2907675 the Basics of Current Loop

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DMS APPLICATION NOTE

4 -2 -2 0 m A Cu C u r r e n t L o o p Pr Pr i m e r Introduction 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.” Configurations.”

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.

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.

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!

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 such as temperature, pressure, pressure, speed, liquid flow rates, etc. Transmitting sensor information via a

PROCESS MONITOR/CONTROLLER

TRANSMITTER POWER SUPPLY +

SENSOR

+

–

–

4-20mA

+

–

–

+

Figure 1. Typical Components Used in a Loop Powered Application DATEL, Inc., Mansfield, MA 02048 (USA)

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Tel: (508)339-3000, (800)233-2765 Fax: (508)339-6356

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Email: [email protected]

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Internet: www.datel.com 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, 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.

DMS APPLICATION NOTE 20

Current Loop Components

Loop Drops

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

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 presents to the transmitter’s transmitter’s output driver. Most transmitter’s data sheets specify the maximum loop resistance the


Current Loop Components

Loop Drops

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).

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 presents to the transmitter’s 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).

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.

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=IxR 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 250 Ω resistance yields a maximum loop drop of :

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.

250Ω x 0.020A = 5.0V

DMS-20PC-4/20S

–

–

20mA 5V

250 Ω +

+

Loop Drop = 250

Ω

x .020A = 5V

Figure 2. Calculating Loop Drops

DATEL, Inc., Mansfield, MA 02048 (USA)

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Tel: (508)339-3000, (800)233-2765 Fax: (508)339-6356

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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?

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


Transmitter Ratings

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.

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.

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 Ω per 1000 feet, the one-way voltage dropped by the wire when the transmitter’s output is 20mA is equal to:

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:

E = 0.020 Amperes x [2000 feet x (40.8 Ω /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 has a resistance of 40.8 Ω per 1000 feet, yielding a total loop resistance (R) equal to 4000 feet x (40.8 Ω /1000 feet) = 163.2 Ω. The total voltage dropped over the 4000 feet of wiring is therefore:

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.

E = 0.020A x 163.2 Ω 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.

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

2000 feet (660 meters)

TRANSMITTER

PROCESS MONITOR

POWER SUPPLY +

+

–

81.6 Ω

SENSOR

20mA 8V(min.)

–

1.64 V

24 V dc

5V 20mA

1.64 V +

–

–

+

–

+

–

81.6 Ω

+

Figure 3. Wiring Resistance Effects

DATEL, Inc., Mansfield, MA 02048 (USA)

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Tel: (508)339-3000, (800)233-2765 Fax: (508)339-6356

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Current Loop Application Note Document No. CLAN1495

International Headquarter B&B Electronics Electronics Mfg. Co. C o. Inc. 707 Dayton Dayton Road - - P.O. Box Box 1040 -- Ottawa, Ottawa , IL 61350 USA Phone (815) 433-5100 -- General Fax (815) 433-5105 Home Page: www.bb-elec.com www.bb-ele c.com Sales e-mail: [email protected] -- Fax (815) 433-5109 Technical Support e-mail: [email protected] -- Fax (815) 433-5104

European Headquarter B&B Electronics Ltd. W estlink Commercial Park, Oranmore, Co. Galway, Galway, Ireland Phone (353) 91 792444 79244 4 -- Fax (353) 91 792445 79244 5 Home Page: www.bb-europe.com Sales e-mail: [email protected] Technical Support e-mail: [email protected]

Copyright 1995 B&B Electronics -- January 1995

Current Loop Application Note 1495

Cover Page

Copyright 1995 B&B Electronics -- January 1995 B&B ELECTRONICS MFG CO – 707 DAYTON RD - PO BOX 1040 - OTTAWA IL 61350 - PH 815-433-5100 - FAX 815-433-5104 B&B ELECTRONIC S LTD – WESTLINK COMM. PK – ORANMORE, GALWAY, IRELAND – PH 353-91-792444 – FAX 353-91-792445

TABLE OF CONTENTS 1.0 1. 0

INTR IN TROD ODUC UCTIO TION N .... ...... .... .... .... .... .... .... .... .... .... .... .... .... ....... ....... .... .... .... .... ....1 ..1

2.0 2. 0

THE BA BASI SICS CS OF CUR CURREN RENT T LOO LOOP.. P.... .... .... .... .... .... .... ....3 ..3 2.1 Full-duplex Full-duplex 20 mA Circuit........... Circuit................. ............ ............ ......... ... ...... 3 2.2 The Simplex 20 mA Circuit................ Circuit..................... ........... ........... ..... .... 4 2.3 Problems Problems with 20 mA Current Loop ..... ......... ......... .......... ....... .. 5 2.4 Current Current Regulation Regulation in Current Current Loops ..... .......... .......... ......... .... 6 2.4.1 Constant Constant Current Current Genera Generator tor Current Current Source Source ..... ....... .. 7 2.4.2 Transmitt Transmitter er Current Current Limi Limiter ter ...... ........... ........... ........... ........... .......... .... . 8 2.4.3 Receiver Receiver Current Current Limiter Limiter...... ............ ............ ............ ............ ........ .. ..... 8

3.0

CURRENT LOOP INTERFACE INTERFACE CONNECTIONS CONNECTIONS ..... .......... ........... .........9 ...9 3.1 Connection Connection to to an Active Active Current Loop Port..... Port ......... .... 9 3.2 Connection Connection to a Passive Passive Current Loop Port ...... 10 3.3 Interconnecti Interconnection on of 2 Current Current Loop Converte Converters rs .. 10

4.0 4. 0

WHAT WH AT IS ISN’ N’T T DIGI DIGITA TAL L CUR CURRE RENT NT LOOP... LOOP..... .... ..11 11 4.1 4 to 20 mA Analog Current Current Loop ..... .......... .......... ......... ......... ..... 11 4.2 HART® 4 to 20 mA Current Current Loops Loops ..... .......... ......... ......... ........ ... 11


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1.0 Introduction The purpose of this current loop application note is to introduce the reader to the physical aspects of 20 mA current loop communications. Until the early 1960’s, military teleprinters teleprinters used 60 ma current loops to communicate over long distances. In 1962, the Model 33 teletype was introduced and 20 mA current loop interfaces became widely widely used. Throughout the 60’s, 70’s, and early 80’s, 20 mA current loop interfaces were applied in many types of equipment. Current loop interfaces became popular at this time because they offered the most cost effective approach to long distance, noise immune data transmission. The 20 mA current loop is suitable for distances to 2000 feet at data rates up to 19.2k baud with careful attention to interface design. It can be used at longer distances when data rates are as low as 300 baud. When the EIA 422 Standard (December (December 1978) and the EIA 485 Standard (April 1983) brought forth the application of balanced differential digital data transmission, the popularity of 20 mA current loop rapidly diminished.


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2.0 The Basics of of Current Loop 2.1 Full-duplex 20 mA Circuit 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 is possible to have one of the two current generators in one current loop interface and the other current generator g enerator in the other interface. For example, the original IBM PC serial adapter card had a current loop interface that contained only one current generator. When you made a correct connection to this current loop interface, the second current loop device would need to provide one current loop generator.

Receiver Transmitter 20 ma current loop generator

Receiver

Transmitter 20 ma current loop generator Active side

Passive side

Figure 1. Full duplex 20 ma ma circuit

2



2.2 The Simplex Simplex 20 mA Circuit Figure 2 is a diagram of a simplex 20 mA current loop circuit. The fundamental elements of a 20 mA current loop are a current source, a current switch, switch, and a current detector. The transmitter is the current switch and the receiver is the current detector. The interface that contains the current source is called the active unit and all other units are referred to as passive units. Figure 3 is a diagram of the levels in an RS-232 interface and how they relate to the presence and absence of current in a 20 mA current loop circuit. In a 20 mA loop the current flows when w hen the loop is idle (no data being transmitted). transmitted). In a simplex type circuit a number of transmitters and receivers are put in series in a current loop. As long as only one transmitter sends data, all receivers receive the data. (Only one device can transmit at a time) Receiver

Transmitter (Current is ON when data is not transmitted)

Receiver

Transmitter (Current is ON when data is not transmitted)

passive side 20 ma current loop generator active side Figure 2. Simplex 20 mA Circuit


3


+15 volts

Flow of 20-mA curren is interrupted

SPACE +3 volts 0 volts - 3 volts

Transistion region

MARK

20-mA current flows in circuit

RS-232 Circuit

20-mA circuit

-15 volts

Figure 3. Comparison of signal levels in an RS-232 Circuit and a 20-mA Current loop Circuit

2.3 Problems with 20 mA Current Loop The main problem with 20 mA current loop is that there is no mechanical or electrical standard defined defined for for this interface. This makes every interface somewhat unique and the user must know some of the technical details about the circuits used in the interface.

Data In

U1

T+

T+

TR+

U2

Data Out

Current switch (Transmitter) Vs

Current generator

Current detector (Receiver)

Rs R-

Figure 4. Simplified one direction current loop

Figure 4 is a simplified one-way current loop implemented with two optocouplers, op tocouplers, a voltage source, and a resistor. Optocoupler Optocoupler U1 is the transmitter and optocoupler U2 is the receiver. The value of the loop current in this circuit is:

4



I loop = (Vs - V transmitter tr ansmitter -V receiver)/Rs for typical optocouplers V transmitter (U1) = .2 v when turned ON V receiver

(U2) = 1.8 v

when input LED is conducting If 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 to demonstrate that the loop currents circuit can vary by considerable amounts, if Vs is varied. varied. Likewise, Likewise, ifif Rs was changed the loop currents could 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 of current in a current loop circuit. This section will illustrate several common methods of regulating regulating the current in a current loop.


5


2.4.1 Constant Current Current Generator Generator Current Current Source Figure 5 is a circuit that uses a linear voltage regulator integrated circuit circuit to serve as a constant current source. Almost any fixed or adjustable voltage voltage regulator can be used. The example shown in Figure 6 uses an LM317 adjustable regulator because is provides a low amount of voltage drop (3 volts) across the current regulator circuit. circuit. For example, if Vs was 12 volts in this circuit, then the maximum voltage that the constant current regulator could drive would be 9 volts. The 62 ohm, Rg resistor sets the regulator current because there is an internal voltage reference in the LM317 between VO and the ADJ pins of 1.25 volts. LM317 VI

Adj V O

Rg 62

I loop = 1.25/Rg = 1.25/62 = 20 mA Vs

I loop Maximum output voltage equal to (Vs - 3v)

Figure 5. Constant Current Generator for a 20 mA Current Loop In a current loop, the sum of all the voltage drops across all the 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 voltage drop across it. For instance, a typical transistor switch can have typically 0.2 volts drop across it. For most of B&B Electronics converters, the voltage drop across the transmitters can be as much 2.3 volts when the switch is turned ON. The reason for this is that the transmitter switch must provide for the reverse bias of the internal photo detector d etector diode inside the optocoupler. An optocoupler used as a current detector will have from 1.2 to 2.0 volts drop across it.

6



T+ Rb U1

Q1

I loop max. =

Q2 Rg

.7 Rg

T-

Figure 6. Current Limiter built into Tranmitter 2.4.2 Transmitter Current Limiter Some current loop interfaces incorporate current limiting into the transmitter (current switch) switch) itself. Figure 6 is an example of a circuit that has built-in current limiting so that the loop current cannot exceed 20 mA. In this circuit Rg provides a source of bias current for Q2 Q2 so that if the loop current tries to exceed exceed 20 mA Q2 will shunt Q1 base bias current so that Q1 will not conduct more than 20 mA. R+

U1

I emitter (max.) =

.7 Rg

Q1 Rg R-

Figure 7. Current Limiter built into Receiver 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 the optocoupler, U1. This is done because some optocouplers require less than 20 mA to operate at maximum speed. Transistor Q1 is used to shunt some of the loop current around the emitter of optocoupler, optocoupler, U1. Current Loop Application Note 1495

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3.0 Current Loop Interface Connections To connect our converter to an existing current loop port, you must first determine if the port is active active or passive. What this means is: is: does the port have an internal power supply that provides the current (active) for the transmitter, the receiver, or both (transmitter and receiver). The simplest way to determine this is to break the loop (disconnect it) and see if there is any DC voltage across the output or input pairs. pairs. If you have access access to the instruction manual for the unit you can also also look in there for the information. Current loop interfaces interfaces normally consist of four wires. They are usually labeled T+, T-, R+, and R-. T+ and TT- are the transmit transmit plus and transmit minus lines lines and data is output from that device on those lines. The R+ and R- lines are the receive receive plus plus and receive minus lines lines and data is input into that device on these lines. Interconnection of the two current loop devices is different depending 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 your units R+ and R- lines go to our units u nits T+ and T- lines. See the following drawing. N o t e : T h e R+ R+ & R - i n d e n t i f i c a t io io n d o e s n ' t i m pl pl y a direct connection across the receiver.

T+

RS-232 to Current Loop Converter

TR+ R-

Vs

R+ RT+

Vs

T-

Active Current Loop Port N o t e : T h e T+ T+ & T - i n d e n t i f i c a t i o n d o e s n ' t i m p l y a direct connection across the transmitter.

Figure 8. Connection to an Active Current Loop

8



3.2 Connection to a Passive Current Loop Port Connection to a passive current loop port is a little harder. You must use a 12 VDC power supply with the 470 ohm resistors inside of our converter to "create" a 20 ma current source. See the following drawing. RS232 TO CURRENT LOOP CONVERTER T+ T-

PASSIVE CURRENT LOOP CONVERTER R+

14 19

R6

470

9

+12VDC

21 470

22

R+ R-

T+

25

TYOUR EQUIPMENT

23

Figure 9. Connection to a Passive Current Loop

3.3 Interconnection of Two Current Cu rrent Loop Converters Interconnection of two B&B current loop converters also requires the use of a 12 VDC power supply since they are both passive port. See the following drawing. T+ T-

14

14

19

19

6 470

9

T+ T-

6 +12VDC

9

470

21

21

22

22

R+

25

25

R+

R-

23

23

R-

470

470

Figure 10. Interconnection of Two Current Loop converters


9


4.0 What Isn’t Digital Current Loop 4.1 4 to 20 mA Analog Current Loop The diagram shown in Figure 11 is an analog 4 to 20 mA current loop. This circuit is mentioned here because it is sometimes confused with 20 mA digital current loop. The purpose of 4 to 20 mA analog current loop is to transmit the signal s ignal from an analog sensor over some distance in the form of current signal. Only two wires are required to send the analog signal and also supply power to the sensor. A loop supply voltage (24 volts in Figure 11) is used to power the remote sensor. The remote sensor regulates the loop current such that the loop current represents the value of the parameter being measured by the sensor. A series resistor RL at loop power supply converts this current to a voltage that can be used by the electronics to record or distribute the parameter being measured. V+ never drops below +12 v V+

Loop Supply 24 v

Cable to Sensor

Remote sensor electronics Co m

Sensor Output Signal

R

sensor

L

may draw up to 4 ma

Figure 11. 4 to 20 mA analog current loop 4.2 HART® 4 to 20 mA Current Loops Figure 12 is another example example of a type of 4 to 20 mA combined analog analog & digital current loop. This current loop uses HART® Communications protocol. The HART® (Highway Addressable Addressable Remote Transducer) protocol is used for SMART remote transducers that are compatible with 4 to 20 mA analog current loops but also have digital communications on the same two wires. This is accomplished by superimposing a two-tone Frequency Shift Keyed (FSK) digital current signal on the 4 to 20 mA analog signal. 10



Control system Remote Smart sensor with H A R T c o m m u n i c a ti ti o n s protocol

Cable to Sensor

4 to 20 ma analog plus digital communications

sensor +0.5mA Analog Signal

0

Frequency Shift Keying (FSK) Modulation used to add digital information to the analog 4 to 20 ma signal

-0.5mA 1,200 Hz "1"

2,200 Hz "0"

Figure 12. 12. 4 to 20 20 mA analog analog current current loop with digital communications using HART protocol


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2907675 the Basics of Current Loop

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