How a current loop works
A typical 4-20 mA current loop circuit is composed of four individual elements: a sensor / transducer; a voltage to current converter (commonly referred to as a transmitter and / or signal conditioner); a loop power supply; and a receiver / monitor. In loop-fed applications, the four elements are connected in a closed circuit, series, loop configuration.
First, there must be some kind of sensor that measures a process variable. A sensor usually measures temperature, humidity, flow, level or pressure. The technology that enters the sensor will vary drastically depending on what exactly it is intended to measure, but this is not relevant to this discussion.
Second, regardless of what the sensor is monitoring, there must be a way to convert your measurement into a current signal, between four and twenty milliamps. This is where a transmitter will come into play. If, for example, a sensor measured the height of a fifty-foot tank, the transmitter would need to translate zero feet as the empty tank and then transmit a signal of four milliamps. On the contrary, it would translate fifty feet as the tank is full and then transmit a signal of twenty milliamps. If the tank were half full, the transmitter would emit a signal at half, or twelve milliamps.
For a signal to occur, there must be a source of energy, just as in the analogy of the water system, there must be a source of water pressure. Remember that the power supply must emit a direct current (which means that the current only flows in one direction). There are many common voltages that are used with 4-20 mA current loops (9, 12, 24, etc.) depending on the particular configuration. When deciding which voltage of the power supply will be used for your particular configuration, be sure to consider that the voltage of the power supply must be at least 10% greater than the total voltage drop of the connected components (the transmitter, the receiver and even the cable). The use of inadequate power supplies can cause equipment failure.
In addition to a suitable VDC supply, there must also be a loop, which refers to the actual cable that connects the sensor to the device that receives the 4-20 mA signal and then back to the transmitter. The current signal in the circuit is regulated by the transmitter according to the sensor measurement. This component is usually overlooked in a current loop configuration because the cable is so intrinsic to any modern electronic system, but it should be considered in our exploration of the fundamentals. While the cable itself is a source of resistance that causes a voltage drop in the system, it is usually not a concern, since the voltage drop of a cable section is minuscule. However, over long distances (more than 1,000 feet) it can add up to a significant amount, depending on the thickness (gauge) of the cable.
Finally, somewhere on the circuit there will be a device that can receive and interpret the current signal. This current signal must be translated into units that operators can easily understand, such as feet of liquid in a tank or degrees Celsius of a liquid. This device also needs to show the information received (for monitoring purposes) or do something automatically with that information. Digital displays, controllers, actuators and valves are common devices to incorporate into the circuit. These components are all that is needed to complete a 4-20 mA current loop. The sensor measures a process variable, the transmitter translates that measurement into a current signal, the signal travels through a cable loop to a receiver, and the receiver shows or performs an action with that signal