TL 431 Shunt Regulator. Design Note 11

TL 431 is an Adjustable Precision Zener Shunt regulator that can be used to get a desired output voltage from a power supply. It’s output voltage can be set to any value between 2.5 volts and 36 volts using a voltage divider comprising two resistors. It can be used to design, Battery Monitors, Battery chargers etc. Along with two resistors, TL431 give a regulated precise output voltage. Here we discuss how TL431 is used to make a Battery monitor for Solar powered Inverter and a 5 volt power supply.


TL431 shunt regulator is a tiny IC similar to transistor BC547 with three pins. Pin 1 is Reference pin, Pin 2 Anode and Pin 3 Cathode. It is internally referenced at 2.5 volts, so the selection of the resistors for the voltage divider is important. The Resistors in the voltage divider can be named as R1 and R2. R1. Let us see the method of selection.


The output voltage can be called as High Limit. The High limit means the maximum voltage at which Zener conducts and reducing the voltage to Zero. So this High Limit can be calculated using the Formula
High Limit = Vref (1 + R1 / R2). Vref is fixed as 2.5 volts.
For example, fix R1 as 1.8 K. That is 1800 Ohms. Consider R2 as 1 K (1000 Ohms)
So the High limit will be

2.5 (1+ 1800 / 1000) = 7 V

Let us see another example

R1is selected as 10 K and R2 as 1K. Then

2.5 (1+10,000 / 1000) = 27.5 Volts

This formula gives the output voltage when we select R1 and R1 randomly. Suppose we need a specific output voltage. For this another formula is there which can be used to select R2.

R2 = R1 / ((High Limit Voltage / Vref) – 1)

Suppose we need 7 volt as High limit to monitor a 6 Volt battery. Now select R1 and R2 for 7 volt High limit. Let us fix R1 as 1.8 K and using the formula

R2 = 1800 / ((7 / 2.5) – 1) = 1800 / 1.8 = 1800 / 1800 = 1K



Fig. 1 shows a 12 volt battery monitor circuit using TL431 Shunt regulator. It lights a Green LED when the voltage of the battery rises above 12 volts. As per the above equations, let us select the High limit as 13.8 volts which the voltage of a 12 volt Lead Acid battery after fully charged. So let us first fix R1 as 10K. Now select R2 for High limit 13.8 V

R2 = 10K / ((13.8 / 2.5)-1) = 10,000 (13.8 / 2.5)-1) .

That is ( 13.8 / 2.5 )-1) = 4.52 = 10K / 4.52 = 10,000 / 4520 = 2200 Ohms or 2.2K.

Resistor R3 limits LED current to 12 mA. For high brightness use 470 Ohms as R3. Resistor R4 in parallel with the LED prevents the LED from glowing softly when the input voltage is still below the switch on voltage.

But if you use a 4.7 K preset connected in series with R2, then the High limit voltage at which LED lights can be adjusted from 6.1 V to 13.8V.


Fig. 2 shows an efficient 5 volt power supply using Shunt regulator. Resistor R1 and R2 acts as a potential divider to set the output voltage as 5 volts. Resistor R3 limits the current to IC1. Output voltage can be calculated using the formula
Vout = ( 1 + R1/R2) Vref
Here the output voltage Vout = ( 1+ 10K / 10K)x 2.5 = (1+10000 Ohms /10000Ohms ) x 2.5 = 5V



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