I wrote a about a simple shunt voltage regulator design with TL431 but circuit is usually not recommended for moderate to high current application or battery powered circuits. In the shunt mode, the TL431 will draw current from the source even if the load is not connected or at light load. And there are other issues like power dissipation, limited output current. A better alternative is to use a series transistor to build TL431 Series Pass Voltage Regulator.
About TL431
The TL431 is a versatile programmable shunt voltage reference widely used in electronics for precision voltage regulation and feedback control. Common applications include acting as an adjustable Zener diode for generating stable reference voltages, serving in SMPS (switch-mode power supply) feedback loops—often paired with an optocoupler for isolated voltage regulation—driving pass transistors in adjustable linear regulators, providing accurate overvoltage or undervoltage protection circuits, and functioning in battery chargers for precise cutoff control. It’s also used in precision current sources, DC-DC converters, and instrumentation where a stable and adjustable voltage reference is essential. Here I wanted to write note on the working, circuit and application of series voltage regulator design with TL431.
Circuit Schematic
Below is TL431 Series Pass Voltage Regulator circuit diagram:
Circuit Operation
In the TL431 + TIP31 series pass voltage regulator, the TL431 compares the output voltage—sampled through the R1–R2 divider—to its 2.495 V internal reference. When the divided voltage exceeds 2.495 V, the TL431 sinks more current from the TIP31 base, reducing conduction and lowering the output voltage; when the divided voltage is below 2.495 V, it sinks less, allowing the TIP31 to pass more current from the input to the output. In this series pass voltage regulator design, R1 is a 10 kΩ potentiometer and R2 is 10 kΩ fixed, which sets the adjustable range according to \(V_{out} = 2.495 \times (1 + \frac{R1}{R2})\) (about 2.5 V to 5 V for your values). R3 (220 Ω) is the base current-limiting resistor that prevents excessive current into the TIP31’s base; it is chosen so that at maximum load, \(I_B \approx I_{load} / h_{FE}\) is supplied without saturating the transistor, using \(R3 \approx \frac{V_{in} - V_{be} - V_{TL431}}{I_B}\). C1 (4.7 nF) is placed between the TL431 REF and cathode pins for loop compensation—its value is chosen from \(f_p \approx \frac{1}{2 \pi R_{th} C1}\), where \(R_{th}\) is the Thevenin resistance of the divider (for 10 kΩ and 10 kΩ, \(R_{th} = 5\) kΩ), giving a pole around 6–7 kHz for stability. This arrangement allows smooth voltage adjustment and stable operation with reduced oscillation risk.
