mv to ma current amplifier design

[cps13]

Hi,

I am currently working on a instrumentation amplifier design to accept mV inputs and provide a 4-20mA output. It is to be used with various industrial sensors like load cells (load is in tonnes/kg not voltage).

The problem I am facing is that I do not know for sure what instrumentation the amplifier will be connected to, and therefore do not have a setup load resistance for the device measuring the 4-20mA output. Two examples are a panel meter with a resistance of 11.1ohms and a PLC with a resistance of 280ohms.

I need to build my amp such that it can be connected (within reason) to any meter. Can anyone point me in the right direction for what to read up on? I have found brief information about Norton circuits but cannot see how to make them apply to my application.

Thanks,

 

[duke37]

The idea of a current loop is that it will work independantly of the load resistance. The AC mains power supply is constant voltage and you can connect loads of different resistances within reason without greatly affecting the voltage.

There are several circuits on the web (try 4-20mA transmitter) for the current loop transmitter. The way this is done is to control the voltage across a resistor (100Ω ?) and the current goes into the loop. An op-amp is used to make the output current match the input voltage.

Since the current is controlled, the resistance it is driving is not a problem so long as sufficient voltage is available. The receiver has to supply the voltage and several non powered receivers can be added to the circuit. The receiver has a resistor which generates a voltage proportional to the current.

Wikipedia gives an explanation.

 

[cps13]

@duke37

Thanks for your reply - in my circuit it is DC powered, would this have an effect?

thanks

 

[duke37]

All the electronics will be powered with DC even with an AC mains supply.

 

[Jouellet]

for instrument amplifier, I normally use the INA125AP . Extremely easy to use

for the 4-20 transmitter, try one of these: AD694, XTR101AP, XTR105AP, XTR112AP, XTR112AP,

2 ICs and a handfull of resistors/capacitors will do it ....