TL431A pin out block diagram.
Fig. 1 TL431A pin out block diagram.

TL431 Precision Current Regulator Circuits

by Lewis Loflin

Here we will explore the TL431 with 2 NPN transistors forming a precision current regulator. This is a tested variation taken from the TL431 spec sheet. This is known as a series-pass regulator.

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

TL431 precision current regulator circuit.
Fig. 2 TL431 precision current regulator circuit.

Fig. 2 illustrates the working circuit as seen in the video. The TL431 has an advantage over a plain Zener diode is feedback through the Ref pin. The 100K potentiometer (R1) and R2 create a small current to Ref that controls Ik cathode current. Ik determined the voltage drop across a 680 resistor that through Q1 and Q2 determines the output current.

The Darlington configuration of Q1 and Q2 produces high current gain for Vo. When operating with a 10 ohm resistor from Vo to ground a continuous 0.5 amp output produces a 5-volt output. Any variation in input voltage (16V) or load current the feedback to Ref will change Ik to compensate.

Transistor Q2 will get hot and must be heat-sinked. Both in the test circuit in the video are TIP41s.

This produced a stable output voltage-current due to the low temperature drift of the TL431.