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
4-20mA Current Loop
Configurations and FAQ's
Figure 1. Typical Connections for Single-Ended Transmitters Driving Single-Ended +24V Powered Meters.
Introduction
As described in DMS Application Note 20, a current loop
transmitter's primary function is to convert a sensor's output into a
proportional 4-20mA current which can then be fed to a remote process
monitor and/or controller. Transmitters are available with either floating
outputs (typically used in loop-powered applications) or with single-ended
outputs (typically used in locally-powered applications). This application
note will describe the key differences between loop- and locally-powered
configurations and depict some real-world schematics using DATEL's
process monitors.
Selecting a Process Monitor
When upgrading existing applications, the choice of which process
monitor type to use is normally dictated by the installed transmitter. For
new installations, factors to be considered when deciding which type of
transmitter and process monitor to select include the available supply
voltage (usually +5V, +12V, or +24V), the desired display technology
(LED or LCD), and the physical distance between the transmitter and
process monitor (ranging from several inches to, at times, thousands of
feet). Loop-powered devices, with their simple 2-wire hookup, usually
require higher supply voltages but are preferred in applications involving
a single supply and/or long wiring runs.
Process monitors which must be readable in direct sunlight should
use LCD displays; whereas indoor applications can usually take
advantage of an LED display's superior legibility. Outdoor applications
that must also be viewable at night or in dimly lit areas should use
locally-powered backlit LCD displays
Since loop-powered process monitors must function accurately
with currents between 4 and 20mA, low-power liquid crystal displays
(LCD's), similar to the type found in inexpensive hand-held calculators,
have traditionally been the only type used in loop-powered process-
monitors. However, the availability of super-efficient red LED displays,
specially selected and driven by proprietary circuits, has allowed
DATEL to offer the loop-powered DMS-20PC-4/20 Series, 3 digit,
LED-display process monitors. LED displays offer superior legibility,
all-angle viewing, and can be viewed in either bright or dimly-lit
environments.
If digit size is an important requirement, locally-powered LED-
display process monitors--with their two individual power supply
inputs--are the better choice. However, at the present time, large,
super-bright LED's can not be adequately illuminated with currents
much below 60mA. Because locally-powered process monitors' power
consumption is not limited to the 4-20mA current circulating in the loop,
they are readily available with large, 0.56 inch (14.2mm) high digits,
super-bright LED displays.
Single-Ended, Locally-Powered Transmitters and Process
Monitors
Figure 1 shows a 4-20mA transmitter driving DATEL's DMS-30PC-
4/20S-24RL locally-powered, single-ended input, process monitor. As
shown in the illustration, locally-powered process monitors have two
power-supply input terminals and two current-loop input terminals. In
most of these configurations, the supply (typically +24V) powers both
the transmitter and the process monitor, with the low side of the
DMS-30PC-4/20S-24RL
+
+V
24V GROUND
SINGLE-ENDED
TRANSMITTER
+24V
4-20mA
(TB1-2)
(TB1-1)
(TB2-1)
V (TB2-2)
+
+
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
Figure 3. Typical Connection for Loop-Powered Transmitters Driving Loop-Powered Process Monitors
DMS-30PC-4/20S-24RL-I
+V (TB2-1)
+
SINGLE-ENDED
TRANSMITTER
+24V
4-20mA
(TB1-1)
24V GROUND
PLC or OTHER
SINGLE-ENDED
DEVICE
+
+
+
(TB1-2)
V
(TB2-2)
4-20mA
transmitter's "" terminal connected to the system ground inside the
transmitter and process monitor. This type of ground-referenced
transmitter and process monitor input/output configuration is commonly
referred to as "single-ended".
Most single-ended input process monitors must be the first device
in the loop, i.e., the device electrically closest to the transmitter's
4-20mA "" output terminal since their "" input terminal is internally tied
to the system ground. Any additional devices in the loop must have
their "" 4-20mA input connection tied to the "+" terminal (TB1-1) of the
DMS-30PC-4/20S-24RL.
The above requirement poses an important restriction on these
additional loop devices in that they must either be powered from
separate, transformer-isolated power supplies, or they must be floating
loop-powered devices (described in next section). However, caution
must be exercised when using a single-ended transmitter with a floating
loop-powered process monitor. The transmitter must have enough
output compliance to accommodate the additional voltage drop imposed
by the loop-powered monitor.
Another option when using multiple series-connected process
monitors and controllers is to use devices which provide built-in
isolation between the loop current and the system ground. DATEL's
DMS-30PC-4/20S-24RL-I and DMS-40PC-4/20S-24RL-I process
monitors incorporate built-in dc/dc converters that effectively isolate
the process monitors' loop-input circuitry from the +24V system supply.
Figure 2 illustrates a typical wiring diagram for the DMS-30PC-4/20S-
24RL-I.
Loop-Powered Process Monitors and Transmitters
Loop-powered 4-20mA input process monitors and transmitters,
commonly referred to as "self-powered", obtain all of their operating
power directly from the current circulating in the loop. These loop-
powered devices are easily identified because they have only two input
connections: a "+" input where the loop current enters, and a "" input
where the loop current exists. Loop-powered process monitors produce
no drop in the magnitude of the 4-20mA current, but instead drop a
voltage (or "loop drop") equal to their loop resistance ( or "burden")
multiplied by the instantaneous loop current. DMS Applicaton Note 20
covers the topic of loop drops in greater detail.
Series-connected loop-powered transmitters similar the one shown
in Figure 3 modulate the two wires comprising their series loop with a 4-
20mA signal which is directly proportional to the sensor's output. As is
the case with single-ended transmitters, loop-powered transmitters also
DMS-30LCD-4/20S
+
LOOP-POWERED
TRANSMITTER
+
+24V
4-20mA
(TB1-1)
(TB1-2)
4-20mA
24V GROUND
Figure 2. Typical Connection for Isolated-Supply Meters in Series with an Auxillary Device
DMS APPLICATION NOTE
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
contain signal-conditioning electronics. However, unlike the single-ended
types which could draw power from the full +24V supply, loop-powered
transmitters must draw all of their operating power from currents as low
as 4mA--a condition which limits their use to relatively low-power
applications.
DATEL now offers loop-powered LCD-display process monitors
which feature super-low loop drops: from the 2.9V drop of the DMS-
40LCD-4/20 Series, to the 1.8V drop of the DMS-20LCD-4/20 Series.
In most applications, these low loop-drops allow three or more process
monitors to be used in a current loop that also uses a loop
controller/PLC.
4-20mA FAQ's (Frequently Asked Questions)
Question: Why does my process monitor's display reading stay
fixed at "000" as the 4-20mA signal is varied?
The most common cause for a process monitor not responding to
changes in the loop current occurrs when a single-ended transmitter is
driving two single-ended input process monitors (see Figure 4). The
second process monitor's grounded negative () input effectively shunts
the transmitter's output current around the first monitor, that is, no loop
current passes through the DMS-30PC-4/20s-24RL's input circuit. The
solution is to either power the second process monitor from an isolated
dc power supply or use one of DATEL's "-I" isolated-power process
monitors (see Figure 2).
Question: How do I hook up a +24V supply to a loop-powered
DMS-20LCD-4/20S?
You don't! The two input terminals on DMS-20LCD/4/20S can only be
connected to the modulated 4-20mA transmitter output current. The
system power supply's two "+" and "" output leads should never be
connected directly to a loop-powered device's "+" and "" input
terminals; doing so will place the full supply voltage (sometimes in
excess of 36Vdc!) across the monitor's input resistors. These input
resistors typically measure less than 30 Ohms, and can easily be
damaged by the resulting excessive power dissipation.
Question: What do the `S', `B', `P' and `I' 4/20 suffixes in DATEL's
4-20mA process monitor part numbers stand for?
The `S', `B', `P', and `I' suffixes in the 4/20 part number field are used to
denote the monitor's display-readings capabilities. For example, the
DMS-20PC-4/20S standard unipolar-reading model's gain and offset
adjustments are optimized to display "000" with a 4mA input, and up to
"1999" with a 20mA input. The DMS-20PC-4/2B bipolar reading model
is designed to display a negative number with 4mA, "000" with 12mA,
and a positive number with 20mA. The `4/20P' positive-reading model is
optimized to display positive numbers with all inputs between 4 and
20mA.
The inverse-reading `4/20I' model is designed to display inverse
readings, that is, a 4mA input will display a full-scale positive number,
while a 20mA input will display "000". The `4/20I' model uses the same
DIP-switch settings tables as its `4/20S' counterpart, the only difference
is its full-scale and zero display-readings are reversed.
Question: How does ambient light affect the selection of a
process monitor?
Process monitors which must be readable in direct sunlight should use
LCD displays, while indoor applications can usually take advantage of
an LED display's superior legibility. Outdoor applications that must also
be viewable at night, or in dimly lit areas, should use locally-powered
process monitors with backlit LCD displays.
At the present time, high-intensity LED's can not be illuminated
adequately with currents much below 60mA. Therefore, if display
legibility is an important requirement, locally-powered LED display
process monitors are the better choice for indoor applications since
their power consumption is not limited to the 4-20mA currents
circulating in the loop.
Figure 4. Where Did the Loop Current Go?
DMS-30PC-4/20S-24RL
+V (TB2-1)
+
SINGLE-ENDED
TRANSMITTER
+24V
4-20mA
(TB1-1)
24V GROUND
PLC or OTHER
SINGLE-ENDED
DEVICE
+
+
+
(TB1-2)
V
(TB2-2)
4-20mA
0V
DS-0471 03/2000
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