What Is the Name of the Circuit Converting 4-20mA to 1-5V Using an Op Amp?

[ionut1010]

I have found this circuit in my travels on the internet and it should suit my needs. I need to know what this configuration is called so I can calculate the resistors for the gain needed.

If anyone knows of a similar circuit that has trimpots to adjust the 4mA to 1v and the 20mA t0 5v that would be greatly appreciated.

 

[dlgoff]

Welcome to PF ionut

If you are wanting to convert a 4-20ma source to 1-5volts, why not just use take the voltage across a 250ohm resistor?

 

[uart]

R5 is a "burden" resistor and the rest of the circuit is a differential ampifier. You should make R5 a fairly low resistance (around one hundred ohms) and the other four resistors much larger. Make R1=R3 and R2=R4 and R5*R1/R2=250

 

[uart]

Welcome to PF ionut

If you are wanting to convert a 4-20ma source to 1-5volts, why not just use take the voltage across a 250ohm resistor?

Yeah I was thinking the same thing, but perhaps he wants to have the current loop terminals "floating" (well not strictly floating, but non-grounded).

 

[ionut1010]

Thanks for the replies.

Yes I need the input to be non-grounded. That is why I want to measure the voltage across the resistor(R5). I need to tap into the loop and then feed the mA to the ADC of a microcontroller.

Where can a put a trimpot to adjust the 4mA to be 1 v and the 20mA to be 5v. I was thinking that this will need some adjustment for zero(4mA) and span(20mA).

 

[ionut1010]

Does anyone have a circuit where the 4mA(zero) and 20mA(span) can be adjusted. I would like a way to adjust the two.
Or can anyone point me to where I can find such a circuit. I have search the internet for a circuit but can not find one that suits my needs.

I am new to 4 to 20ma signals and need to convert one for input to an adc. I can not use the 250 ohm resistor method due to loop requirements.

 

[berkeman]

Does anyone have a circuit where the 4mA(zero) and 20mA(span) can be adjusted. I would like a way to adjust the two.
Or can anyone point me to where I can find such a circuit. I have search the internet for a circuit but can not find one that suits my needs.

I am new to 4 to 20ma signals and need to convert one for input to an adc. I can not use the 250 ohm resistor method due to loop requirements.

Why not just do the conversion after the ADC? You can easily adjust for zero once the 0-20mA signal is digitized. Just set 20mA as full scale, and do the conversion math in the uC after the ADC?

And what do you mean you can't use the 250 Ohm resistor due to loop requirements. Could you please explain that a bit more?

 

[ionut1010]

I do not want any reference to ground.
I need to put the least amount of load on the loop due to wire length and other devices on the loop. With the schematic above I can use a 100 ohm or lower resistor for this.

I do not want to set the zero and span in software when I can have two trimpots for this purpose. I know that there is a way to accomplish this but I do not know how and where to find it. All current loop devices have an adjustment for zero and span depending on what the load is on the loop.

I have found that I need the following adjustments in the op amp.
'zero' is offset.
'span' is gain.

I found the circuit above doing searches for a few days but there are no means for adjustment.

Maybe I am just searching the wrong terminology and that is why I can not find it !

 

[dlgoff]

I'm curious ionut. Where are you getting the 4-20ma signal? What is it's source? If it from some sort of transducer; most have zero and full scale adjustment provided.

 

[ionut1010]

Yes one is a pressure sensor and the other is temperature sensor. Two separate loops.
I can not adjust the whole loop with the transducer/ transmitter since it will affect the whole loop. I need my unit to have span and zero adjustment. I am trying to build the circuit myself with an op amp. I do not want to buy an off the shelf product. Call it a learning experience on how stuff works.

 

[dlgoff]

You have pressure and temperature in one loop?
Oh well, I'll look around for some loop conditioner circuits.

 

[ionut1010]

First line...

Yes one is a pressure sensor and the other is temperature sensor. Two separate loops.

I guess my eyes are not the only ones getting crossed

 

[ionut1010]

Can anyone point me in the right direction ??

 

[dlgoff]

Omega makes http://www.omega.com/vhpc/" that claim to be up to 1800Vdc isolation. And this is what they say about their outputs:

iDRN SERIES ANALOG OUTPUT
The iDRN series feature a 0-10 Vdc, or 0-20 mA (including 4-20 mA) analog output signal that is typically scaled to be directly proportional to the input signal. It is an ideal component in a system with PLC. s or PC. s with analog data acquisition boards.
The iDRN series are an excellent choice for applications that demand an extra measure of accuracy and performance that is not possible with conventional "analog" signal conditioners or transmitters. Unlike conventional analog devices that are scaled by adjusting zero and span pots, the programmable, micro-processor based iDRN instruments are scaled precisely on a PC with free and simple Windows software. The iDRN modules connect directly to a PC. s RS-232 Serial Communications port for programming and scaling. Once the module is configured, the parameters are saved in nonvolatile memory. The device can be disconnected from the PC, or the RS-232 output from the module can be used for continuous data acquisition in addition to the analog output.
Alternatively, the iDRN signal conditioner can be connected to a PC. s Ethernet port or an Ethernet network using the EIS-2B module as a Serial/Ethernet bridge.

Note:

Unlike conventional analog devices that are scaled by adjusting zero and span pots, ...