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TL431A-Q1: TL431A

Udayakumar Perumal
Prodigy 40 points

Part Number: TL431A-Q1

Hi ,

I am working on a Design using TL431 shunt regulator and optocoupler.If input is given 4.7 or less system should conduct and optocoupler collector and emitter should be short .If voltage is greater than 4.7V then system should not conduct.But in my case i have did exactly opposite can any one suggest me to get proper output .PFA of my circuit .

  • 0
    TI__Expert 3835 points

    Hi Udayakumar,

    Can you share the resistor values of this design? I want to calculate the voltage at the REF pin. If it is close to the bandgap reference (2.5V), the TL431 may start conducting.

    Thanks,

    Ethan

  • 0 in reply to Ethan T
    Prodigy 40 points

    Res 148 + R154 = 10k

    Res 158 + R159 = 11.21k

    R149 = 10k

    R145 = 120R

    ..

    can you suggest me whats issue in this circuit

  • 0 in reply to Udayakumar Perumal
    TI__Expert 3835 points

    Hi Udayakumar,

    The forward voltage of the optocoupler is 1.2 V typical which means that the parallel resistor R149 = 10K, is only conducting 0.120 mA. Try changing this resistor to 1K to make sure the TL431 is properly biased.

    I also wanted to clarify your question after noticing that the logic may be reverse. If your trip point is 4.7V, then any voltage higher will cause the TL431-Q1 to conduct (Vref > 2.5 V). Voltage less than the bandgap reference will cause the TL431 to be high impedance (Vref < 2.5 V). See application section 9.2.1 in the TL431A-Q1 datasheet.

    Hope this helps,

    Ethan

  • 0 in reply to Ethan T
    Prodigy 40 points

    Hi Ethan,

    Thanks for the reply.

    I solved this issue by adding 4.7k pull up at the collector side of optocoupler .

    and by removing R149 as you suggested.

    Ethan can you say why after adding pullup at the collector side this circuit is working.Reason .

    Thanks 

    Udayakumar P