Fig. 1

# TL431 Based Current Limiter and Constant Current Source Circuits

by Lewis Loflin

In the past this website has explored constant current circuits using the LM317 adjustable regulator. These circuits used a PNP power transistor such as the MJ2955. A far more simple solution is the TL431 used with a NPN transistor such as a TIP41 or 2N3055. The circuit was built, tested, and yielded good results.

In simple terms the TL431A acts a temperature compensated variable/adjustable Zener diode. It can also act as a voltage reference. In addition to good temperature compensation and stability, the TL431A has a voltage range of 2.5 volts to 36 volts and a current range from 1mA to 100mA.

For basic information on the TL431 see Experiments with TL431A Shunt Regulator

Fig. 2 Basic TL431 current limiter circuit.

Fig. 2 illustrates a pass-series constant current regulator using a TL431 shunt regulator. While there is a formula for R1 it doesn't seem very critical. RCL is the big factor in output current with Vref / RCL. Vref = 2.5 volts.

Fig. 3 Basic TL431 current limiter circuit operation.

Fig 3 is the live operating circuit in the video. With RCL = 10 ohms the output =~ 250mA. There is some small additional current Ik and Ib. The voltage across RCL (Ref & Anode) determined the voltage drop across R1 thus the voltage-current though the base-emitter junction of Q1 a TIP41 NPN bipolar transistor.

The voltage out is dependant on load resistance based on a fixed current from Q1. Ik and Ref change with variation in input voltage or output load.

This known as a series pass circuit.

- Experiments with TL431 Shunt Regulator
- TL431A Precision Current Regulator Circuits
- TL431A Based Current Limiter Constant Current Source Circuits
- TL431A Shunt Regulator Circuits

- Arduino Measures Current from Constant Current Source
- Constant Current Source Theory Testing
- Arduino Controlled Power Constant Current Source

- LM317 Adjustable Current Boost Power Supply
- Constant Current Circuits LM334, LM317
- Build LM317 0-34 Volt Power Supply
- LM334 Constant Current Source with Resistive Sensors
- LM317 High Power Constant Current Source Circuit
- LM317 Constant Current Source Circuits
- Test SCRs and Triacs
- Basic MOSFET Transistor Test Circuits
- High Voltage MOSFET Switching Circuits
- 3 Amp LM741 Op-Amp Constant Current Source
- Current Limiter Testing of Zener Diodes
- Current Limiter for Opto-Coupler Inputs
- LM317 CCS for Light Emitting Diodes

## Other Circuits

- Hall Effect Magnetic Switches and Sensors
- Comparator Theory Circuits Tutorial
- ULN2003A Darlington Transistor Array with Circuit Examples
- Transistor-Zener Diode Regulator Circuits
- AC Power Supply Rectification
- Coils for Highly Selective Crystal Radio
- Neon (NE-2) Circuits You Can Build
- Photodiode Circuits Operation and Uses
- Photodiode Op-Amp Circuits Tutorial

**Web site Copyright Lewis Loflin, All rights reserved. If using this material on another site, please provide a link back to my site.**