19V+5V to 12V PWM Fan Adapter

This is an adapter board for using a standard 4-pin 12V PWM fan instead of a 5V one, when there is a nominal 19V (15V - 25V) power available. It must share the ground with the host fan connector ground, or Bad Things Will Happen.

When R1 (11.3 kOhm, marked as 11k3* on the board) is not used, the host PWM is pulled to 3.3V. When it is used, the host PWM is pulled to 5V.

To omit host PWM pull-up altogether, do not use any of the three resistors (3k83, 10k, and 11k3*) closest to the host 4-pin connector.

Board is 38.1mm × 25.4mm (1.5"×1") with all components on topside. There are four 2.5mm (0.098") mounting holes with a 5.5mm (0.216") diameter strengthening ring, their centers 3.048mm from the nearest edges. The hole centers form a 32.0mm×19.3mm or 1.26"×0.76" rectangle.

The components are sourced from Mouser, and Mouser part numbers are included. While the resistors in the BOM are all 0603 packages, you can instead use 0805 resistors: the pads are suitable for both package sizes.  Similarly, the inductor footprint suits both 5×5mm (VLS5045EX-101M) and 6×6mm (VLS6045EX-101M) inductors.

Here are the top and bottom sides of the board:

And here are a couple of 3D renders of the board: