By John Matlack, VP of Sales and Marketing, Alliance Sensors

A 4-20mA current loop has become the standard for signal transmission and electronic control in most analogue control systems; see Figure 1 – for analogue sensor data transmission, it is a very common method to convey the sensor data acquired.

In a 4-20mA current loop, current is drawn from a DC loop power supply, then flows through the transmitter using field wiring connected to a loop load resistor in the receiver or controller, and then back to the loop supply, with all elements being connected in a series circuit. All current-loop-based measuring systems use at least these four elements.

Figure 1: Typical 4-20mA current loop

Advantages of a current loop

But, an obvious question arises: Why use a 4-20mA current loop to transmit the analogue data from a sensor? The answer is that a 4-20mA current loop offers several benefits for such sensor data transmission:

• A major reason is that the loop current does not vary with long field wiring, as long as the voltage developed in the loop, called “compliance voltage”, can sustain the maximum loop current.
• Another benefit is that the current loop has a low impedance and is not particularly susceptible to noise or EMI at large.
• A third advantage is the live-zero feature of the loop (the 4mA low limit), which makes the loop self-diagnostic if there is a break or bad connection in the loop or a loop power supply failure.
• A current loop permits other current operated devices such as a remote readout or a recorder to be put in series with the loop, within the constraints permitted by the loop’s compliance voltage.
• The low level of maximum loop current (20 mA) allows the use of relatively simple safety barriers to limit loop current to an “intrinsically safe” level that prevents ignition in a hazardous location.

Loop power supply and compliance voltage

When current is transmitted in the loop, there are voltage drops due to the field wiring conductors and any connected devices. However, these voltage drops do not affect the current in the loop as long as the total loop voltage is sufficient to maintain the maximum loop current. The element responsible for maintaining a stable current in the loop (as shown in Figure 1) is the loop DC power supply. The range of voltage over which the loop will function is its “compliance voltage”. Common values for 4-20mA loop supplies are 24 or 36VDC. The voltage chosen by a designer depends on the number of elements connected in series with the loop, because the loop power supply voltage must always be higher than the sum of all the voltage drops in the circuit, including the field wiring voltage drop. The sum of all these voltage drops is known as the loop’s minimum compliance voltage. There are certain requirements that the compliance voltage must be able to fulfill, the two most important of which are: 

• The power supply voltage must be able to power all the devices in the loop, including the field wiring voltage drop, when the current is at its maximum value, normally 20mA.
• The loop power supply maximum voltage output must be equal to or lower than the maximum voltage rating of any device in the loop.

Transmitter

A sensor or transducer that measures a physical parameter, such as temperature, pressure, position, or fluid flow, is connected to a signal conditioning circuit that converts the measured parameter value to an electrical output signal such as a voltage or current proportional to the measured physical parameter. If this electrical signal is a 4-20mA DC output connected to a current loop, the hardware and electronics system that sends a current into the loop is called a transmitter. A transmitter may consist of a single device containing a sensing element and some electronics, or it may use a sensor or transducer connected to separate signal conditioning electronics configured as a 4-20mA current transmitter.

Sensors or transducers are usually designed to measure a range of values of the measured parameter, which is known as the “measurand”. The measurand value must be converted to current within the measuring device in such a way that the current in the loop will be proportional to the measurand value. The range of the loop current, 4-20mA, is called the “span of the transmitter”. The transmitter is typically configured so that one end point of the measurement value will correspond to 4mA and the other to 20mA.

Field wiring

Field wiring conductors are used in the loop to connect the transmitter to the process monitoring or control hardware. It is important to see them as an element of the loop because they have some resistance and produce a voltage drop, just like any other element in the loop. If the sum of all the voltage drops is higher than the loop power supply compliance voltage, the current will not be proportional to the measured parameter and the system will produce unusable data.

The distance between the sensor-transmitter combination and the process controller or readout can be several hundreds of feet or more. The resistance of the field wiring conductors is normally given in Ohms per length, typically Ohms per 1000 feet, so the total resistance is the product of this value times the length of the wires divided by 1000. Note that the wire length includes the loop conductor going out and the loop conductor for the current return, which is twice the individual conductor length. The total wiring resistance is represented by the symbol R_w, as is shown in Figure 1. The voltage drop due to the field wiring is given by Ohm’s law:

Receiver or process controller 

After the loop current is generated, it must usually be further processed in the system; for example, the current could be used as feedback to a valve controller to open, close, or modulate the valve in order to initiate or control a process. It is easier to perform control functions with a voltage rather than a current. The receiver is the part of the loop that converts current into voltage. In Figure 1, the receiver is a simple resistor that is in series with the loop, so from Ohm’s Law, the voltage developed across it is directly proportional to the measured physical parameter, or measurand.

The load resistor used in a 4-20mA current loop is not an arbitrary value. For any specified compliance voltage, there is a maximum loop load resistance that will permit full current to be developed in the loop. Exceeding the maximum loop resistance, which must include the resistance of the field wiring, prevents the system from providing the full 20mA output current in the loop. In the case of a typical current output sensor, whose loop load graph is shown in Figure 2, at 18V input, the total loop load can be as high as 550 Ohms. At 24V input, total loop load can be as high as 850 Ohms, and at the system’s maximum input of 32V, the total loop load can be 1200 Ohms.

Figure 2 Loop load resistance vs. loop supply voltage for a typical current loop output sensor

Choosing the right loop load resistor

The choice of loop load resistor usually depends on the input signal voltage the receiver system requires for good resolution. A 4-20mA loop current will develop 2-10VDC across a 500-Ohm load resistor (E = IR). If the receiver system will work satisfactorily with a lower input voltage, the 4-20mA loop current will develop 1-5VDC across a 250-Ohm load resistor, which is the most common loop load. Note that a loop load resistor is quite often already built into the receiver input terminal connections. Check the specifications of the receiving device to determine if there is a loop load resistor supplied at its input.

It is very important that the loop load resistor power rating is sufficient to ensure that any heating caused by current flowing through it won’t change its value and thereby change the voltage developed across it. Recall that the wattage dissipated by the resistor is I² x R. For a 500-Ohm load resistor, the power dissipated at 20mA is 0.2W. A good choice for the resistor’s power rating is at least 2W because such a load will not heat up very much. Even at full loop current, there won’t be a voltage change across the load resistor due to heat from power dissipation instead of actual loop current changes. Wire-wound resistors usually have lower temperature coefficients than metallised resistors.

Transmitter types

There are several different varieties of current transmitters used for 4-20mA current loops. In general, they conform to the following categories, delineated by the number of connections required for operation:

• Two-wire transmitters, which usually function as loop-powered current sinking devices.
• Three-wire transmitters, which are independently-powered loop current sourcing devices.
• Four-wire transmitters, which are normally independently-powered devices used when loop isolation is needed for noise or ground loop elimination, or for operation in hazardous locations (hazlocs).
• Derivatives of four-wire transmitters such as loop isolators or current loop repeaters. In some cases, these devices are incorporated into national-agency-approved safety barriers for intrinsically safe (IS) systems that can be safely operated in specific hazardous location environments.

Two-wire current loop powered transmitters

Two-wire loop-powered transmitters are electronic devices that can be connected in a current loop without having a separate or independent power source. They are designed to take their power from the loop current. Typical loop-powered devices include sensors, transducers, transmitters, repeaters, isolators, meters, recorders, indicators, data loggers, monitors, and many types of field instruments.

Loop-powered devices are important because for some systems it is difficult to supply separate power to all the devices and instruments in the loop. The device might be located in an enclosure where access might be difficult, or in a hazardous location (hazloc) where power cannot be allowed or must be limited.

Figure 1 shows a two-wire loop-powered device connected to a current loop. It is considered a current-sinking device. The power to drive it is supplied entirely by the unused current below 4mA in the loop. Two-wire loop powered transmitters are popular, but usually more costly than three-wire.

Three-wire current transmitters

Three-wire transmitters are often less costly than two-wire ones. They are different from loop-powered transmitters because their loop current is developed from a DC power supply that supplies more current than just the loop current. The entire transmitter operates off this supply and may consume much more current than typical two-wire loop-powered devices. However, a three-wire system is a sourcing element, so it supplies the current loop despite what it uses itself; see Figure 3.

It is important to note that a three-wire transmitter should never be connected to any two-wire loop powered system.

Figure 3 Typical current loop using a three-wire transmitter

Notice the high side of the power supply is not directly connected to the loop, but that the return side of the power supply is connected via a grounded point, so a three-wire transmitter requires careful consideration of grounding issues to prevent potential ground loops. If an application using a three-wire transmitter requires isolation in the loop, there are several paths to follow.

One way is to use a separate DC power supply for each three-wire loop output device, so that there is no interaction with other current loops. Another way is to use a loop isolator module. These devices use various methods to achieve galvanic isolation, typically using transformers or optical couplers. They accept a 4-20mA signal, function as a repeater or re-transmitter, and deliver a reconstituted 4-20mA current loop signal that is fully isolated. A third way is to use a four-wire transmitter which has isolation already built in.

Four-wire current transmitters

Four-wire transmitters offer the current sourcing advantages of a three-wire device, but also provide galvanic isolation for the current loop output. Four-wire devices are substantially more expensive than three-wire devices. For this reason, they are generally used where the isolation is needed, or they are part of a combination device with an approved safety barrier for current loop operation in a specific hazardous location.

A four-wire transmitter block diagram is shown in Figure 4. A notable item is that the four-wire device itself uses a separate DC power supply for operation, just like a three-wire transmitter, and supplies loop current that way.

Figure 4 Typical current loop using a four-wire transmitter

Review of 4-20mA data transmission

It is useful to conclude with a summary of the features and benefits of the 4-20mA data transmission process, as well as its limitations.

Advantages include:

•  The 4-20mA current loop is recognised as the simplest analogue data transmission method to connect and configure and is the dominant data transmission standard in many industries.
•    It uses less wiring and connections than other methods, greatly reducing startup/setup costs.
•    It is superior over long distances, as current won’t diminish over long connections like voltage.
•    It is relatively insensitive to most electrical noise and related electromagnetic interference.
•    It allows both local and remote readout or monitoring devices to be inserted into the loop.
•  It is easy to detect a fault in the measuring system because 4mA is equal to 0% system output, so any loop current substantially lower than 4mA becomes a direct indicator of a loop fault.

Limitations include:

•  4-20mA current loops can only transmit one specific sensor or process signal per loop.
•  Multiple loops are required for applications where there are many sensor or process outputs that must be transmitted. A lot of field wiring will be needed, which can lead to serious issues with ground loops if the independent current loops are not properly isolated from each other.
•  Isolation requirements become exponentially more complicated with a larger number of loops.