Dankdryer improvements

In an earlier article, I designed and constructed a high-temperature filament dryer. Before I was even done putting together the first design, I was thinking of improvements for reliability, efficiency, cost, and ease of assembly. I've put some of them into effect, and the results are most pleasing.

Problems with v1

• It was very difficult to keep the spool level between the impact of the worm gear and the load cell structure.
• The motor was loud.
• It was difficult to print the bottom chamber without corner warping, which further threw off the spool leveling.
• Lack of a standard power hookup required mutilating three-prong cables.

Models

• I raised the air shield to cover the entirety of the load cell.
• Cut most of the midsection of the bottom chamber wire channel away. It's easier to work with if you can see/touch the wires in the middle of the channel.
• Cut the bottom chamber down mercilessly to save mass, aiming to get the sum of both chambers below 500g using my recommended settings. Haven't reached it yet, but we're close. Most noticeably, I changed the motor mount from a rectangular to a trapezoidal prism, and then cut an inverted trapezoidal prism out from its center.
• I separated the bottom of the control chamber from its walls, and added corners to the bottom of the latter so the two can be fastened with screws. This ought (1) eliminate warping when printing with polycarbonate and (2) allow more rapid iteration on the control chamber, since the bottom represented significant printing time, yet stayed pretty much constant throughout.

Electronics

• Let's toss the TB6612FNG motor controller. We only need one direction of rotation, so we control the motor with an RFP30N06LE N-channel MOSFET, a 1N5817 Schottky diode, and a 10K resistor. The n-FET goes on the ground side of the motor, and the diode runs in parallel with the load (i.e. is connected to the motor's two pins). The resistor pulls down the gate lead. This eliminates four net wires (AIN1, AIN2, STBY, AO2, APWM go away; we add GATE) while adding one resistor, a net reduction of seven joints. I'm not certain that it's actually any cheaper, though; it might actually be more expensive.
  • It's not easy finding a good throughhole logic level MOSFET for 3.3v (surface mount are readily available)! The RFP30N06LE will only pass about 20% of its rated amperage at 3.3v...but that's more than enough for our needs (the TB6612FNG could only sustain 1.2A, after all). I already had the necessary MOSFET thanks to this cute little set.
• The LM35 is operating in a nasty thermal environment, and on about ten centimeters of poorly-shielded AWG22. Let's give it some bigger boxing gloves. Put a 0.01µF bypass capacitor across its power and ground leads. Put a 75Ω resistor and a 0.22µF capacitor in series between the signal and the ground. These recommendations come directly from the datasheet:

• We should use the AVDD analog power input of the HX711 at 5V.

PCB considerations

• We're ditching the Sparkfun NAU7802 breakout board, so we'll need add 2.2kΩ pullups on both I2C lines, 0.1µF and 1µF capacitors on VDDA, and 47Ω resistors on VNN1N and VNN1P with a 0.1µF bypass capacitor across them. See the schematic:

• The first version took in 120VAC, 12VDC, and 5VDC (i.e. there was a discrete buck converter from 12 → 5VDC). Let's build that circuit into the PCB, so we're only working with 120VAC and 12VDC (the native output of the AC adapter) inputs. At the same time, let's move the ESP32 over to 3.3V input (at least when we're not powering it with external USB), so we don't exercise its low-efficiency voltage regulator. We'll still want 5V to the LM35 and the HX711's AVDD. Since these are both precision analog devices requiring very small currents, we'll use a voltage regulator from 12->5; this ought be less noisy than a switching transformer.

Thoughts

• Rather than mounting the RC522 directly onto the bottom of the top chamber, we print a short mount for it (building the mount into the hotbox would make it very difficult to print). This reduces the heat directly transferred to the RC522, without adding new connectors. The necessary screw holes into the bottom of the chamber already exist.
• I cut out chunks from the eight "fat corners" of the hotbox. I'd like to make these siting areas for dessicants. This added weight, presumably due to more walls vs sparse infill, so I changed it back.
• I experimented with adding a kinda inverted eggholder (actually just a bunch of inverted frustra) to the underside of the bottom chamber, in the hope of eliminating warping. It didn't really pan out (a good number detached from the build plate), and you need defeat warping for the top chamber anyway, so I removed them.
• I'm experimenting with a hypoid gear to replace the less efficient worm drive, and also hoping to reduce the choppiness of the platter rotation.
  • Can I not just mount the motor rotor-up, and use regular gears, or even a belt?