Voltage to Current Loop Converter recommendation needed

[ahdavewest751]

Hello, Can anyone recommend a simple Voltage to Current Loop Converter to take a DC voltage and output a current between 4-20mA?

I have tried everywhere but I cant find anything.

 

[Reloadron]

You don't mention the voltage to convert to a 4-20 mA loop?

Ron

 

[ahdavewest751]

I have a 4 strain gauge bridge circuit AC output voltage that has been amplified by a 3 op-amp inst amplifier and converted to a DC voltage of 1 volt. I want to take this and convert to a current for data logging.

 

[Reloadron]

Most data loggers accept a voltage input? Anyway a voltage to current conversion is pretty easy and common. Also so you come off a strain gaiuge, amplified and I assume 0 to 1 volt equal your strain? The op amp circuit linked to is the start of a common design. Keep in mind if the op amp chosen can't source 20 mA you need to drive a transistor with it.

Ron

 

[ahdavewest751]

Thing is I dont know the typical changes in resistance so I randomly altered the gauge resistances on the simulation. I dont really know much about this stuff and Im not really sure what to expect at the output. Should there be only a current at the output or will there be a voltage as well?

I also found:

**broken link removed**

but Im not sure what the wires are for going through the center of the 1st op-amp.

 

[Reloadron]

When you do a voltage to current conversion circuit the output is current. Typically this signal (let's say 4 to 20 mA) is passed to a data logger. Most data loggers have a voltage input, however, for those designed for a current input the input resistance is low like either 250 or 500 ohms. This results in an actual voltage input of 1 to 5 or 2 to 10 volts. So when you look at the circuit and see Rl or Rload or load that is the actual input to the data logger and not a circuit component.

In the circuit you posted the first op amp is just a buffer but allows an offset adjust. The first op amp has a unity gain of 1 so Vin = Vout. If Vin were shorted to ground then the 20K offset pot would be adjusted for 0 Volts out of the first op amp. The second op amp serves as the current source driving the mosfet. If I have time later I might be able to put it together in a simulation. Just a tad busy right now.

Ron

 

[Reloadron]

You may also want to look at the AD694 as a solution. There are several ways to go about it.

Ron