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Display Magazine

Back to Basics: The Fundamentals of 4-20 mA Current
Loops

Click
Here for Spanish Version

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.

Related: Why a 0 mA Signal is Not Practical

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:

It is very expensive to engineer 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.

How Does a 4-20 mA Current Loop Work?

In order to understand what a 4-20 mA direct current (DC) loop is and how it
works, we will need to know a little bit of math. Don't worry; we won't be
delving into any advanced electrical engineering formulas. In fact, the formula
we need is relatively simple: V = I x R. This is Ohm's Law. What this is saying
is that the voltage (V) is equal to the current (I) multiplied by the resistance
(R) ("I" stands for Intensité de Courant, French for Current Intensity). This is
the fundamental equation in electrical engineering.

Figure 1. Simple DC Circuit

Consider the simple DC circuit above, consisting of a power supply and three
loads. A current loop requires voltage to drive the current. This is provided by
the power supply, with the voltage of the supply labeled as Vtot. Current then
flows through the loop, passing through each load. The voltage drop at each load
can be calculated from Ohm's Law. The voltage drop V1 across R1 is:

Figure 2. Ohm's
Law

Every element in the loop either provides voltage or has a voltage drop.
However, the current, "I" is the same everywhere in the loop. This is the
critical principle of the 4-20 mA loop. Current is the same in all places
throughout the loop. It may be difficult to understand why the current remains
constant, so consider your home's water system as a comparison. There is a
certain amount of pressure in the water pipes pushing the water towards your
house.

Figure 3. Current /
Water Flow Analogy

Voltage, in a similar fashion, acts as a pressure, pushing current through
the circuit. When a tap inside your home is turned on, there is a subsequent
flow of water. The flow of water is analogous to the flow of electrons, or
current. The ability of the pressure to push the water through the pipes is
limited by bends and restrictions in the pipe. These restrictions limit the
amount of flow in the pipe, similar to how a resistor limits the current. The
flow through the pipe, and likewise the current through the wire, remains
constant throughout the system, even though pressure, and likewise voltage, will
drop at various points. This is why using current as a means of conveying
process information is so reliable.

Featured: PD9502 Low-Cost Signal
Generator

Components of a 4-20 mA Current Loop

Now that you have an understanding of how and why current is used, you can
begin to understand what exactly the loop is for.

Figure 4. Components
of a 4-20 mA Current Loop

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

Part of the challenge of working in an industry which requires process
control is determining if the pros outweigh the cons. Making the right decision
can save both time and money.

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.

Featured: PD420 Panel
Mount 4-20 mA Set-Point Generator

4-20 mA Current Loops Overview

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.

By Simon Paonessa - Technical Writer
Bruce McDuffee - Technical
Writer
Precision Digital Corporation

Download White Paper

Read
Part 2 - The Fundamentals of Loop-Powered Devices

Watch: The Fundamentals of 4-20 mA Current Loops

Watch this recorded webinar designed as an introductory class for those who
have to deal with process signals. Gain a strong understanding of the
fundamentals of 4-20 mA current loops, and be better equipped to decide when,
where, or if a 4-20 mA current loop solution is right for your
application.

You may be interested in

PD9502 Low-Cost Signal Generator

A low-cost, compact, simple to use 4-20 mA or 0-10 VDC signal
generator.

PD420 Panel Mount 4-20 mA Set-Point Generator

Provides a convenient way to generate a 4-20 mA signal that can be used to
control another
device.

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