Replacing LM336Z-5.0 with TL431A

[fjpompeo]

How do I replace LM336Z5.0 5V voltage reference in this circuit?

Regards,

Fernando

 

[nguyennam]

It is 5.0V voltage reference, you can use TL431 only when you do not need a high precision voltage there.

To replace LM336, you need TL431 with 2 resistors which are calculated according to TL431 datasheet, beside, you need to reduce R1 value to ensure the current through TL431 within range of 1 to 100 mA.

nguyennam

 

[fjpompeo]

Hi nguyennam,

Very thank you for your prompt reply. I have used three 4k7 resistors one between anode and adjust, one between kathode and adjust and one replacing 10k. It simply does't work was thinking abaout the resistance between gnd and kathode is 4k7 and other LM741 input is close to 10K. Should I use another 4k7 in series to balance both input?

Regards,

Fernando

 

[nguyennam]

Oh yea,
This circuit doesn't work, or exactly, the opamp output (pin 6) always ties to +Vcc. It is not because the resistor as you said.

1. You use the opamp in comparator mode, due to no feedback resistor.
2. You use the split power supply (of course, for 741), and it should not be exceed +/-12V, I guess you use any within +/-9V to +/-12V.

But regardless the supply voltage, you can easily realize, the voltage at (-) input (pin 2) is always less than the the voltage at (+) input (pin 3).

For example, suppose you use +/-12V,
so (-) input voltage is (12V-5V=+7V),
(+) input voltage: minimum = (12V/(6.8+1+3.3))*6.8 = 7.35V
maximum = (12V/(6,8+3.3))*6.8 = 8.08V

and the opamp will not change output status when you adjust the potentiometer.

To use the circuit in this comparator mode, you should re-calculate the resistor divider network so that the (-) input voltage is within the (+) input voltage range (minimum to maximum).

nguyennam