TL431 as Variable Zener Diode
Category: Linear Power Supplies
Designer: RANA MUHAMMAD ADNAN KHALIL
Notes
This circuit is designed and simulated in Proteus 7.10. In this circuit you will learn that how LM431 or TL431 can be used as variable zener diode. Before understanding this circuit, see "Constant Linear Voltage Regulator" in Linear Power Supplies category. This is a very simple circuit and working of this circuit is given below. 1. TL431 is a 3 terminal device. Pin 1 is Reference, pin 2 is anode and pin 3 is cathode. 2. The maximum sink capability of TL431 is 100mA. 3. In this circuit a TL431, a variable resistor of 10K, a series resistor of 100ohm and R2 10k ohm are used to make potential divider for reference pin. 4. The input of this circuit is constant which is 12Vdc and desired output is 2.75Vdc to 11.8Vdc (see animation) 5. The formula of calculating output voltage is Vo = (1+RV1/R2)*2.5V lets assume RV1 is 1k and after puting values of RV1 and R2 we get Vo = (1+0.1)*2.5V = 2.75V, which can be seen in animation 6. The current flowing through resistor R3 can be calculated as I = (Vin - Vo)/R For max. Vo = 12V I = (12 - 11.8)/100 = 2mA and for min. Vo = 3V I = (12 - 2.75)/100 = 92.5mA 7. You can also make fixed voltage zener diode, for this constant voltage application, see previous circuit. Applications: 1. This circuit can be used to understand the working of TL431. 2. This circuit can be used as low power variable zener diode. 3. This circuit can be used as low power variable linear voltage regulator. Note: 1. Come back daily to get new, amazing and exciting electronic circuits. 2. Also share this app to your friends.