Current Loop Voltage Calculator

4-20 mA current loop systems need sufficient voltage supply to function. When the voltage supply is insufficient, the maximum current may not be reached. When the maximum current is not reached the measured value is not transferred correct which can cause serious system errors. Therefore it is necessary to check the power supply, cable length and cross section and load to make sure the loop is working as expected.

As all electrical systems 4-20 mA loops also comply to Kirchhoff's law. Kirchhoff`s second law states the sum of all voltages in a circuit is 0. Due to resistance in the loop, the voltage varies with the current flowing through these resistors. This means when the voltage supply is insufficient, the maximum current can not be reached.

The voltage calculator below shows a typical current loop circuit.

The current loop voltage diagram consist of the following items:

Power Supply The power supply provides the power to the current loop. Typical power supply for industrial systems is 24VDC.
Receiver The receiver is a PLC, DCS system or some other device such as a I/P converter display etc. Receivers typically have 2 types of inputs which can be selected in the current loop voltage simulation.
1. A fixed resistor, the resistor will convert the current in the loop to a variable voltage which is evaluated by the receiver.
2. A fixed voltage, the internal electronics will convert the current from the loop to a (digital) signal.
Loop tester The loop tester will set the current in the loop. The Loop tester can be replaced by a measurement transmitter such as a pressure or temperature transmitter. The loop tester needs sufficient voltage to operate. When the voltage drops below the minimum required voltage the current in the loop will also drop which will cause measurement errors.
Cable The cable connects the Loop tester/measurement transmitter to the power supply and receiver. Depending on the length and cross section of the cable, the cable has a resistance. This resistance will cause a voltage drop as indicated in the calculator. The voltage drop is depending on the current flowing through the cable. See Ohm's law
Current The Current is the current flowing through the loop. When power supply is sufficient this is the current set by the Loop Tester, if supply is insufficient the current is calculated from Ohm's law.

The current loop voltage calculation is based on the following formula:

$$ U_{power supply} = U_{receiver} + U_{loop tester} + ( 2 * R_{cable} * I_{loop}) $$

Current loop voltage calculator conclusion

When the power supply voltage to the current loop is to low, the required maximum current of 20 mA can not flow.The calculation shows the current will stick at a certain level. The PLC, DCS system or other receiver will not detect the insufficient voltage supply, so the measured current will be used as a normal measured value. Due to the insufficient voltage supply this value is not correct which may cause serious problems in the operation of the plant in which this situation occurs. Hence, alarms may not sound as the trip level is not reached due to the insufficient voltage supply to the current loop. We recommend to perform a loop test according the Namur NE43 current loop range of 3,4-21,5 mA on every loop during commissioning. And to add a voltage monitoring alarm to the power supply which is used to power the 4-20 mA current loop system. This alarm shall signal when the power supply drops below 24 VDC.

© February 22, 2015
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