® ®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 Configurati ons.” 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 suc h as temperature, pres sure, 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: [email protected]• Internet: www.datel.co m DATEL makes no representation that the use of its products in the circuits described herein, or the use of other technical information contained herein, wi ll not infringe upon existing or future patent ri ghts. 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! A-PDF Merger DEMO : Purchase from www.A-PDF.com to remove the watermark
DATEL makes no representation that the use of its products in the circuits described herein, or the use of other technical information contained herein, wi ll not infringe upon existing or future patent ri ghts. The descriptions containedherein 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!
A-PDF Merger DEMO : Purchase from www.A-PDF.com to remove the watermark
DATEL makes no representation that the use of its products in the circuits described herein, or the use of other technical information contained herein, wi ll not infringe upon existing or future patent ri ghts. The descriptions containedherein 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 sincemost 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 Ω
81.6 Ω
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 currentpassing 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 40.8Ω 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 (40.8Ω /1000 feet)]
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 hasa resistance of 40.8Ω per 1000 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 = 0.020A x 163.2Ω
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
2.0 THE BASICS OF CURRENT LOOP..................32.1 Full-duplex 20 mA Circuit................................ ...... 32.2 The Simplex 20 mA Circuit................................ .... 42.3 Problems with 20 mA Current Loop ..................... 52.4 Current Regulation in Current Loops ................... 6
2.4.1 Constant Current Generator Current Source....... 7
2.4.2 Transmitter Current Limiter................................ . 82.4.3 Receiver Current Limiter................................ ..... 8
3.0 CURRENT LOOPINTERFACE CONNECTIONS ...................9
3.1 Connection to an Active Current Loop Port......... 93.2 Connection to a Passive Current Loop Port ...... 103.3 Interconnection of 2 Current Loop Converters .. 10
4.0 WHAT ISN’T DIGITAL CURRENT LOOP.......114.1 4 to 20 mA Analog Current Loop ........................ 114.2 HART® 4 to 20 mA Current Loops...................... 11
The purpose of this current loop application note is tointroduce the reader to the physical aspects of 20 mA current loopcommunications.
Until the early 1960’s, military teleprinters used 60 macurrent loops to communicate over long distances. In 1962, theModel 33 teletype was introduced and 20 mA current loop interfacesbecame widely used. Throughout the 60’s, 70’s, and early 80’s, 20mA current loop interfaces were applied in many types ofequipment. Current loop interfaces became popular at this timebecause they offered the most cost effective approach to long
distance, noise immune data transmission. The 20 mA current loopis suitable for distances to 2000 feet at data rates up to 19.2k baudwith careful attention to interface design. It can be used at longerdistances when data rates are as low as 300 baud.
When the EIA 422 Standard (December 1978) and the EIA485 Standard (April 1983) brought forth the application of balanceddifferential digital data transmission, the popularity of 20 mA current
Figure 1 is a full-duplex 20 mA current loop circuit.Simultaneous two-way communications is possible with this circuit.Two 20 mA current generators are necessary with this circuit. It ispossible to have one of the two current generators in one currentloop interface and the other current generator in the other interface.For example, the original IBM PC serial adapter card had a currentloop interface that contained only one current generator. When youmade a correct connection to this current loop interface, the second
current loop device would need to provide one current loopgenerator.
Figure 2 is a diagram of a simplex 20 mA current loopcircuit. The fundamental elements of a 20 mA current loop are a
current source, a current switch, and a current detector. Thetransmitter is the current switch and the receiver is the currentdetector. The interface that contains the current source is called theactive unit and all other units are referred to as passive units. Figure3 is a diagram of the levels in an RS-232 interface and how theyrelate to the presence and absence of current in a 20 mA currentloop circuit. In a 20 mA loop the current flows when the loop is idle(no data being transmitted). In a simplex type circuit a number of
transmitters and receivers are put in series in a current loop. Aslong as only one transmitter sends data, all receivers receive thedata.
Figure 3. Comparison of signal levels in an RS-232 Circuitand a 20-mA Current loop Circuit
MARK
SPACE
Transistion region
RS-232 Circuit
-15 volts
+15 vo l ts
+3 vo l ts
0 volts
- 3 volts
20-mA currentflows in circuit
Flow of 20-mA currenis interrupted
20-mA circuit
2.3 Problems with 20 mA Current Loop
The main problem with 20 mA current loop is that there isno mechanical or electrical standard defined for this interface. Thismakes every interface somewhat unique and the user must knowsome of the technical details about the circuits used in the interface.
Figure 4 is a simplified one-way current loop implemented
with two optocouplers, a voltage source, and a resistor. OptocouplerU1 is the transmitter and optocoupler U2 is the receiver. The valueof the loop current in this circuit is:
when input LED is conductingIf Vs = 12 volts & Rs = 470 ohms then
I loop = (12v - .2v - 1.8v)/470 ohms
I loop = 10v/470 ohms = 21.3 mA
If we changed Vs = 60 v and left Rs = 470 then
I loop = (60v -.2v -1.8v)/470 ohms = 123 mA
If we changed Vs = 5 v and left RS = 470 ohms then
I loop = (5v -.2v -1.8v)/470 ohms = 6.4 mA
The point of showing these different calculations is todemonstrate that the loop currents circuit can vary by considerableamounts, if Vs is varied. Likewise, if Rs was changed the loopcurrents could also vary considerably. The only way to determine
that currents are near 20 mA is to examine the circuit in detail.
2.4 Current Regulation in Current Loops
Several methods can be used to control the amount ofcurrent in a current loop circuit. This section will illustrate severalcommon methods of regulating the current in a current loop.
Figure 5 is a circuit that uses a linear voltage regulatorintegrated circuit to serve as a constant current source. Almost any
fixed or adjustable voltage regulator can be used. The exampleshown in Figure 6 uses an LM317 adjustable regulator because isprovides a low amount of voltage drop (3 volts) across the currentregulator circuit. For example, if Vs was 12 volts in this circuit, thenthe maximum voltage that the constant current regulator could drivewould be 9 volts. The 62 ohm, Rg resistor sets the regulator currentbecause there is an internal voltage reference in the LM317between VO and the ADJ pins of 1.25 volts.
I loop = 1.25/Rg = 1.25/62 = 20 mA
Figure 5. Constant Current Generatorfor a 20 mA Current Loop
VI
Vs I loop
LM317
VOAdj
Rg
62
Maximum output voltageequal to (Vs - 3v)
In a current loop, the sum of all the voltage drops across allthe devices must be less than the voltage source, Vs driving the
loop. Each device in the current loop whether it is a transmitter(current switch) or receiver (current detector) has some voltagedrop across it. For instance, a typical transistor switch can havetypically 0.2 volts drop across it. For most of B&B Electronicsconverters, the voltage drop across the transmitters can be asmuch 2.3 volts when the switch is turned ON. The reason for this isthat the transmitter switch must provide for the reverse bias of theinternal photo detector diode inside the optocoupler. An optocoupler
used as a current detector will have from 1.2 to 2.0 volts dropacross it.
Some current loop interfaces incorporate current limitinginto the transmitter (current switch) itself. Figure 6 is an example ofa circuit that has built-in current limiting so that the loop currentcannot exceed 20 mA. In this circuit Rg provides a source of biascurrent for Q2 so that if the loop current tries to exceed 20 mA Q2will shunt Q1 base bias current so that Q1 will not conduct morethan 20 mA.
Figure 7. Current Limiter builtinto Receiver
R-
Q1
R+
I emitter (max.) =
Rg
U1
Rg
.7
2.4.3 Receiver Current Limiter
The circuit shown in Figure 7 is used not to regulate the
loop current, but to regulate the maximum emitter current in theoptocoupler, U1. This is done because some optocouplers requireless than 20 mA to operate at maximum speed. Transistor Q1 isused to shunt some of the loop current around the emitter ofoptocoupler, U1.
To connect our converter to an existing current loop port,
you must first determine if the port is active or passive. What thismeans is: does the port have an internal power supply thatprovides the current (active) for the transmitter, the receiver, or both(transmitter and receiver). The simplest way to determine this is tobreak the loop (disconnect it) and see if there is any DC voltageacross the output or input pairs. If you have access to theinstruction manual for the unit you can also look in there for theinformation.
Current loop interfaces normally consist of four wires. Theyare usually labeled T+, T-, R+, and R-. T+ and T- are the transmitplus and transmit minus lines and data is output from that device onthose lines. The R+ and R- lines are the receive plus and receiveminus lines and data is input into that device on these lines.
Interconnection of the two current loop devices is differentdepending on whether your unit is active or passive.
3.1 Connection to an Active Current Loop Port
Connection to an active current loop port is very simple.Your units T+ and T- lines go to our units R+ and R- lines. And yourunits R+ and R- lines go to our units T+ and T- lines. See thefollowing drawing.
Note: The T+ & T- indent i f icat ion doesn ' t imply adirect connection across the transmitter .
Note: The R+ & R- indent i f icat ion doesn ' t imply adirect connection across the receiver.
Connection to a passive current loop port is a little harder.You must use a 12 VDC power supply with the 470 ohm resistorsinside of our converter to "create" a 20 ma current source. See the
following drawing.
+12VDC
Figure 9. Connection to a Passive Current Loop
470
R+
R- 23
25
22
RS232 TO CURRENTLOOP CONVERTER
470
T-19
21
9
6
T+ 14
YOUREQUIPMENT
T-
T+
PASSIVE CURRENTLOOP CONVERTER
R+
R-
3.3 Interconnection of Two Current Loop Converters
Interconnection of two B&B current loop converters alsorequires the use of a 12 VDC power supply since they are bothpassive port. See the following drawing.
Figure 10. Interconnection of Two Current Loop converters
The diagram shown in Figure 11 is an analog 4 to 20 mAcurrent loop. This circuit is mentioned here because it is sometimesconfused with 20 mA digital current loop. The purpose of 4 to 20 mAanalog current loop is to transmit the signal from an analog sensorover some distance in the form of current signal. Only two wires arerequired to send the analog signal and also supply power to thesensor. A loop supply voltage (24 volts in Figure 11) is used to
power the remote sensor. The remote sensor regulates the loopcurrent such that the loop current represents the value of theparameter being measured by the sensor. A series resistor RL atloop power supply converts this current to a voltage that can beused by the electronics to record or distribute the parameter beingmeasured.
may draw up to 4 ma
Cable to Sensor
Figure 11. 4 to 20 mA analog current loop
LoopSupply24 v
SensorOutputSignal
LR
V+ never drops below +12 v
Remotesensorelectronics
sensorCo m
V+
4.2 HART®®®® 4 to 20 mA Current Loops
Figure 12 is another example of a type of 4 to 20 mAcombined analog & digital current loop. This current loop uses
HART® Communications protocol. The HART® (HighwayAddressable Remote Transducer) protocol is used for SMART
remote transducers that are compatible with 4 to 20 mA analogcurrent loops but also have digital communications on the same twowires. This is accomplished by superimposing a two-toneFrequency Shift Keyed (FSK) digital current signal on the 4 to 20mA analog signal.
