Controller- An Insight

Inside the controller, a dependent current source provides the 4-20 mA DC current signal to
the I/P transducer. Like all current sources, its purpose is to maintain current in the “loop”
circuit regardless of circuit resistance or any external voltage sources. Unlike a constant current
source, a “dependent” current source (represented by a diamond shape instead of a circle shape)
varies its current value according to the dictates of some external stimulus. In this case, either the
mathematical function of the controller (Automatic mode) or the arbitrary setting of the human
operator (Manual mode) tells the current source how much DC current it should maintain in the
circuit.
For example, if the operator happened to switch the controller into Manual mode and set the
output value at 50%, the proper amount of DC current for this signal percentage would be 12 mA
(exactly half-way between 4 mA and 20 mA). If everything is working properly, the current in the
“loop” circuit to the I/P transducer should remain exactly at 12 mA regardless of slight changes
in wire resistance, I/P coil resistance, or anything else: the current source inside the controller will
“fight” as hard as it has to in order to maintain this set amount of current. This current, as it flows
through the wire coil of the I/P transducer mechanism, creates a magnetic field inside the I/P to
actuate the pneumatic mechanism and produce a 9 PSI pressure signal output to the control valve
(9 PSI being exactly half-way between 3 PSI and 15 PSI in the 3-15 PSI signal standard range).
This should move the control valve to the half-way position.
 Usually, it takes the form of an operational amplifier circuit driven by the voltage output of a DAC (Digital-to-Analog Converter). The DAC converts a binary number (either from the controller’s automatic calculations, or from the human operator’s manual setting) into a small DC voltage, which then commands the op-amp circuit to regulate output current at a proportional value.