The ZK-4KX is a cheap buck-boost panel module (5–30 V in, 0.5–30 V / 4 A out) that runs hot when you push it. This project turns it into a closed bench supply: a 3D-printed case, a DC jack input, a power switch, 4 mm banana outputs, and a small add-on board that drives a 30 mm fan. The fan stays off until the module's heatsink reaches a temperature you set with a trimmer, then speeds up as it gets hotter. The board is single-sided and through-hole, designed to be etched at home; Gerbers are included if you'd rather order it.

Details

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 script gen_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/