4-20 mAmp circuit problem

[KiranVSutar]

calculating 4-20 milliamp circuit

Hi All,
I had downloaded this circuit somewhere from this site.

Last 4 days I am struggling to get this 4-20 mAmp circuit to work.

I modified the input stage of first opamp. In the original circuit the resistors R3, R4 were absent. Hence the first opamp was working like a comparator.
After putting R3,R4, this first stage is working OK.


I am getting a linear voltage 0~10 vdc at pin 1 of First Opamp.

I have blown 2 transistors but this circuit does not work.



If somebody has a different circuit, then please inform me about it, I need to get to the solution fast.
The requirements are: +12V Supply(single polarity). 0~10Vdc analog input, 4-20 mAmp or 0~20 mAmp output. Opamp to be used: LM324 or LM358.

Thanks from,
Kiran V Sutar.

 

[FvM]

How did you calculate the resistors? The circuit can basically work, but the dimensioning is incorrect. If the R8 value shall be kept (it seems suitable), then R2 must be much lower, eg. 500..800 ohm. Also the R4/R3 ratio must be changed accordingly, to allow an output current of 0 or 4 mA. If you set the range to 4..20 mA. then the supply voltage is acting as a reference and must have sufficient stability.

 

[KiranVSutar]

How did you calculate the resistors? The circuit can basically work, but the dimensioning is incorrect. If the R8 value shall be kept (it seems suitable), then R2 must be much lower, eg. 500..800 ohm. Also the R4/R3 ratio must be changed accordingly, to allow an output current of 0 or 4 mA. If you set the range to 4..20 mA. then the supply voltage is acting as a reference and must have sufficient stability.

So what should be the values of resistors ? I cannot calculate them. Can you please inform me about it?

After posting this problem, I referred to this link:
#1160321

I just changed the Output transistor from TIP127(PNP) to BC547(NPN) with the necessary changes as in the above link....And the circuit has started working...and with acceptable linearity.


So thank you FvM, for helping me.
I will post the final working circuit with the values of each resistor when I finalyze it tommorow. Its 11:00pm now, family is waiting for me @home.

 

[geex]

For more insights, I found a similar problem on this.

https://forum.allaboutcircuits.com/showthread.php?t=2972

 

[FvM]

If you don't require a ground referenced load, a npn respectively NMOS current source is preferable for it's simplicity.
Also a ground referenced sense resistor promises higher accuray.