[SOLVED] need current to voltage converter ckt which give linar output

[ud23]

Hi all i need to convert current source of 4ma to 20ma in to 0v to 5v using opamp lm324 but now i am not getting linear output of circuit attached with this post please tell any correction required in circuit.any other option available for linear out put of 0vto5v.

The bmp image is 5.5MB and is very inconvenient to download and see, replaced with gif image [alexan_e]

 

[khetanikrunal]

Just clarify,

Current is AC or DC?

Anyway you just need to give give i/p with negative feedback with one resistor and again give o/p to another op amp to invert.

Finally u get positive o/p.

Op-amp positive pin connect to GND.

May this help.

 

[FvM]

I guess, there have been 25 or more working 4 to 20 mA I/V converters posted at edaboard during the last years. Why did you design a new non working?

Problems involved with your design:
- it has to connect the negative current source output to the amplifier and the positive to ground, opposite to the industry standard. If any standard equipment has to be connected, you would want a positive current input.
- the circuit doesn't implement the necessary offset to get 0V at 4 mA.

I suggest a 50 or 100 ohm shunt with a suceeding differential amplifier, it can provide the factor 5 or 10 gain along with the offset. A stable reference voltage will be needed for this purpose. If you think your VCC supply is stable enough for this purpose, you have to tell the voltage, a least.

P.S.: It would be a good idea to post images in a compressed (gif or jpg format) to save server space and download time. Even the trivial Windows Paint tool can convert it.

 

[JayantD]

For your information, RCV420 is a single chip solution.
https://focus.ti.com/lit/ds/symlink/rcv420.pdf

 

[ud23]

@kunal:ya i need to convert pressure transducer output 4to20ma Dc current in to 0to5v dc voltage.
@Fvm : can you give me link of working I/v circuit posted in this fourm. I try to find it but not find any working solutions.

 

[khetanikrunal]

Find the Attachment

for this you need to connect Zener (5v) at o/p for Protection.

 

[JayantD]

@khetanikrunal:
The requirement is to output 0 to 5V for an input of 4 to 20mA, as against 1 to 5V suggested by you.

 

[khetanikrunal]

Yes you are right Mr. JayantD

I am sorry for that...

 

[ud23]

hi i am attaching jpg file as alexan_e suggested
@khetanikrunal : As given in your circuit you giving +10 v at pin number 11 which is GNd pin according datasheet. and pin 4 is VCC so how your circuit works I tried in multisim its not working.

 

[khetanikrunal]

i just gave basic diagram and it is not with pin diagram

and it is the common thing that VCC is +10 V

Image circuit is tested with circuit maker.

4 mA @1V
20mA@5V

 

[FvM]

can you give me link of working I/v circuit posted in this fourm. I try to find it but not find any working solutions.

I see, that there's a problem that edaboard doesn't allow an effective search of respective threads. I just noticed them, as they pass by. Here's a principle circuit, there are much more.
https://www.edaboard.com/threads/188930/#post789845
As I said, it can be implemented with a single OP as well. You just have to play around with the resistor dimensioning.

By extracting the internal circuit of the suggested RCV420 (only the +In variant), you'll also get a single OP converter.

 

[JayantD]

You need to tackle one more issue. With a single supply, you can never get 0 volt output. There will always be an offset at the output. Use dual supply.

 

[FvM]

With a single supply, you can never get 0 volt output.

Basically correct. You get down to a few mV with LM324, if the output don't need to sink current, or better, using a recent rail-to-rail OP.

With LM324, a pulldown resistor can reduce the output saturation voltage (it's not exactly an offset). If you need exactly zero, use dual supply.

 

[ud23]

how to generate 4to20ma constant current source to test I/V circuit in lab?

 

[FvM]

You can of course use a true current source or a 4-20 mA calibrator, but a fixed voltage with a variable resistor or a variable voltage with a fixed resistor should work perfectly in most cases.

 

[JayantD]

@ud23:

XTR115 / XTR116.
Use a potentiometer between pins 8 and 3.
Please work out the values by yourself. See https://focus.ti.com/lit/ds/sbos124a/sbos124a.pdf for details.

Regards,
JayantD

 

[ud23]

thnx all for your suggestions.
@Fvm : I tried with fixed voltage variable current source and get 4to20ma current source(passive).and its helpful to test I/V circuit.