DMS APPLICATION NOTE
Introduction
This application note's primary goal is to provide an easy-to-
understand 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 locally-
powered 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
4-20mA Current Loop Primer
Figure 1. Typical Components Used in a Loop Powered Application
TRANSMITTER
SENSOR
+
+
POWER SUPPLY
PROCESS MONITOR/CONTROLLER
+
+
4-20mA
DATEL, Inc., Mansfield, MA 02048 (USA)
Tel: (508)339-3000, (800)233-2765 Fax: (508)339-6356
Email: sales@datel.com
Internet: www.datel.com
DATEL makes no representation that the use of its products in the circuits described herein, or the use of other technical information contained herein, will not infringe upon existing or future patent rights. The descriptions contained
herein do not imply the granting of licenses to make, use, or sell equipment constructed in accordance therewith. Specifications are subject to change without notice. The DATEL logo is a registered DATEL, Inc. trademark.
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 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!
DATEL, Inc., Mansfield, MA 02048 (USA)
Tel: (508)339-3000, (800)233-2765 Fax: (508)339-6356
Email: sales@datel.com
Internet: www.datel.com
DATEL makes no representation that the use of its products in the circuits described herein, or the use of other technical information contained herein, will not infringe upon existing or future patent rights. The descriptions contained
herein do not imply the granting of licenses to make, use, or sell equipment constructed in accordance therewith. Specifications are subject to change without notice. The DATEL logo is a registered DATEL, Inc. trademark.
DMS APPLICATION NOTE 20
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 (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.
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 electrome-
chanical 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:
E = I x R
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
`
' 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 250
resistance yields a maximum
loop drop of :
250
x 0.020A = 5.0V
Figure 2. Calculating Loop Drops
DMS-20PC-4/20S
20mA
250
+
+
Loop Drop = 250
x .020A = 5V
5V
DMS APPLICATION NOTE 20
DATEL, Inc., Mansfield, MA 02048 (USA)
Tel: (508)339-3000, (800)233-2765 Fax: (508)339-6356
Email: sales@datel.com
Internet: www.datel.com
DATEL makes no representation that the use of its products in the circuits described herein, or the use of other technical information contained herein, will not infringe upon existing or future patent rights. The descriptions contained
herein do not imply the granting of licenses to make, use, or sell equipment constructed in accordance therewith. Specifications are subject to change without notice. The DATEL logo is a registered DATEL, Inc. trademark.
Figure 3. Wiring Resistance Effects
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 dissipa-
tion 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 propor-
tional 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
TRANSMITTER
SENSOR
+
20mA
20mA
+
POWER SUPPLY
PROCESS MONITOR
+
81.6
81.6
8V(min.)
24 V dc
1.64 V
1.64 V
+
5V
+
+
2000 feet (660 meters)
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 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 40.8
per
1000 feet, the one-way voltage dropped by the wire when the
transmitter's output is 20mA is equal to:
E = 0.020 Amperes x [2000 feet x (40.8
/1000 feet)]
E = 0.020A x 81.6
= 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 40.8
per 1000 feet, yielding a total loop resistance
(R) equal to 4000 feet x (40.8
/1000 feet) = 163.2
. The total
voltage dropped over the 4000 feet of wiring is therefore:
E = 0.020A x 163.2
E
= 3.27V.
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.
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