Meanwell LRS-350 RSP-320 PSU cover for Prusa Mk3/Mk4 with rear AC/Mains receptacle

Introduction

This model incorporates design elements from 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?

You probably don't need to upgrade your power supply. But if the original is aging out or you got a no-name clone PSU, why not replace with a better-than-OEM model? Prusa includes a 240W Meanwell or Delta PSU. When heating up the print-bed and extruder from 15°C or lower, the load at the AC/Mains outlet can exceed 325W. At ~80% efficiency (typical for this style PSU) that is 260W output power (8% above the 240W spec). With warmer temperatures closer to 20°C, the output power is closer to 230-245W. 

The PSU can run reliably at these wattages as it has generous over-current (OCP) and over-power (OPP) limits. But the internal components age quicker under these conditions reducing long-term reliability. As the print-bed and extruder heat up, the load on the power supply decreases until it reaches steady-state. For ABS/PETG printing, the average load will be on the order of 90-150W while PLA would be 75-125W.

By upgrading to a 320-350W PSU, not only does the PSU efficiency improve (output power is closer to the 50-75% plateau where efficiently is at it's peak), but the PSU will probably last longer, especially in an enclosure, 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 or add an inline resistor to lower the fan voltage/speed. For power supplies with 12V fans like in the LRS-350 model, you can add an inline ~20 ohm ½ watt resistor to safely reduce the fan speed without impacting longevity of the PSU. Adding a sound-baffle to the fan inlet/outlet is not recommended as it will create too much back-pressure and the fan won't flow well.

IMHO, the RSP-320-24 is superior to the LRS-350-24 even though it has 30W less capacity. The RSP models have a newer PCB design with active PFC, quieter PWM fan speed control, full-range AC/Mains input instead of a 120/240 switch and are rated for ~90% efficiency rather than ~80%. 

Changes compared to the original cover design by Sneaks

• thickened upper PSU mounting area to fit 30mm “slim” PSUs such as LRS-350-24 and RSP-320-24

• moved power switch closer to AC/mains receptacle for simpler wire routing

• added relief lines near 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

Requirements

Electronics

Older Mk3 printers with the silver PSU come with the required switch and AC/Mains receptacle. For newer Mk3S/S+/Mk4 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 

Fasteners

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

Tools

• hex wrench kit

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

Print settings

• PETG filament

• 0.25mm layer height

• 3 perimeters

• 15% infill adaptive cubic or gyroid

• 0.6mm nozzle (0.4mm nozzle OK)

• no supports

• disable thin-wall detection

Installation

• 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

• Optional: extend original power cables by 2cm

• Place 3D-printed part roughly where it needs to be on the Prusa frame

• Place PSU, switch, AC/Mains etc within the 3D printed part roughly where they need to be but don't 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.