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.

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