query regarding the TL431 and opamp

[piyushpandey]

Hello guys

I have two questions one is regarding the TL431 and the other is regarding the OPAMP .

My first question is that I want to get the refrence voltage of 2.5 volts from the TL431 but in the datasheet it is given that it can be regulated to any value from 2.6 to 36 V so can't I get the 2.5 V and the minimum regulated voltage which I can have is of 2.6 V


I am using these two schematics so please tell me that which one will provide the 2.5 or 2.6 refrence voltage.

the pics are here:






First one which I think will not serve the purpose as it is ground. and second one will provide the refrence voltage.

I just want to be sure that what I am thinking is correct by asking you expert people.



Now second question is that I want to measure the sinusoidal current coming to my home through an ammeter I am designing and for that I am using the cheap shunt , I know that my purpose can be solved very easily by the use of the current transformer but I am opting for the shunt because of two reasons first it is cheap and second I want to learn how shunt works.

So since the resistance of the shunt is very less approximately .01 ohm so the voltage drop for a 5 A of sinusoidal current would be very low in the miliamp range and therefore I am using the OPAMP for it .


But what I realised that the opamp will no be able to read the negative value of the input voltage signal as the voltage range to the OPAMP is from V+ = 5v to V- = 0V so it will measure the positive value but not the negative value , so what I have to do is that I have to give it a bias and therefore I am providing it the bias of above mentioned 2.5 V at the input signal .

So am I doing right or something elese has to be done or the input signal should be given without any bias to the OPAMP.

the schematic is as follows:






So I want to know that my circuit is right now the first one and in the second one I have put the bias at the signal input end of the opamp i.e. at the CT_o/p end of the circuit.


So please tell me if I have added the bias correctly and also I must tell there is no other power supply and the opamp has only the 0 to 5 V of the power supply and also the opamp is operating in the non-inverting mode.




Thanks

Piyush Pandey

 

[FvM]

Minimum regulated output voltage of 2.6V is just another word for "connecting cathode and reference input doesn't work". You need to refer to a different shunt regulator device.

Your OP circuit is missing a DC bias for the N.I. input and doesn't work. Simply connect a high impedance voltage divider to Vcc/2. R2 can be omitted because it has no function at all.

 

[alexan_e]

Minimum regulated output voltage of 2.6V is just another word for "connecting cathode and reference input doesn't work".

I don't understand , why can't the device be used by connecting cathode and reference pins to give 2.5v?
I have checked several TL431 datasheets and they all contain a test circuit connected this way to give 2.5v and there is no warning that this shouldn't be used or that it doesn't work.

For example www.ti.com/lit/ds/symlink/tl431.pdf

 

[FvM]

You are right. Unfortunately I believed the statement in the original post without checking the datasheet. It's not true for original TL431, may be for some other regulators. Thanks for the correction.

 

[piyushpandey]

Minimum regulated output voltage of 2.6V is just another word for "connecting cathode and reference input doesn't work". You need to refer to a different shunt regulator device.

Your OP circuit is missing a DC bias for the N.I. input and doesn't work. Simply connect a high impedance voltage divider to Vcc/2. R2 can be omitted because it has no function at all.

Hello FVM


you are right and that whats I am doing as I said that the second circuit regarding the tl431 regulator in which the V25REF value has been taken can be used as the refrence purpose as it would be stable one.

and in the first since it is grounded to the analog ground therefore it is going to be a zero voltage across the Vref and the ground right.


Also as you told that get the DC bias for the opamp from the high impedance potetial divider , what I suggested that I am taking the V25REF from the tl431 circuit as the DC bias for the opamp circuit.


Thanks

- - - Updated - - -

Also I want to know that if rather than providing the 2.5V refrence to the non-inverting end of the opamp I provide it at the Vref pin of the pic16f688 micrcontroller than would I be able to get the negative value reading of the analog voltage.


The range is same from 0 to 5 V.



Thanks

 

[goldsmith]

Dear Piyush
Hi
Regarding your first question : Did you reed it's datasheet carefully ? i have used this IC for 2.5 volts , many times . why you have connected it's cathode to the ground ??

But what I realised that the opamp will no be able to read the negative value of the input voltage signal

Yes it is able ! you should use -vcc then your op amp will be able .
How ever if you don't want use minus supply there are several ways ! but not that circuit !
Let me try to help you in better way . why do you need to measure both cycles ? i think your input is sinusoidal . hence both cycles are the same together . (exactly the same ) . so one of the cycles is enough . and another thing ! you can use a differential amplifier with op amp ( subtract ) and then a precise rectifier ( ideal diode w) with op amp and then an RC network . it is pretty simple !
Best Wishes and Good luck
Goldsmith

 

[D.A.(Tony)Stewart]

Specs from TI support your requirements of 2.50V

Use their recommended design for higher voltage or current starting p28 of spec

PARAMETER TEST CIRCUIT TEST CONDITIONS UNIT Vref, IKA = 10 mA
MIN TYP MAX Vref
2.440 2.495 2.550 V