Single Supply OPAMP instead of 741

[Roff]

Here is one way to do it.

 

[bnfdvn]

Thank you for your time and effort in coming up with this design. I have ordered the parts and will connect everything up. Thanks again! I really appreciate it!!

 

[ericgibbs]

Thank you for your time and effort in coming up with this design. I have ordered the parts and will connect everything up. Thanks again! I really appreciate it!!

hi,
I know that you have decided to a self build 4 to 20mA, but for reference only, the AD694 ic, it has a number options.

 

[Roff]

Thank you for your time and effort in coming up with this design. I have ordered the parts and will connect everything up. Thanks again! I really appreciate it!!

One caveat (a little late). The design worked in simulation. I did not test it in hardware.

 

[bnfdvn]

I have used the circuit presented by Roff and it works except for one little problem. When I adjust the pot to set the output to 4.00 mA, I sometimes am unable to set to to 4.00. On the board I am making now, it only goes to 4.02. Would it work to lower the two 10K resistors and use a bigger adjustment pot?? I looked at the AD694 and it looks like it would do the deed but I already have 15-20 boards in use and want to stay with the same design. Thanks!

 

[Roff]

I have used the circuit presented by Roff and it works except for one little problem. When I adjust the pot to set the output to 4.00 mA, I sometimes am unable to set to to 4.00. On the board I am making now, it only goes to 4.02. Would it work to lower the two 10K resistors and use a bigger adjustment pot?? I looked at the AD694 and it looks like it would do the deed but I already have 15-20 boards in use and want to stay with the same design. Thanks!

Sorry about that.
If you have good access to 1% resistors, change R4 to 9.53k, and change the pot to 1k. This should give you about ±200uA adjustment range at the low end.

EDIT: I forgot that the complete transfer function is
Iout=Vin*(R4+Rpot)/(R3*R1).
Solving for Rpot,
Rpot=(Iout*R3*R1/Vin) - R4
Rpot=200Ω nominal, which is ≈half of R7.
As you said below, ignore my previous suggestion and use 1% resistors.

 

[audioguru]

Ron's circuit needs a dual opamp but a LM324 quad opamp was used. Why not use an LM358 dual opamp that has opamps that are identical to the ones in an LM324?

 

[bnfdvn]

There is certainly nothing to apologize for. I didn't have 1% resistors readily available and used 5% resistors. From your response, I can see that this is the cause for my predicament. If I had followed your design, it would have worked as I wanted. I will try your suggestion and use the 1% resistors. Thanks for your help!!

 

[Roff]

There is certainly nothing to apologize for. I didn't have 1% resistors readily available and used 5% resistors. From your response, I can see that this is the cause for my predicament. If I had followed your design, it would have worked as I wanted. I will try your suggestion and use the 1% resistors. Thanks for your help!!

See the edit to my previous post. And Audioguru is right, as usual. I used the LM324 spice model. I should have changed the posted schematic to use an LM358. Too late now, I suppose.

 

[Roff]

If you really want to use 5% resistors, we can come up with a working design.

 

[bnfdvn]

I have already updated the documentation for the 1% resistors. I am using several other 10K resistors on the board and for simplicity (and probably laziness) I added these resistors as 5% originally. The circuit works great for my needs. Thanks for the offer and for providing the initial design. I really appreciate it!!

 

[Sy0211]

You can use IC741 by using a single power supply . All you need to do is to make a potential divider circuit taking both resistor value high in order to limit the current entering the IC and connect the junction of 2 resistors to ground . The other terminal of resistors are connected to positive and negative terminals of power supply respectively.