TROUBLESHOOTING CONTROL LOOP Analog 4-20MA. When we verbalize about a process we mean an installation in which the input component is processed as a semi-final or final product in a controlled and automated process. In most cases, some supervision and control are exercised in a house or other building where physical production occurs.


The control system activity in industrial processes is predicted in 420mA loops which form a common analog communication interface that enables the transmission and reading of a single quantitative signal. TROUBLESHOOTING CONTROL LOOP For several years there has been one of the most vigilant values of the industrial control system, having been bought and used for dozens of years.

  • The control loop system typically consists of the following components:
  • Transmitter input to which the signal is connected to the sensor;
  • Transmitter outputs where we get 4-20mA signal;
  • 24V power supply;
  • 250-ohm termination resistor;
  • Process controller.


Statistics show that automation engineers spend about 70% on problem-solving in 4-20mA loops, 20% on commissioning incipient installations, and only 10% on process calibration. In this article, we will take a closer look at the problem-solving aspects optically.

It should be noted that the basic diagnostics of the 4-20mA loop consist of the following steps:

  • Measurements – voltage and current;
  • Simulation – input signal to the transmitter;
  • Use the source – 24 V voltage;
  • The most common condyles in 4-20mA loops include:

Wiring Quandaries: Poor consummation, inadequate insulation, corrosion, and oxidation in the connectors can adversely affect the cable performance of the 4 to 20 mA loop.

24V Loop Power Supply: Faulty or overloaded power supply may cause the MA loop to malfunction or fail instantly.

Loss of controller inputs: If the MA signal is vertical, the controller does not interpret it correctly, process control is impaired.

Loss of transmitter: The amount of sensors/sensors if the transmitter does not transmit the MA signal to respond correctly to the prescribed signals, then in the above case the control mechanism is disturbed.

Faulty sensor.

One of the first steps in testing is to provide a supply of pus. If we find the full 24V when measuring the output of the virtual supply, it is probably working properly. If the potency supply is overloaded, it may be a source of confusion with the control loop (Figure 3).

This is great for verifying the thermal conditions under which the power supply works. The effect of the ambient temperature at which he works will have a negative impact on the durability and reliability of his work.


If it is suspected that the 24V loop power supply is faulty, the next step is to utilize the Poisson source from the control loop calibrator. To do this, disconnect the wires from the output of the Poisson supply and connect them to the connection terminals of the calibrator (Figure 4). Please note that this type of diagnosis should not be made permanent. More flexible calibrators can be overloaded with longer 24V signals.


Let us postulate that the power supply is working together, so the next step in diagnostics is quantifying the signal at the transmitter output. Remember before we start this measurement, we need to call the sender first to stop this element of the process. For what? To determine this amount the calibrator needs to be connected in series with the loop (Figure 5). After connecting a multimeter or calibrator, the loop closes again and the process can be resumed to determine a quantity. TROUBLESHOOTING CONTROL LOOP


It can be onerous to stop and commence the process to quantify loop current in 4-20mA. However, this will sanction a homogeneous situation to the one in which the process is run perpetually. Quantifications in the 4-20mA loop can be carried out utilizing dedicated clamps habituated to quantify the DC signal and diminutive values. In this case, the loop remains intact, the process perpetuates perpetually, and the quantified value can be read from the authentic situation (Picture 6).


The last step of the tracking tract in the 4-20mA loop is examining the transmitter input (Figure 7). For this port, a calibrator should be used, which enables simulation of the sensor signal with which we are communicating in a particular situation. Many of the calibrators allow you to set threshold values for a given type of sensor. This will allow for precise process monitoring and signal simulations from 0% (4 mA) to 100% (20 mA).


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