4-20mA Current Loop Primer

Application Note
4-20mA Current Loop Primer
This application note’s primary goal is to provide an easy-tounderstand primer
for users who are not familiar with 4-20mA current-loops and their applications. Some of the many topics discussed include: why, and where, 4-20mA
current loops are used; the functions of the four components found in a typical application; the electrical terminology and basic theory needed to understand current loop operation. Users looking for product-specific information
and/or typical wiring diagrams for DATEL’s 4-20mA loop- and locallypowered
process monitors are referred to DMS Application Note 21, titled “Transmitter
Types and Loop Configurations.”
Despite the fact that the currents (4-20mA) and voltages (+12 to +24V)
present in a typical current loop application are relatively low, please keep in
mind that all local and national wiring codes, along with any applicable safety
regulations, must be observed. Also, this application note is intended to be
used as a supplement to all pertinent equipment-manufacturers’ published
data sheets, including the sensor/transducer, the transmitter, the loop power
supply, and the display instrumentation.
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 first 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 electricallynoisy
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, seriescircuit, loop configuration (see Figure 1).
Sensors provide an output voltage whose value represents the physical
parameter being measured. (For example, a thermocouple is a type of sensor
which provides a very low-level output voltage that is proportional to its
ambient temperature.) The transmitter amplifies and conditions the sensor’s
output, and then converts this voltage to a proportional 4-20mA dc-current
that circulates within the closed series-loop. The receiver/monitor, normally a
subsection of a panel meter or data acquisition system, converts the 4-20mA
current back into a voltage which can be further processed and/or displayed.
Figure 1. Typical Components Used in a Loop Powered Application
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Application Note
The loop power-supply generally provides all operating power to the transmitter and receiver, and any other loop components that require a well-regulated
dc voltage. In loop-powered applications, the power supply’s internal elements
also furnish a path for closing the series loop. +24V is still the most widely
used power supply voltage in 4-20mA process monitoring applications. This is
due to the fact that +24V is also used to power many other instruments and
electromechanical components commonly found in industrial environments.
Lower supply voltages, such as +12V, are also popular since they are used in
computer-based systems.
Loop Drops
One of a process monitor’s most important specifications—be it a loop-powered or locally powered device—is the total resistance (or “burden”) it presents to the transmitter’s output driver. Most transmitter’s data sheets specify
the maximum loop resistance the transmitter can drive while still providing a
full-scale 20mA output (the worst-case level with regards to loop burden).
Ohm’s Law states that the voltage drop developed across a current-carrying
resistor can be found by multiplying the resistor’s value by the current passing
through it. Stated in mathematical terms:
where E is the voltage drop in volts, I is the current through the resistor in
amperes, and R is the resistor’s value in Ohms (the ‘Ω’ symbol is commonly
used to represent Ohms).
The sum of the voltage drops around a series loop has to be equal to the
supply voltage. For example, when a loop-powered application is powered
from a 24V power source, the sum of all the voltage drops around the series
loop has to also equal 24V. Every component through which the 4-20mA loop
current passes develops a maximum voltage drop equal to that component’s
resistance multiplied by 0.020 Amperes (20mA). For example, referring to
Figure 2 the DMS-20PC-4/20S’s 250W resistance yields a maximum loop drop of :
250Ω x 0.020A = 5.0V
Transmitter Ratings
With the above loop-drop theory in mind, and assuming a +24V loop-powered
application in which the transmitter’s minimum operating voltage is 8V, and the
process monitor drops only 4V, a logical question which arises is what happens
to the “extra” 12V? The extra 12V has to be dropped entirely by the transmitter
since most process monitors have purely resistive inputs combined with zener
diodes that limit their maximum voltage drop.
Transmitters usually state both minimum and maximum operating voltages.
The minimum voltage is that which is required to ensure proper transmitter
operation, while the maximum voltage is determined by its maximum rated
power-dissipation, as well as by its semiconductors’ breakdown ratings. A
transmitter’s power dissipation can be determined by multiplying its loop drop
by the highest anticipated output current, usually, but not always, 20mA. For
example, if a transmitter drops 30V at an overrange output level of 30mA, its
power dissipation is:
30V x 0.030A = 0.9 watts
Wiring Resistance
Because copper wires exhibit a dc-resistance directly proportional to their
length and gauge (diameter), this application note would not be complete
without discussing the important topic of wiring—specifically the effects wiring
resistance has on overall system performance.
Applications in which two or more loop-monitoring devices are connected
over very long, 2-way wiring distances (1000-2000 feet) normally use +24V
supplies because many transmitters require a minimum 8V-supply for proper
operation. When this 8-volt minimum is added to the typical 3-4 volts dropped
by each process monitor and the 2-4 volts dropped in the system wiring and
interconnects, the required minimum supply voltage can easily exceed 16V.
The following worked-out example will illustrate these important concepts.
The voltage drop developed along a given length of wire is found by multiplying
the wire’s total resistance by the current passing through it. The wire’s total
resistance is found by looking up its resistance (usually expressed in Ohms per
250 7
Loop Drop = 250 7 x .020A = 5V
Figure 2. Calculating Loop Drops
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Application Note
1000 feet) in a wire specifications table. Referring to Figure 3 if a transmitter’s
output is delivered to a remote process monitor using 2000 feet (660 meters)
of 26-guage, solid copper wire having a resistance of 40.8Ω per 1000 feet, the
one-way voltage dropped by the wire when the transmitter’s output is 20mA is
equal to:
feet, yielding a total loop resistance (R) equal to 4000 feet x (40.8Ω /1000 feet)
= 163.2Ω. The total voltage dropped over the 4000 feet of wiring is therefore:
E = 0.020 Amperes x [2000 feet x (40.8Ω /1000 feet)]
Looking down the loop towards the remote process monitor, the transmitter
sees the sum of the 3.27V wire drop and the 5.0V process-monitor drop, for
a total loop-drop of 8.27V. If the transmitter itself requires a minimum of 8V
(this is also considered a voltage drop) for proper operation, the lowest power
supply voltage required for the system shown in Figure 3 is 16.3V.
E = 0.020A x 81.6Ω = 1.63V
However, the current must travel 2000 feet down to the process monitor and
another 2000 feet back to the transmitter’s “+” output terminal, for a total of
4000 feet. As noted above, 26-gauge wire has a resistance of 40.8Ω per 1000
E = 3.27V.
2000 feet (660 meters)
+ –
E = 0.020A x 163.2Ω
24 V dc
81.6 7
1.64 V
1.64 V
81.6 7
Figure 3. Wiring Resistance Effects
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