LM2596 Adjustable Buck Supply
The blue-board buck regulator every desk has — 12 V down to anything from 1.5 V to 35 V at up to 3 A.
Difficulty: Through-hole. Estimated build time: about 45 minutes. Estimated parts cost: about US$6.00. 6-line bill of materials. Compare supplier offers when available. An adjustable step-down switching supply built around the LM2596S-ADJ, the same regulator used in the ubiquitous blue "buck converter" modules sold by every electronics vendor on the planet.
Lay out the TO-220 regulator
Place the LM2596S-ADJ (TO-220-5 package) in the centre of the breadboard or PCB. Note the pin order — pin 1 is VIN, pin 2 is OUTPUT (the SW node, which is noisy), pin 3 is GROUND, pin 4 is FEEDBACK, pin 5 is /ON-OFF. Bend the leads so pin 3 reaches a sturdy ground rail; the GND pin is also the heatsink tab.
Place the input bulk capacitor
Solder a 220 µF, 35 V or higher electrolytic capacitor between pin 1 (VIN) and pin 3 (GND), as close to the chip body as physically possible. This is the most important component for stability — long input traces or undersized input caps cause the SW node to ring and the regulator to oscillate.
Wire the inductor and catch diode
Connect a 33 µH, 3 A or higher power inductor between pin 2 (OUTPUT/SW) and the output rail. Then solder a 1N5822 Schottky diode from pin 2 to ground, with its cathode (banded end) at pin 2. The diode catches the inductor current when the internal switch turns off — without it the back-EMF would destroy the chip in microseconds.
Build the output filter and feedback divider
Solder a 220 µF, 35 V electrolytic between the output rail and ground for the output bulk capacitor. Set the output voltage with a divider from VOUT to feedback (pin 4) to ground. Use a 10 kΩ trim potentiometer in the lower leg with a fixed 1 kΩ on top: VOUT = 1.23 × (1 + R_top/R_bot). With the pot from 0 to 10 k, this gives ~1.23 V at one end and ~13.5 V at the other (assuming enough input headroom).
Power up and measure efficiency
Set the input to 12 V. Turn the pot to mid-range and verify VOUT changes smoothly between roughly 1.5 V and ~10 V. Probe pin 2 with a scope — you should see a clean 150 kHz square wave swinging from ground to roughly VIN. Now load the output with a 5 Ω power resistor and measure both input and output current at, say, 5 V output: efficiency should be 70–85% depending on load and output voltage. Tie pin 5 (/ON-OFF) to ground to enable the regulator; pull it high to shut it down.