Hello, so basically one day i had to leave after starting a short print. Whole day i thought how the printer is sitting there doing nothing and consuming electricity. So I thought about simple circuit to switch itself off from mains after print is complete. Also during power supply disassembly i noticed that power contacts are rubbing to low voltage outputs so i added a little spacer to keep them further away.

Step 1: Things You Will Need

For this modification there are very few things you will need. I tried to keep it as simple as possible.

1. 24 V relay for switching mains.I used similar to this that i had lying around.

https://www.rapidonline.com/finder-40-31-7-024-000...

2. Micro switch. Long arm gives more freedom of adjustment.

https://www.railwayscenics.com/miniature-v4-micros...

3. Safety diode for relay.

1N4007 or similar

4. Some wires. thicker gauge for mains and thinner for micro switch.

Printable parts from thingverse:

https://www.thingiverse.com/thing:3972464

1. micro switch mount. For (19,8*6,4*10,2) dimensions.

2.Power contacts spacer:

Step 2: Modification

Remove power supply from printer and undo the plastic that is covering input and output contacts. When disconnecting contacts, take note what goes where. Now we can prepare the relay like shown in the picture or according to schematics.Soldering or wiring crimping is necessary. Cover terminals with heat shrink tube. The relay sits nicely in the upper corner of plastic cover attached with hot glue. I also added glue to contacts so that they didn't bend easily. Put everything back adding the spacer to keep the contacts further away. Only two new small gauge wires come out of the power supply.

Step 3: Final Step

Now what is left is attaching the micro switch in the bracket and adjusting its height. The way i approached this i made my end G code like this:

G91 ;Relative positionning
G1 E-2 F2700 ;Retract a bit

G1 E-2 Z0.2 F2400 ;Retract and raise Z

G1 X5 Y5 F3000 ;Wipe out

G1 Z10 ;Raise Z more

G90 ;Absolute positionning

G1 X0 Y{machine_depth} ;Present print

M106 S0 ;Turn-off fan

M104 S0 ;Turn-off hotend

M140 S0 ;Turn-off bed

M84 X Y E ;Disable all steppers but Z

M109 R100; wait for hotend to cool

G1 Z250; Raise Z to activate shutoff switch

Everything works like this. You turn on the power supply with the switch. Then relay activates from 24V supply output. At this point if you want printer to be able to shut down you must turn the switch to off position. Now all power goes through relay. After the print is complete g code waits for the nozzle temperature to cool to 100 deg and then rises the Z axis to 250mm (max for ender 3) where the micro switch is actuated disengaging relay and therefore the power to the printer until it is turned on by switch again.