Experiments with TL431A Shunt Regulator
by Lewis Loflin (lloflin2@gmail.com)
In simple terms, the TL431A acts as a temperature-compensated, programmable Zener diode. It is an essential component for any electronics hobbyist, serving as an adjustable voltage reference or the control element for a precision constant current source.
Device Overview
The TL431A is a three-terminal programmable shunt regulator. Unlike a standard Zener diode, which has a fixed breakdown voltage, the TL431A allows you to program the output voltage from 2.5V up to 36V using only two external resistors. Key features include:
- Stable Reference: 2.5V internal bandgap reference.
- Dynamic Impedance: Extremely low, typically 0.22 Ohms.
- Current Range: Operating currents from 1mA to 100mA.
- Accuracy: High precision, making it an excellent replacement for traditional Zener diodes in sensitive applications.
Circuit Configurations
As shown in Fig. 2, the basic configuration provides a stable 2.5V reference. By introducing a voltage divider (Fig. 3), you can program a specific reference voltage; setting this to 5V is as simple as using two 10K resistors. Replacing the fixed divider with a potentiometer (Fig. 4) creates a versatile, adjustable-voltage Zener replacement.
For applications requiring higher current, we can implement a series pass transistor (Fig. 5). When using this method, remember to factor in the 0.6V Base-Emitter (Vbe) drop. For even higher current demands, a Darlington pass transistor configuration (Fig. 6) is preferred, though this requires accounting for a total Vbe drop of approximately 1.2V.
Resources
- Zener Diode Regulator Circuits
- Tricks for LM78XX Regulators
- TL431A Based Current Limiter Constant Current Source Circuits
- TL431A Shunt Regulator Circuits
References
Fig. 1: TL431A Pinout
Fig. 2: Simple 2.5V Reference
Fig. 3: 5V Reference via Voltage Divider
Fig. 4: Variable Voltage Reference
Fig. 5: Current Boost with Series Pass Transistor
Fig. 6: Higher Current with Darlington Pair
Reference Library
For further study, I have compiled these core resources:
- LM317 Constant Current Source Circuits
- LM334 Constant Current Source with Resistive Sensors
- TL431A Precision Current Regulator Circuits
- 3 Amp LM741 Op-Amp Constant Current Source
- LM317 Adjustable Voltage, Current Boost Power Supply
- Current Limiter Testing of Zener Diodes
- TL431A Lithium-Ion Cell Charging Circuits
- Arduino Measures Current from Constant Current Source
- Arduino Controlled Power Constant Current Source
- Basic MOSFET Transistor Test Circuits
- Current Limiter for Opto-Coupler Inputs
- LM317 CCS for Light Emitting Diodes
- Test SCRs and Triacs
- High Voltage MOSFET Switching Circuits
- Overview: Constant Current Circuits