Printer-powered Bento Box for X1C with automatic on/off

It's a Bento Box that automatically turns on when you start printing, and turns off after you're done. No external PSU needed, no overly complicated electronics, no loss of functionality.

DISCLAIMER: I'm not an electrical engineer, nor an electrician. I'm just a guy with a screwdriver. The below worked for me, but do not assume that I'm correct about anything, or that I'm capable of communicating what's important in a straightforward manner. Even small mistakes can lead to very unpleasant results. 

Download, print, and assemble the required parts for the original Bento Box V2:

https://www.printables.com/model/272525-bentobox-v20-carbon-filter-for-bambu-lab-x1c-enclo

Instead of the original fan case I recommend you use my model as it provides extra clearance for gaskets, washers, and a through hole for fan cables. You can also use my model for the fan duct as it provides a convenient cable channel. Print two copies of the “wire clip” model, these are used for attaching the probe to the underside of the bed.

BOM, choices, reasoning

1. FANS
   
   You need two of them: size 4020 or 4028, voltage 12V or 24V. I strongly recommend a pair of this 24V 4028 SanACE:
   
   https://www.digikey.com/en/products/detail/sanyo-denki-america-inc/9WPA0424H3001/16720008
   
   Tremendous pressure at >38 mm H₂O. Pretty silent at 48 dBA (for a 18500 rpm part, that is). It draws less than 4 watts. The only con is that it's on the expensive side. With that said, people use much less powerful fans with the Bento (such as 12V 4020s) and have good results, so fan choice is really up to you.
    
2. GASKETS & WASHERS (optional)
   
   Get some silicone gaskets to give the fans something soft to sit on. This will greatly reduce noise. Some silicone washers up top - between the screw head and the fan - will also be helpful. Examples:
   
   https://www.digikey.com/en/products/detail/essentra-components/FGA-40/3811938
   
   https://www.mcmaster.com/catalog/130/3690/99604A141
    
3. THREADLOCKER (optional)
   
   A threadlocker will help the fan mounting screws resist vibration and becoming noisier over time. Vibra-Tite VC3 or Loctite 425 are both great and play nice with the plastics involved.
    
4. "NORMALLY OPEN" TEMPERATURE SENSOR/SWITCH, 55°C
   
   We'll be mounting a temperature switch  on the plastic sheet covering the underside of the bed. This part is easy to access, gets hot quickly, cools down quickly, and transfers heat to the sensor well enough. This works great for me:
   
   https://www.amazon.com/gp/product/B0BLL8B4SL
   
   I chose a part with 55°C trigger temperature; it turns the fans on before my bed reaches 90°C for ASA/ABS, and turns off about 15 minutes after the print is done. You could pick up a few different sensors and play around with them; do note that most of them are low quality parts with high variance on set & reset temperatures. Reset usually isn't triggered until 15 or 20 degrees below the set temperature, which is one of the reasons why a sensor measuring chamber air temperature - that would be simpler to wire - is a frustrating experience. We can use this weakness to our advantage as a delayed off switch, so in this case it's actually an advantage.
    
5. WIRES
   
   Nothing special. I used 18 AWG (0.75 mm²) copper wiring. Preferably red, black, and a third color I'll refer to as yellow going forward. You will need one end of red & black terminated with M3 or M4 ring terminals; either purchase the wire like this or get the ring terminals and crimp them yourself.
   
   You'll probably want wire on the thicker side like 18 or 20 AWG purely because these sizes make the ring terminals crimpable, but otherwise it doesn't matter much as we're only drawing ⅓ of an amp even with the two SanACE fans. Consult here for reference: https://www.powerstream.com/Wire_Size.htm
    
6. CONNECTING WIRES
   
   If you're comforable doing everything with a soldering iron, just do that - but you can also use these instead:
   
   https://www.amazon.com/gp/product/B0957K36WQ
   
   These are butt connectors that solder two wires together with the help of your heat gun. I used them to hook up the sensor's pigtails. 
   
   https://www.amazon.com/gp/product/B0BN6MBR7X
   
   WAGO lever nuts are great for quickly connecting two or more wires together. No soldering, no crimping ferrules, just strip the insulation and click the cable in. A huge quality of life improvement.
    
7. OTHER STUFF
   
   You'll need some Kapton tape, a little wiring harness tape (optional but recommended), and a few pieces of heatshrink tubing.

Bento Box Assembly

This goes as per the original design. When you install the fans, use the silicone gaskets on their underside, silicone washers on the topside, and some threadlocker on the tips of the screws. All of these are optional but go a long way towards managing vibrations. The original model doesn't have clearance for these items so print the “fan case” model instead that's part of this project.

If you have fans with more than two leads, it's OK to snip the sense and PWM wires here and just keep VCC and GND (usually red and black) going forward. Use some heatshrink tubing to tidy up the cut ends.

If you're considering linking the fans up in series (either because they're 12V or you just want to downvolt them for the extra quiet) slip on some heatshrinks on the individual wire pairs so you can distinguish them later:

Running Wiring

Before turning the printer off, raise the bed about 50mm below the maximum Z height, this will let you work on the underside of the bed while still having easy access to the back cable guide attached to it.

Unplug the power cord & PTFE tubes, then remove the back panel from the X1C and also remove the PSU covers underneath. This Bambu guide, while it's for the X1E, has the steps required. You only need to perform steps 1 & 2, then remove the plastic covers shielding the PSU terminals.

https://wiki.bambulab.com/en/x1/maintenance/x1e-power-supply

Install your ring-terminated black & red wires onto the two empty PSU terminals. Red goes to V+ and black to V-. 

Reinstall both plastic parts guarding the screw terminals on the PSU and make sure you have about 6-10 inches of your black & red wires dangling freely.

Attach your yellow leads to the temperature probe pigtails.

(In the above picture I used heatshrink solder butt connectors, but the straight-through type WAGOs would work just as well.)

Using a liberal amount of Kapton tape, fasten the probe to the underside of the bottom plastic cover of the print bed. Make sure it's flush against the plastic. Clean the area with an IPA wipe before, and give it a minute to evaporate. Use the “wire clip” model in this project to affix the yellow probe cables to the metal bar holding the print bed; secure them with more Kapton tape.

Run the yellow wire through the cable guides on the underside of the bed near the back Z leadscrew. The picture below is looking up towards the bottom of the bed.

Put some wiring harness tape or heatshrink tubing on the yellow cable right at the spot where it will be flexing as the bed moves up and down. Be VERY CAREFUL not to pinch any wires with the screw-on metal retainer visible below:

Running The Fan Wires to The Back

You have two options here: one that's cleaner but requires some easy drilling, and one that's a bit less involved.

ALTERNATIVE 1: Put the bottommost piece of the Bento Box in its place and use a sharpie to mark the spot where the side cutout meets the chamber's back panel, then use a small drill bit in a Dremel or an electric screwdriver to punch a hole here. The plastic is very soft and thin, it will come apart easy. Mind the mains cables on the other side! You're a few inches away from them at this exact spot so you're OK to drill but do be careful. The hole needs to be just big enough for your fan leads to pass through. Clean up the hole's jagged edges with a deburring tool if you have one handy, and/or use heatshrink or wiring harness tape to protect the cables as they pass through the hole.

ALTERNATIVE 2: print the “fan duct” model included in this project and use the cable channel on its bottom to guide the fan wires up to the back wall, then run the cables up to the back leadscrew to have them exit the main chamber and enter the back wiring area. Use some wiring harness tape to secure the fan wires in the cable channel:

Install the Bento Box's bottom piece in the printer, and secure it with an M3x8 self-tapping screw. 

Finishing Wiring

Run the yellow sensor wires together with the preexisting black bed wire harness all the way to the right side of the printer, then down and back to the center around the edge of the chassis as shown on the picture above. Utilize the existing wiring clips that Bambu has put in place.

At this point all three of your cable bundles should meet in the bottom back area, roughly in the middle:

1. The four fan wires (2 red, 2 black)
2. VCC+ and GND from the PSU (1 red, 1 black)
3. The sensor wire loop (2 yellow)

If running the fans in parallel (24V fans ONLY):

1. Connect VCC+ (red) to one end of the yellow sensor wire.
2. Connect the other yellow sensor wire to the two red wires coming from your fans
3. Connect GND (black) from the PSU to the two black wires coming from the fans

If running the fans in series (a MUST for 12V fans):

1. Connect a red fan wire to the black wire from the other fan. (You did remember to mark them with some heatshrink tubing, right?)
2. Connect VCC+ to one end of the yellow sensor wire.
3. Connect the other end of the yellow sensor loop to the remaining free red fan wire
4. Connect the remaining black fan wire to GND from the PSU.

Use the ample cabling clips Bambu provided and some wiring harness tape or zip ties to tidy things up. 

Put the printer back together, power it on, run a self-test then set the print bed temperature to 90°C and wait a couple of minutes; the fans should power on.

Q&A 

Q: Does the printer's PSU have enough power to handle this?

A: Yes. The two fans I'm using draw almost 8 watts and I've printed over two hundred hours with them on without any apparent issues. I also added a cheap but precise (& calibrated) voltage meter to my PSU just to be sure, and the reading never budges from 24.1 V:

https://www.amazon.com/gp/product/B00YALV0NG is the voltmeter, and the models for its housing are part of this project; it needs two M2 heat inserts to assemble and can be mounted with some double-sided tape if you want to add it to your printer.