LM317 Adjustable 1.25–25 V Bench Supply
Two resistors set any output voltage you want from a single regulator IC — the most-built linear power supply in electronics history.
Difficulty: Through-hole. Estimated build time: about 45 minutes. Estimated parts cost: about US$4.80. 7-line bill of materials. Compare supplier offers when available. A linear adjustable bench supply built around the LM317 — a 3-pin regulator that holds 1.25 V across its output-to-adjust pins regardless of what is happening at the input.
Identify the LM317 pinout
TO-220 package, looking at the printed face: pin 1 = ADJ (left), pin 2 = OUT (middle, also tied to the metal tab), pin 3 = IN (right). The middle pin and the tab are connected — clamp it to a heatsink (electrically isolated from anything you don't want shorted to OUT).
Wire the input filter
Connect your unregulated supply (a 12–24 V DC wall wart works fine for first builds) to pin 3 (IN). Drop a 100 µF electrolytic and a 100 nF ceramic in parallel from IN to GND, right at the chip. The big cap eats line-frequency ripple; the ceramic kills high-frequency noise the big cap misses.
Build the feedback divider
From OUT (pin 2) to ADJ (pin 1), place R1 = 240 Ω (the LM317's reference resistor — the one TI specifies). From ADJ to GND, place a 5 kΩ trimmer (R2). The output voltage is Vout = 1.25 × (1 + R2/240). With the trimmer at zero, Vout = 1.25 V. Crank it to 5 kΩ and Vout climbs to 27 V.
Add the output cap and a protection diode
From OUT to GND, add a 10 µF cap for transient response. From OUT to IN (cathode at IN), place a 1N4007 diode — this protects the LM317 if your output is connected to a charged cap and the input drops faster than the output. It's optional below 25 V but free insurance.
Calibrate against a multimeter
Power the input. Probe the output with a DMM. Turn the trimmer slowly — output voltage tracks it smoothly from 1.25 V upward. Mark a few common values (5 V, 9 V, 12 V) on the trimmer face. The supply is now a real bench tool. Loaded with a 100 Ω resistor at 5 V (50 mA), the LM317's tab gets warm but not hot; loaded at 1 A you absolutely need a heatsink — the tab dissipates roughly (Vin − Vout) × Iout.