Why Use a Current Loop?

A Little Bit of History

Before the advent of electronic circuitry, process control was a wholly mechanical endeavor. Facilities used pneumatic control signals where controllers were powered by varying pressures of compressed air. Ultimately, air compression of 3-15 psi became the standard for a few reasons:

• Very expensive to systems detecting pressure signals under 3 psi
• Signals below 3 psi would be unrecognizable
• Easier to differentiate a live zero (3 psi) signal from a failure in the system (0 psi)

In the 1950s, as electronic systems became less expensive, current input became the preferred and more efficient process control signal. The 4-20 mA range later became the standard for similar reasons as 3-15 psi did.

Introduction

In the world of process control, there are a myriad of different types of process inputs. Thermocouples and RTDs provide direct temperature reading while digital signals such as Modbus? provide exacting control over process variables and display. Analog signals, where information about the process is transmitted via varying amounts of voltage or current, are the predominant type of input in industries requiring process control today. Of all possible analog signals that can be used to transmit process information, the 4-20 mA loop is, by far, the dominant standard in the industry.

As major as the 4-20 mA loop standard has become in the process control industry, many do not understand the fundamentals of its setup and use. Not knowing the basics could potentially cost you money when it comes time to make decisions about process display and control. Having a grasp on the history, workings, pros and cons of the 4-20 mA loop will help you to understand why it is the dominant standard for the industry and allow you to make informed decisions about your process control.

4-20mA Current Loop

The 4-20 mA current loop is one of the most robust sensor signaling standard. Current loops are ideal for data transmission because of their inherent insensitivity to electrical noise. In a 4-20 mA current loop, all the signaling current flows through all components; the same current flows even if the wire terminations are less than perfect. All the components in the loop drop voltage due to the signaling current flowing through them. The signaling 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 signaling current of 20 mA.

Transmitting sensor information via a current loop is particularly useful when the information has to be sent to a remote location over long distances (500 meters, or more). The loop’s operation is straightforward: a sensor’s output voltage is first converted to a proportional current, with 4 mA normally representing the sensor’s zero-level output, and 20 mA representing the sensor’s full scale output. Then, a receiver at the remote end converts the 4-20 mA current back into a voltage which in turn can be further processed by a processor/ IO card.

Why Use a Current Loop?

The 4-20mA current loop shown in Figure 1 is a common method of transmitting sensor information in many industrial process-monitoring applications. A sensor is a device used to measure physical parameters such as temperature, pressure, speed, liquid flow rates, etc. Transmitting sensor information via a current loop is particularly useful when the information has to be sent to a remote location over long distances (1000 feet, or more). The loop’s operation is straightforward: a sensor’s output voltage is fi rst converted to a proportional current, with 4mA normally representing the sensor’s zero-level output, and 20mA representing the sensor’s full-scale output. Then, a receiver at the remote end converts the 4-20mA current back into a voltage which in turn can be further processed by a computer or display module.

However, transmitting a sensor’s output as a voltage over long distances has several drawbacks. Unless very high input-impedance devices are used, transmitting voltages over long distances produces correspondingly lower voltages at the receiving end due to wiring and interconnect resistances. However, high-impedance instruments can be sensitive to noise pickup since the lengthy signal-carrying wires often run in close proximity to other electrically noisy system wiring. Shielded wires can be used to minimize noise pickup, but their high cost may be prohibitive when long distances are involved.

Sending a current over long distances produces voltage losses proportional to the wiring’s length. However, these voltage losses— also known as “loop drops”—do not reduce the 4-20mA current as long as the transmitter and loop supply can compensate for these drops. The magnitude of the current in the loop is not affected by voltage drops in the system wiring since all of the current (i.e., electrons) originating at the negative (-) terminal of the loop power supply has to return back to its positive (+) terminal—fortunately, electrons cannot easily jump out of wires!

Current Loop Components

A typical 4-20mA current-loop circuit is made up of four individual elements: a sensor/transducer; a voltage-to-current converter (commonly referred to as a transmitter and/or signal conditioner); a loop power supply; and a receiver/ monitor. In loop powered applications, all four elements are connected in a closed, series circuit, loop configuration.

1. Sensor

First, there needs to be some sort of sensor which measures a process variable. A sensor typically measures temperature, humidity, flow, level or pressure. The technology that goes into the sensor will vary drastically depending on what exactly it is intended to measure, but this is not relevant for this discussion.

2. Transmitter

Second, whatever the sensor is monitoring, there needs to be a way to convert its measurement into a current signal, between four and twenty milliamps. This is where a transmitter will come into play. If, for instance, a sensor was measuring the height of a fifty foot tank, the transmitter would need to translate zero feet as the tank being empty and then transmit a four milliamp signal. Conversely, it would translate fifty feet as the tank being full and would then transmit a twenty milliamp signal. If the tank were half full the transmitter would signal at the halfway point, or twelve milliamps.

3. Power Source

In order for a signal to be produced, there needs to be a source of power, just as in the water system analogy there needed to be a source of water pressure. Remember that the power supply must output a DC current (meaning that the current is only flowing in one direction). There are many common voltages that are used with 4-20 mA current loops (9, 12, 24, etc.) depending on the particular setup. When deciding on what voltage of power supply to use for your particular setup, be sure to consider that the power supply voltage must be at least 10% greater than the total voltage drop of the attached components (the transmitter, receiver and even wire). The use of improper power supplies can lead to equipment failure.

4. Loop

In addition to an adequate VDC supply, there also needs to be a loop, which refers to the actual wire connecting the sensor to the device receiving the 4-20 mA signal and then back to the transmitter. The current signal on the loop is regulated by the transmitter according to the sensor's measurement. This component is typically overlooked in a current loop setup because wire is so intrinsic to any modern electronic system, but should be considered in our exploration of the fundamentals. While the wire itself is a source of resistance that causes a voltage drop on the system, it is normally not a concern, as the voltage drop of a section of wire is minuscule. However, over long distances (greater than 1,000 feet) it can add up to a significant amount, depending on the thickness (gauge) of the wire.

5. Receiver

Finally, at someplace in the loop there will be a device which can receive and interpret the current signal. This current signal must be translated into units that can be easily understood by operators, such as the feet of liquid in a tank or the degrees Celsius of a liquid. This device also needs to either display the information received (for monitoring purposes) or automatically do something with that information. Digital displays, controllers, actuators, and valves are common devices to incorporate into the loop. These components are all it takes to complete a 4-20 mA current loop. The sensor measures a process variable, the transmitter translates that measurement into a current signal, the signal travels through a wire loop to a receiver, and the receiver displays or performs an action with that signal.

Pros & Cons of 4-20 mA Loops

Pros

• The 4-20 mA current loop is the dominant standard in many industries.
• It is the simplest option to connect and configure.
• It uses less wiring and connections than other signals, greatly reducing initial setup costs.
• Better for traveling long distances, as current does not degrade over long connections like voltage.
• It is less sensitive to background electrical noise.
• Since 4 mA is equal to 0% output, it is incredibly simple to detect a fault in the system.

Cons

• Current loops can only transmit one particular process signal.
• Multiple loops must be created in situations where there are numerous process variables that require transmission. Running so much wire could lead to problems with ground loops if independent loops are not properly isolated.
• These isolation requirements become exponentially more complicated as the number of loops increases.

Conclusion:

The 4-20 mA current loop is the prevailing process control signal in many industries. It is an ideal method of transferring process information because current does not change as it travels from transmitter to receiver. It is also much simpler and cost effective. However, voltage drops and the number of process variables that need to be monitored can impact its cost and complexity. By knowing these fundamentals you will be able to make more informed decisions about process control in your facility which could affect your bottom line.

Article by:

S Bharadwaj Reddy

Website: InstrumentationTools.com