0-10V to 4-20mA

[jebediah]

Hi, i know this question has been asked many times. I want to achieve the 4-20mA without chips(XTR115/116 etc). I know someone said that you can do that using op-amps but nothing specific was said. Could someone help by giving me specific details. Thank you.

 

[dhwilliams]

Here's a circuit I made years ago loosely based on the AD694, see

http://www.analog.com/UploadedFiles/Data_Sheets/390142776AD694_b.pdf

I used an 8-bit port on a PIC to generate 0-255 which I converted to 0-10V that fed the V to I circuit. The output was 0 to 25.5mA.
Hope this helps.

(I did notice that if no load was connected the op-amp got hot! Never did find out why.)

Dave

 

[jebediah]

I have no problem getting 0-20mA. but it seems pretty hard to get an offset of 4mA, using like a 741 op-amp. I tried inputting a 0-16V and adding a 4V into a summing amplifier. to get 4-20V and running it thru 1K to get 4-20mA, but my summing amplifier doesnt work. I dont know why, its such a simple circuit but I cant sum any inputs.( i switched 741 chips and used multiple configurations)

 

[dhwilliams]

I don't design circuits very often so I may be wrong with the following. I usually mess around with the components until I get something working but it's too late to start now (11:50pm UK time), I might have a go tomorrow! Anyway, my starting point would be the circuit I posted before with the total emitter resistance for the BC184 about 2Kohm, that should give you 0 to 16mA output. Then I would add another BC184 with its collector connected to the collector of the first BC184, a 1Kohm resistor from emitter to ground and the base would be held at a constant voltage of about 1.9V. This would give a constant current of about 1.2mA through the transistor which should add about 4mA to the output. Use a combination of fixed and variable resistors for the BC184 emitters and adjust the second to get 4mA output with an input of zero Volts then adjust the first to get 20mA output with an input of 10V.

If it works, you should then have a poor version of the AD694!

 

[xnuke]

Here's a link to a transconductance amplifier, but you'll need to convert your 0-10 V input to 1-5 V for this to work:

http://www.allaboutcircuits.com/vol_3/chpt_8/7.html

xnuke

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[lcsjk]

The op-amp gets hot with no load because its output is having to sink the total current that would normally go to the load.

A higher gain transistor, such as a darlington, and a resistor from op amp to transistor base will solve the problem.