Basics of The 4 - 20mA Current Loop ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Basics of The 4 - 20mA Current Loop

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The 4-20mA current loop is a very robust and popular sensor signalling standard. Current loops are ideal for data transmission because of their inherent insensitivity to electrical noise. In a 4-20mA current loop, all the signalling current flows through all devices. All the devices in the loop drop voltage due to the signal current flowing through them. The signalling current is not affected by these voltage drops as long as the power supply voltage is greater than the sum of the voltage drops around the loop at the maximum signal current of 20mA.

As shown in the diagram above, current supplied from the power supply flows through the loop wires with resistance, RW, to the transmitter and the 4 – 20mA transmitter regulates the current flow within the loop. The current allowed by the transmitter is called the loop current and it is proportional to the parameter that is being measured. The loop current flows back to the controller through the wire, and then flow through resistor, R, to ground and returns to the power supply. The current flowing through R produces a voltage that is easily measured by the analog input of a controller. For a 250 ohm resistor, the voltage will be 1VDC at 4mA and 5VDC at 20mA.

As the diagram above shows, there are four basic components in the 4-20mA current loop namely:
(a) The power supply
(b) The 2-Wire Transmitter
(c) A receiver resistor, R that converts the loop current into a voltage
(d) The loop wires that interconnects all devices or components in the loop.

The Power Supply
Power supply package for 4 – 20mA, 2-wire transmitters must always be DC because the change in current flow is representative of the parameter that is being measured. For 4-20mA loops with 2-wire transmitters, common power supply voltages are 36VDC, 24VDC, 15VDC and 12VDC. Current loops using 3-wire transmitters can have either AC or DC power supplies. The most common AC power supply is the 24 VAC control transformer. Be sure to check your transmitter’s installation manual for the proper voltage requirements.

The 4 – 20mA, 2 – Wire Transmitter
The transmitter is the heart of the 4-20mA signal system. It converts a physical property such as temperature, flow or pressure into an electrical signal. This electrical signal is a current proportional to the temperature, flow or pressure being measured. In a 4-20mA loop, 4mA represents the low end of the measurement range and 20mA represents the high end.

The voltage specification for most transmitters comes in a range. For example if the voltage of a 2 – wire transmitter is specified as 15 to 24VDC, the lower voltage is the minimum voltage necessary to guarantee proper transmitter operation. The higher voltage is the maximum voltage the transmitter can withstand and operate to its stated specifications without damage or adverse consequences.

The Receiver Resistor, R
In engineering, it is much easier to measure a voltage than it is to measure a current. Therefore, many current loop circuits use a Receiver Resistor (R in our case) to convert the current into a voltage. In the diagram above, R is a 250 ohm resistor. The current flowing through it produces a voltage that is easily measured by the analog input of a controller. For the 250 ohm resistor, the voltage will be 1VDC at 4mA of loop current and 5VDC at 20mA of loop current. The most common receiver resistor in a 4-20mA loop is 250 ohm; however, depending upon the particular application, resistances of 100Ω to 750Ω may be used.

The Loop Wires
When current flows through a wire, it produces a voltage drop proportional to the length and thickness (gauge) of the wire. All the loop wires have resistance, usually expressed in Ohms per length. The voltage drop can be calculated using Ohm’s law:
E = I x RW
E = the voltage across the resistor in volts;
I = the current flowing through the loop wires in amperes;
RW = the loop wire’s resistance in Ohms.
However, because the current flowing in a typical 4 – 20mA loop is small, voltage drops are usually small although the loop wire runs should be considered when running instrumentation wiring to bring down voltage drops.
These basic components exist in any 4 – 20mA loop that you will deal with. To successfully troubleshoot this loop you need to be familiar with all these components that have been discussed.