Catalin Serafimescu
Features
- Fan off below a start temperature set with a trimmer (about 15–52 °C), full speed about 18 °C above it.
- Fan voltage never exceeds 5 V, whatever the input voltage or a fault.
- Runs from the same DC input as the ZK-4KX: 9 V / 2 A adapter tested, up to about 28 V supported.
- Single-sided PCB, 90 × 66 mm, no vias, no jumpers — made for toner-transfer etching.
- Through-hole parts only: LM35, LM358, LM317, 78L05 and one trimmer.
- Printed case, 98 × 128 × 68 mm, magnetic lid, prints without supports.
How it works
An LM35 on the ZK-4KX heatsink measures temperature. One half of an LM358 buffers the start-temperature voltage from the trimmer; the other half amplifies the difference: V_ADJ = 11 × V_LM35 − 10 × V_REF. That drives the ADJ pin of an LM317, so the fan gets V_ADJ + 1.25 V: 1.25 V (stopped) up to 4.75 V (full speed). A 78L05 powers the op-amp and the sensor, which also caps the fan voltage.
Fail-safes: if the trimmer wiper opens, the fan starts at about 17 °C; if the LM35 is disconnected, the fan runs at full speed.
Design and checks
- Designed in KiCad 10; ERC and DRC clean.
- Simulated in both ngspice and LTspice: nominal, trimmer at both ends, input sweep, power-up hot and cold, open wiper, open sensor, op-amp offset worst cases.
- Enclosure and solder-mask jig modelled in FreeCAD and fit-checked against models of the real parts.
- Measured on the built unit: the fan starts at about 2.8 V.
Files in the project
- Enclosure base and cover, ready to print (in print orientation):
mechanical/enclosure/Enclosure_base.3mf,Enclosure_cover.3mf - Bambu Studio project for the Bambu Lab A1 (0.20 mm, PLA Basic, two plates):
mechanical/enclosure/Enclosure_A1_project.3mf - Enclosure CAD for remixing:
mechanical/enclosure/Enclosure_base.step,Enclosure_cover.step, and the FreeCAD scriptgen_enclosure.py - Solder-mask jig (optional):
mechanical/mask_jig/Mask_jig.stl/.step - PCB print for toner transfer and the solder-mask film, A4, print at 100 %:
KiCAD/toner_B.Cu_1to1.pdf - Gerbers and drill files for ordering the PCB:
KiCAD/gerber/SursaTensiune_gerbers.zip - Schematic:
KiCAD/SursaTensiune_sch.pdf - KiCad 10 project:
KiCAD/ - Bill of materials:
bom.md - Full build guide:
instruction.md - SPICE simulations (ngspice and LTspice):
simulation/ - Build photos:
photos/