Voltage to Current converter


Thread Starter

Dipu Damodaran

I am working on a project involving conversion of voltage output of sensors 4-20 mA. Can any body please suggest a cheap circuit, which can be easily designed.
You will need 2 individual opamps with offset adjustment, some precision resistors and a plus minus 15 v power supply.
Input will be a 150 ohms 0.1 % 1 watt resistor. This will convert the 4-20 ma to 600 to 3000 milli volts. Measure using the non inverting input of the first opamp with a series resistor of 100 k ohms and this does not affect the accuracy of the circuit .The inverting section is a gain circuit with bias adjustment. With no input and zero volt bias adust offset so that the output of the first opamp is 0.000 volts. With 4.000 ma input adjust the BIAS on the inverting input that the output is zero volts and with 20.000 ma adjust gain to 5000 milli volts.Repeat as necessary.You will need 10 turn pots for these. Now your output from the first opamp will be 0.000 to 5000 milli volts for a range of 4-20 ma. The second opamp is a voltage amp to amplify the first opamp output to the required output of say 0.000 to 10.000 volts. The non inverting input is connected to the output of the first opamp. with 4 ma input and AFTER first opamp adjusted to zero offset adjust the offset for second opamp to zero volts. With 4 ma and 20 ma adjust output to your required span. Use metal can type IC and metal film 1 watt resistors. Do NOT use dual opamps.All calibration adjustments are fine adjustment type.
Hope this helps .
Best regards.
I am not clear of ur application, can you detail?
I have certain delicate and reliable designs of current to voltage converters which i used in my optical power conversion projects.
i also need the details of ur sensor and its resposivity too for suggesting the design.
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the other "link" thread is excellent - should give a good starting point. I noticed most indicate to buy a major part then add on etc. Maybe you should look at a finished one - the circuit is tiny surface mount and I'm wondering if it is cheaper just to make one? I think if you want to have some fun and learn it would be better to build your own unit - if not look at:

you will see a hook up diagram as well that may give you some idea of how to build one.

Thanks Bob Hogg
you can use an ordinary OPamp to do this acocording to this matter that between to input
pin there is an virtual connection
you can find examples in electronics books such as OPAMPs COOKBOOK

This is actually a simple solution. Assuming your voltage signal is 0-10V, you will connect an external 500 Ohm resistor. Based on Ohm's law, a .2V signal = 4mA, 4.8V=10mA, and 9.5V =20MA.
If your voltage signal is 0-5V, you should use a 250 Ohm resistor. You can find prepackaged terminal block solutions at www.entrelec.com
Use the search tab and search by page number for page 819. It will connect you to .pdf data sheet.
Good Luck!!

Brandon Ellis

If I understand your question correctly, you have a sensors with a 4-20mA output and you wish to convert that to voltage. Just put a resistor across the outputs and there your go. Resistor size is based upon the voltage range you wish to read. Is that simple enough?

No way to design anything, simple or complicated, without knowledge of precision, stability vs. temperature and some other parameters. Do you
have these parameters somewhere defined directly or indirectly?

Mike Virgiliev. mailto:[email protected]