As like the input current loops, output current loops are also 4-20mA current loop. 4 mA to produce minimum control action and 20mA for maximum control action.
A process controller produces output in relation to the input to the controller. Here, the controller supplies both the electrical power and signal information to the final control element, which acts as an electrical load. To illustrate, consider the example of a controller sending a 4-20 mA signal to an I/P (current to- pressure) signal converter.
The controller can work in automatic and manual mode.
There will be a dependent current source in the controller o/p module, which provides both the current source to the I to P converter. Its basic purpose is to maintain the constant current in the loop circuit.
If the final control element has to be controlled to minimum working condition, then the output of the controller will be 4mA and 20mA for maximum controller action. For 50% output, 12mA current produced.
Here in the above-shown figure, the final control element is air to open control valve. The minimum current is to close the valve and maximum current to fully open the valve.
Here in the above figure, the valve is air to close the valve. The I to P transducer with induction load transfer current to pressure. To produce maximum output in the line, a minimum current is produced from the controller. To produce minimum output in the line, a maximum current is produced.