Pumps & Systems, September 2008
The 4-20mA Current Loop
The 4-20mA current loop is used to transmit analog data representing various process variables like level, pressure, temperature or flow. Although conceptually simple, a 4-20mA current loop can be tricky to troubleshoot. Current flows in a single direction through each device in the loop (see Figure 1).
There are two types of instruments used in a current loop: loop-powered (2-wire) and non-loop-powered (4-wire) instruments. Loop-powered instruments are powered entirely from a DC voltage supply placed within the 4-20mA loop. Non-loop-powered devices require connection to a separate voltage source for the instrument to be used in the current loop.
Take care when setting up the loop to ensure that the impedance of the loop instruments and the available DC voltage supply allows for the full 20mA to flow through the loop. To ensure your loop will work properly, add up the voltage drops across each device in the loop, and make sure this number does not exceed the DC power supply voltage used to power the loop. While some controllers have internal DC power supplies for use with an analog level transducer input, these power supplies may not be sufficient when additional instruments are added into the current loop.
For a simple loop containing a controller and a transducer (see Figure 1), the internal supply shown is more than adequate to power the loop. For the example in Figure 2, however, the controller's internal 20VDC power supply is insufficient to power the loop. Most transducers require a minimum of 10-12VDC. A typical analog level input for a controller will have a voltage drop of 5VDC at 20mA. A dual-channel intrinsically safe barrier with a clamping voltage of 28VDC may have a voltage drop greater than 7 VDC at 20mA.
The loop in Figure 2 would require an external DC power supply rated at 24VDC for this application. If an external power supply with a DC voltage greater than 24VDC is selected, then use caution to ensure the clamp voltage of the barrier zener diodes is not exceeded when there is 4mA of loop current. A zener diode is a semiconductor device that is essentially off until the voltage across the diode reaches its zener, or clamping voltage. As the voltage level approaches the clamping rating, the zener diode begins to conduct, clamping the voltage at the voltage rating of the zener diode.
Two of the most common obstacles encountered when working with a 4-20mA loop are polarity and unintentional grounding. If an instrument in the loop is connected with an incorrect polarity, current may not flow in the loop. Grounding the loop in more than one place also creates a problem since the current does not flow through the entire loop.
When troubleshooting a 4-20mA current loop, carefully select the analog intrinsically safe barrier. Ensure the clamping voltage rating of the barrier is higher than the power supply voltage powering the loop. If the DC voltage powering the loop is greater than the clamping rating of the barrier, the loop will not achieve the full range of 4-20mA current flow due to the zener diodes of the barrier going into the conduction mode.
Isolated Versus Non-Isolated Analog Inputs
Isolation with regard to a controller's analog input has to do with whether the 
analog input (-) common is connected to the device's internal power supply common, which is typically connected to control panel ground.
The isolated analog input common (-) will be free from any connection to control panel ground and any connection to the common of any other analog loop. This fact is important, as a separately powered loop instrument may have its common tied to control panel ground in another loop.
For the majority of 4-20mA analog level input applications, isolation for the level input of the controller is not required. Figure 1 shows the level signal coming from a loop-powered transducer. Since the loop-powered transducer does not have a ground connection, no isolation is required at the controller analog input.
Here are four terms additional terms needed for a better understanding of the 4-20mA loop:
- Analog Common: The negative (-) terminal of the 4-20mA loop.
- Power Supply Common: The negative (-) terminal of the DC power supply associated with the source of power for the analog loop.
- Power Supply Ground: The ground connection for the DC supply. The power supply common may or may not be internally connected to the power supply ground.
- Control Panel Ground: The ground connection for the incoming power to the control panel.
If the 4-20mA level input current loop contains any externally powered instruments, ensure that the analog signal from this device is either isolated or that the analog common from this remotely powered instrument is not connected to a panel ground. If any of these externally powered instruments have their common lead for the 4-20mA circuit referenced to a control panel ground, then the analog level input for the controller should be specified as an isolated input.
Figure 3 shows an example of a 4-20mA loop with a remotely powered device that references the analog common to a panel ground. The analog input common of the controller is referenced to its internal power supply common, which in turn is connected to power supply ground and panel ground.
While the current flow in the 4-20mA loop would be read at the remote display, the current flow would bypass the controller analog input because the return path for the loop current would be the panel ground connections. To remedy the loop shown in Figure 3, the remote analog common must be disconnected from control panel ground, or the remotely powered device should be changed to a unit with an isolated analog circuit. Otherwise, the controller would require an isolated analog input circuit.
Another similar symptom requiring an isolated analog level input is an incorrectly displayed value. The analog input level signal, due to the difference in ground potentials, would divert some of the current loop into
