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TL431A-Q1: Shunt Resistor Value recommend

Nicky Lim
Genius 3195 points

Part Number: TL431A-Q1

Hello,

Could you recommend shunt resistor value of reference circuit with below 2 conditions?

1. 12V(9~16V @ Max 50mA), output V(5V)

2, 24V(18~32V @ Max 25mA) output V(5V)  

Vo is used for NXP S9KEAZN8AMTG, Zenttler AZ8A series.

 

Is there any resistor value which can meet above 2 conditions ?

or resistor values will be different for each conditions?

And also let me know the reason why you recommend resistor value....

Regards,

Nicky

  • 0
    TI__Mastermind 19580 points
    Hi Nicky,

    The resistors R1 and R2 control the Vout. Since Vout = 5V for both conditions, the same R1 and R2 will work for both.

    Using a simplified equation of
    Vka = VRef * (1+R1/R2)
    We can work it down to
    R1 = R2.
    So if R1 = R2 then the output will be ~5V. R1 = R2 = 20K Ohm resistors will work.

    -MZ

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


    If you feel this is resolved, please selected Answered.
  • 0 in reply to Marcoo Z
    Genius 3195 points

    Hi MZ,

    Could you recommend below Resistor's value which can meet above 2 conditions? Not R1 & R2 value.

    Regards,

    Nicky

  • 0 in reply to Nicky Lim
    TI__Mastermind 19580 points
    Hi Nicky,

    I see, lets call that resistor Rin. To calculate this, you need to calculate the voltage drop across Rin at the highest current you want to let through.

    1. 12V(9~16V @ Max 50mA), output V(5V)
    Rin = (12-5)/0.05A = 140 Ohms

    2, 24V(18~32V @ Max 25mA) output V(5V)
    Rin = (24-5)/.025A = 760 Ohms

    Out of these I would pick the value of 760 Ohms as it will meet the conditions of the 2nd section. But this value will limit the current in condition 1 to 9.2mA instead of 50mA. It's not possible to get 50mA at 12V and 25mA at 24V with the same resistor.

    -MZ