L7812 12 V Fan Supply
A classic three-terminal linear regulator turns a noisy 18 V wall-wart into a clean 12 V supply for cooling fans and small motors.
Difficulty: Breadboard. Estimated build time: about 25 minutes. Estimated parts cost: about US$2.90. 4-line bill of materials. Compare supplier offers when available. A clean 12 V power supply built around the L7812CV, the 12 V member of the legendary 78xx family of three-terminal linear regulators.
Identify the TO-220 pinout
Hold the L7812CV with the metal tab facing away from you and the printed face toward you. Left-to-right pins are: 1 = INPUT, 2 = GROUND (common to the heatsink tab), 3 = OUTPUT. The tab is electrically connected to GND, which is convenient if you bolt it to a grounded chassis but means you must isolate it on a non-grounded heatsink.
Add the input and output decoupling caps
From INPUT to GND, place a 0.33 µF film cap right at the chip — the datasheet calls this out specifically to prevent input oscillation when the power source is 'far' (more than a few inches of wire). From OUTPUT to GND, place a 0.1 µF ceramic for high-frequency stability and a 10 µF electrolytic for transient response. Without these caps the regulator can ring or even latch into a self-oscillation.
Wire a 12 V fan as the load
Connect the OUTPUT pin to the +12 V wire of a small 12 V brushless DC fan, the fan's GND wire to GND. Apply 18 V to INPUT (a benchtop supply or a 18 V wall-wart). The fan should spin and the L7812 will warm noticeably. Probe the OUTPUT to GND with a multimeter — you should see 11.95 to 12.05 V, regardless of whether the fan is loaded or you're varying the input from 14.5 V to 24 V.