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Below is the circuit for constant voltage. The Value of R1 is 330 ohm and when Vin = 3.6V , the Vka = 1.5V And when Vin changes the value for Vka also changes. it should be stable

Why? The Vka should be equal to 2.5V

enter image description here

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    \$\begingroup\$ Show the complete circuit, and tell use the voltages you're using. \$\endgroup\$
    – CL.
    Mar 21, 2017 at 12:22
  • \$\begingroup\$ "Test Circuit 4 for VKA = VREF" and VREF is defined to around 2.5V so clearly you are not giving it enough headroom to regulate with your 3.6V \$\endgroup\$
    – PlasmaHH
    Mar 21, 2017 at 12:32
  • \$\begingroup\$ The configuration in your question forces the TL431 to act like a 2.5V zener diode. According to the datasheet, input voltage can be anything between 2.5V and 36V so there's no problem in your circuit. But your bias current is too high (around 3mA but it should be 1mA max), this shouldn't be the cause though. Maybe you can place a higher resistor like (3.6V-2.5V)/0.5mA = 2k. \$\endgroup\$ Mar 21, 2017 at 13:39
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    \$\begingroup\$ @RohatKılıç: The TL431 needs 1mA minimum, not 1mA max. It won't regulate properly below 1mA. Also, this is not a "zener diode," it's a shunt regulator with much better performance than a zener diode. The configuration as shown is correct, and I believe the values given are all correct, which means the OP is leaving out some important fact that explains the problem. \$\endgroup\$ Mar 21, 2017 at 13:45
  • \$\begingroup\$ Do you have a load on this circuit, or is it being tested unloaded? With a shunt regulator, the proper resistor value depends on the load. \$\endgroup\$ Mar 21, 2017 at 13:49

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