4-20 mA Transmitters (2-wire, 3-wire and 4-wire Types)

Background of 4-20 mA Signal Transmission

Initially signal transmission was dominated by pneumatics before the advent of 4-20 mA signal standard. The standard was 3-15 psi pneumatic signal where the 3 psi was the ‘live zero’ and 15 psi represented the 100 %. Any pressure below 3 psi was considered ‘dead zero’ and an alarm condition. Large compressors drove the 3 -15 psi pneumatic signals throughout the plant, and these pneumatic lines connected to pneumatically controlled valves to drive proportional controls and actuators throughout the plant. With the advent of computerized process control, in the early 1950’s the signal transmission technique shifted from 3-15 psi to 4-20 mA signals where the 4 mA was the ‘live zero’. Note that the ‘real’ dead zero has always been the alarm condition.

Nevertheless, some industrial installations still use pneumatic control today, especially in applications where electrical signals or sparks could ignite combustible materials. Modern I/P converters (current-to-pressure transducers) are available to convert the 4-20 mA current loops to common pneumatic signal ranges 3-15 psi, 1-18 psi, and 6-30 psi and P/I converters (pressure-to-current) that convert pneumatic signals to current signals.

4-20 mA Transmitter

Figure 1(a) below shows the internal circuit blocks of a 4-20 mA transmitter.

These circuits provide the following functions as illustrated by the block diagram above:

• The input signal conditioning circuits provide the suitable interfacing for all types of inputs such as RTDs, thermocouples, strain gauges, AC-DC voltages and currents, etc. Several 4-20 mA modules have smart signal conditioning functionality that provides linearization and mathematical manipulations of the signal.
• The current conversion circuits establish the 4-20 mA current loop signal.
• Power circuits generate all the required internal voltages and are energized either from a local power source or the actual current loop.
• The dashed line in the block diagram illustrates the isolation between the field side and the output loop side. Isolation is an important feature of signal transmission. Signal loops, power supplies and ground should always be completely isolated from each other.

The 4-20 mA Transmitter current loop

Consider the current loop schematic diagram below:

An ideal Norton current source composed of I_S and R_S signal models the 4-20 mA transmitter. The line resistance is given as R_L and V_N represents the random induced loop noise in form of voltage. In this application example, the 500 ohm controller and 240 ohm digital display are connected with the signal current. The loop is powered by a 24 V DC supply.

Key features of the 4-20 mA current loop

• The signal voltage at any load is given by (I_S x R_load), which is independent of supply voltage variations, and the line resistance R_L.
• Random induced loop noise voltage at any load is given by: 

You can notice that, the loop noise at a load is reduced by the factor

• Supply voltage variations are reduced at any load by the same factor illustrated above.
• We can have multiple loads connected in series in a transmitter loop providing a significant control and display opportunities. Typical loads have full scale input requirements of 1 volt, 5 volts, and 10 volts. Generally 4 -20 mA transmitters require a voltage across the output terminals to maintain the device within its operational specifications. This voltage is called compliance voltage for example in figure 1(b) the loop voltage (compliance) available for the transmitter is:

(V supply – all load voltages –line IR drops)

Therefore the compliance voltage or loop power supply voltage must be greater than the maximum voltage in the loop. There are 3 things that must be checked when determining loop compliance or power supply voltage:

-The compliance voltage should be able to power all the devices in the loop.

-The loop power supply should not exceed the maximum voltage rating of any device in the loop.

Types of 4-20 mA Transmitters

We have 3 types of 4-20 mA transmitters, namely:

• 2-wire transmitters
• 3-wire transmitters
• 4-wire transmitters

2-wire Transmitters

In 2-wire 4-20 mA current loops, we use 2-wire transmitters to convert the various process signal representing flow, level, position, pressure, strain, temperature, pH, speed, etc. to 4-20 mA dc for the purpose of transmitting the signal over some distance with little or no loss signal noise.

The figure 1(c) below, illustrates the 2-wire transmitter energized by the loop current.

You can notice from figure 1(c), that the transmitter floats relative to the ground and the dc power in series in the loop. Power supply and receiver shares a common, which is connected to the ground. Note we can only have one ground in 4-20 mA system. Multiple grounds will cause errors in the readings.

3-wire Transmitters

The transmitter and receiver shares common with power supply. We have a separate dc power connection to the transmitter.

4-wire Transmitters

The transmitter and receiver floats. A separate power supply powers the transmitter.

Please note, in most instrumentation installations, the power supply is local either to the transmitter or the receiver.

Why the 2-wire Transmitter is Preferred over the 3-wire and 4-wire Transmitters

The floating connection relative to the ground in the 2-wire transmitters makes them flexible in the way they connect to various receiver devices. The receiver devices can be connected as either sourcing inputs or sinking inputs. Sourcing inputs act as loop receivers and source of current in the loop as illustrated in figure 1(f)

Sinking units do not provide the excitation for the transmitter hence they need a separate power supply in the loop as illustrated in figure 1(g).

Since in most installations the loop power is either local to the transmitter or local to the remote receiver, a shielded pair wiring is often used to connect the longest distance between the field transmitter and remote receiver. The twisted pair provides protection against electrical noise that might cause errors in measurement.

Advantages of 4-20 mA Signal Transmission

• 4-20 mA current loop has a low sensitivity to electrical noise. This is very significant for long distance transmission and in harsh industrial environments.
• Noise signal loss with respect to the transmission media (wire) and the interconnection i.e. connectors.
• The 4 mA ‘’live zero’’ is fail safe. The use of 4 mA as the starting point for the transmitted signal is helpful in troubleshooting as signal integrity is verified with 0% input and output signal. A failed current loop due to break or open device can be immediately discovered as zero current flow, which is a fail-safe level outside of the signal range.
• A ‘live zero’ of 4 mA allows the two-wire current loop to power the transmitter, simplifying the installation and reducing the costs.
• The 4-20 mA current loop allows additional receiver devices to be connected in series in the loop without a loss of signal provided the loop voltage supply has the sufficient capacity to power these additional devices, and this voltage does not exceed the maximum voltage rating of the transmitter.

You can also read: Types of Sensors used in Measurement and Process Control

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