frame)
• 4x M3x6mm Flat screws (Mount Bottom to Cover)
• 2x M3x10mm or 2x M4x10mm Cap or Button screws (Mount Cover to MK3 Frame Y extrusion - size depends on which T-nuts are slotted into the extrusion)
• 2x M3x10mm Flat or Button screws (Mount AC/Mains receptacle to Cover)
• 4x M3 Square nuts (3x Bottom / 1x Cover)
• 2x M3 nyloc nuts (Mount AC/Mains receptacle to cover)
• 1x 4" plastic cable tie

Required electronics:

Printers with silver PSU come with the required switch and AC/Mains receptacle. For printers that came with a black PSU you'll need two parts:

• Mk3 style AC/Mains receptacle with integrated power panic by Polis3D or Fystec: https://amzn.to/4jwGeJ9
• 20x15mm panel mount 250V/6A rocker switch: https://amzn.to/44eAa3e 

Required tools:

• hex wrench kit
• optional: electric drill with 3/16" or 4.5mm drill bit

Recommended print settings:

• PETG filament
• 0.25mm
• 3 perimeters
• 15% infill
• 0.6mm nozzle (0.4mm nozzle OK)
• no supports
• disable thin-wall detection

Install process:

• Disconnect AC/Mains cable
• Disconnect 24V wires from PSU
• Remove old PSU from Prusa frame
• Optional: use a 3/16" or 4.5mm drill bit to drill additional hole in Prusa frame to correspond to mounting hole on the slim PSU.
• Optional: test fit PSU with 1 or 2 M4x10mm button screws, then remove PSU from frame
• Place 3D-printed part roughly where it needs to be on the Prusa frame
• Place PSU, switch, AC/Mains etc within the 3D printed roughly where they need to be but dont screw anything together yet
• Connect all the wires for AC/Mains, power panic pcb ,etc
• Connect AC/Mains power cable
• Test that the printer turns on properly with heat bed and extruder heating up to operating temperature
• reset printer and allow extruder to cool until fan switches off
• Unplug AC/Mains power cable and double-check all power connections ensuring screws are tight and wires are not snagged
• fasten PSU, switch, power panic and AC/Mains receptacle to the main 3D-printed part
• fasten bottom 3D printed cover in place
• Rotate cover+PSU into place on the Prusa frame
• Fasten whole assembly to the chassis

Note if you try to fasten the PSU in place before connecting the wires, there won't be enough room to tighten the screws and you'll find the wires will be a little too short.

For slim Meanwell PSUs