TL431 Precision Voltage Reference
A two-resistor adjustable shunt regulator — 1% accuracy from a part that costs a few cents.
Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$1.50. 6-line bill of materials. Compare supplier offers when available. The TL431 is one of those three-terminal parts that keeps showing up in real designs.
Place the TL431 and series resistor
The TL431 in TO-92 has three pins. With the flat side facing you, the left pin is REF, the middle pin is ANODE (ground), and the right pin is CATHODE (the output). Wire a 1 kΩ series resistor from the input rail (try 9 V) to the CATHODE pin. This resistor sets the shunt current — Ohm's law on (Vin − Vout)/1 kΩ tells you the available current.
Build the feedback divider
Connect a 10 kΩ resistor from the CATHODE pin (output node) to the REF pin, and a 10 kΩ resistor (or a 10 kΩ multi-turn pot wired as a rheostat) from REF to ground. With both 10 kΩ the TL431 will hold the cathode at 2 × 2.495 V = 4.99 V (within 1 %).
Power up and verify
Apply 9 V to the input through your 1 kΩ series resistor and probe the cathode with a multimeter. You should read 4.99 V ± 50 mV. Now sweep the input from 7 V to 12 V — the output should stay locked at 4.99 V the entire time. That is line regulation: the chip is sinking whatever current is needed to maintain the reference.
Replace the bottom divider resistor with a pot
Swap the 10 kΩ resistor between REF and ground for a 10 kΩ multi-turn potentiometer wired as a variable resistor (use the wiper plus one end pin). Now turning the pot continuously varies the divider ratio. The output equation becomes Vout = 2.495 V × (1 + R_top / R_bot). Watch on the multimeter as you tune — turning down R_bot raises the output.
Push the limits
The TL431 can sink up to 100 mA, so it can hold a load too. Wire an LED + 220 Ω from cathode to ground as a load and verify regulation does not budge. Also note the limits: minimum cathode current is 1 mA (drop the series resistor below 1 kΩ for low-V operation), and the chip will not regulate below 2.495 V no matter what your divider does — that is the floor.