Current Loop

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Analog Electronics Instrumentation - Current Loops

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Controlleroutput currentloopsThe simplest formof 4-20 mA current loop is the type used to represent the output of aprocesscontroller, sending a command signalto a nal control element. Here, the controllerboth supplies the electrical poer and regulates the !Ccurrent to the nal control element,hich acts as an electrical load. To illustrate, consider the e"ample of a controller sending a4-20 mA signal to an #$% &current-to-pressure' signal con(erter, hich then pneumaticallydri(es a control (al(e)

This particular controller has to digital displays, one for process (ariable &%*' and one forsetpoint &+%', ith a bargraph for displaying the output (alue &ut'. ne pushbuttonpro(ides the operator ith a ay to sitch beteen Automatic and anual modes &A$',hile to other pushbuttons pro(ide means to decrement and increment either the setpoint(alue &in Automatic mode' or the utput (alue &in anual mode'.

#nside the controller, a dependent current source pro(ides the 4-20 mA !C current signal to

the #$% transducer. i/e all current sources, its purpose is to maintain current in the loop1circuit regardless of circuit resistance or any e"ternal (oltage sources. nli/e a constantcurrent source, a dependent1 current source &represented by a diamond shape instead of acircle shape' (aries its current (alue according to the dictates of some e"ternal stimulus. #nthis case, either the mathematical function of the controller &Automatic mode' or thearbitrary setting of the human operator &anual mode' tells the current source ho much!C current it should maintain in the circuit.

3or e"ample, if the operator happened to sitch the controller into anual mode and set theoutput (alue at 05, the proper amount of !C current for this signal percentage ould be
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62 mA &e"actly half-ay beteen 4 mA and 20 mA'. #f e(erything is or/ing properly, thecurrent in the loop1 circuit to the #$% transducer should remain e"actly at 62 mA regardlessof slight changes in ire resistance, #$% coil resistance, or anything else) the current sourceinside the controller ill ght1 as hard as it has to in order to maintain this set amount ofcurrent. This current, as it 7os through the ire coil of the #$% transducer mechanism,creates a magnetic eld inside the #$% to actuate the pneumatic mechanism and produce a 8

%+# pressure signal output to the control (al(e &8 %+# being e"actly half-ay beteen 9 %+#and 6 %+# in the 9-6 %+# signal standard range'. This should mo(e the control (al(e to thehalf-ay position.

The details of the controller:s internal current source are not terribly important. sually, itta/es the form of an operational ampliercircuit dri(en by the (oltage output of a !AC&!igital-to-Analog Con(erter'. The !AC con(erts a binarynumber &either from the controller:sautomatic calculations, or from the human operator:s manual setting' into a small !C(oltage, hich then commands the op-amp circuit to regulate output current at aproportional (alue.

The scenario is much the same if e replace the #$% and control (al(e ith a (ariable-speedmotor dri(e. 3rom the controller:s perspecti(e, the only di;erence it sees is a resisti(e loadinstead of an inducti(e load. The input resistance of the motor dri(e circuit con(erts the 4-20

mA signal into an analog(oltage signal &typically 6- *, but not alays'. This (oltage signalthen constitutes a command to the rest of the dri(e circuitry, telling it to modulate thepoer going to the electric motor in order to dri(e it at the desired speed)

4-ire &self-poered1' transmittercurrent loops

!C electric current signals may also be used to communicate process measurementinformation from transmitters to controllers, indicators, recorders, alarms, and other inputde(ices. The simplest form of 4-20 mA measurement loop is one here the transmitter hasto terminals for the 4-20 mA signal ires to connect, and to more terminals here apoer source connects. These transmitters arecalled 4-ire1 or self-poered. The current
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signal from the transmitter connects to the process (ariable input terminals of the controllerto complete the loop)

Typically, process controllers are not e


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The ob(ious disad(antage of this scheme is the re


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Here, the transmitter is not really a current source in the sense that a 4-ire transmitter is.#nstead, a 2-ire transmitter:s circuitry is designed to act as a current regulator, limitingcurrent in the series loop to a (alue representing the process measurement, hile relying ona remote source of poer to moti(ate current to 7o. %lease note the direction of the arroin the transmitter:s dependent current source symbol, and ho it relates to the (oltagepolarity mar/s. >efer bac/ to the illustration of a 4-ire transmitter circuit for comparison.

The current source1 in this loop-poered transmitter actually beha(es as an electrical load,hile the current source in the 4-ire transmitter functions as a true electrical source.

A loop-poered transmitter gets its operating poer from the minimum terminal (oltage andcurrent a(ailable at its to terminals. ?ith the typical source (oltage being 24 (olts !C, andthe ma"imum (oltage dropped across the controller:s 20 ohm resistor being (olts !C, thetransmitter should alays ha(e at least 68 (olts a(ailable at its terminals. @i(en the loerend of the 4-20 mA signal range, the transmitter should alays ha(e at least 4 mA of currentto run on. Thus, the transmitter ill alays ha(e a certain minimum amount of electricalpoer a(ailable on hich to operate, hile regulating current to signal the processmeasurement.

#nternally, the loop-poered transmitter circuitry loo/s something li/e this)

All sensing, scaling, and output conditioning circuitry inside the transmitter must be

designed to run on less then 4 mA of !C current, and at a modest terminal (oltage. #n order

to create loop currents e"ceeding 4 mA as the transmitter must do in order to spantheentire 4 to 20 milliamp signal range the transmitter circuitry uses a transistor to shunt

&bypass' e"tra current from one terminal to the other as needed to ma/e the total current

indicati(e of the process measurement. 3or e"ample, if the transmitter:s internal operating

current is only 9.B mA, and it must regulate loop current at a (alue of 6 mA to represent a

condition of D5 process measurement, the transistor ill bypass 62.2 mA of current.
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Early current-based industrial transmitters ere not capable of operating on such lo le(elsof electrical poer, and so used a di;erent current signal standard) 60 to 0 milliamps !C.oop poer supplies for these transmitters ranged upards of 80 (olts to pro(ide enoughpoer for the transmitter. +afety concerns made the 60-0 mA standard unsuitable for someindustrial installations, and modern microelectronic circuitry ith its reduced poerconsumption made the 4-20 mA standard practical for nearly all types of process

transmitters.

Troubleshooting Current Loops

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A fundamental principle in instrumentation system troubleshooting is that e(ery instrumenthas at least one input and at least one output, and that the output&s' should accuratelycorrespond to the input&s'. #f an instrument:s output is not properly corresponding to itsinput according to the instrument:s design function, there must be something rong iththat instrument. Consider the inputs and outputs of se(eral common instruments)

transmitters, controllers, indicators, and control (al(es. Each of these instruments ta/es in&input' data in some form, and generates &output' data in some form. #n any instrumentloop,1 the output of one instrument feeds into the input of the ne"t, such that information ispassed from one instrument to another. Fy intercepting the data communicated beteencomponents of an instrument system, e are able to locate and isolate faults. #n order toproperly understand the intercepted data, e must understand the inputs and outputs of therespecti(e instruments and the basicfunctions of those instruments. The folloingillustrations highlight inputs and outputs for instruments commonly found in controlsystems)
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#n order to chec/ for proper correspondence beteen instrument inputs and outputs, emust be able to use appropriate test e


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+ince the manually-operated bypass (al(e no performs the Gob that the automatic control(al(e used to, a human operator must remain posted at the bypass (al(e to carefullythrottle it and maintain control of the process.

Floc/ and bypass (al(es for a large gas 7o control (al(e may be seen in the folloingphotograph)


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#n consideration of the labor necessary to safely interrupt the current signal to a control(al(e in a li(e process, e see that the seemingly simple tas/ of connecting a milliammeterin series ith a 4-20 mA current signal is not as easy as it may rst appear. Fetter aysmust e"ist, no

Using a clamp-on milliammeter to measure loop current

ne better ay to measure a 4-20 mA signal ithout interrupting it is to do so magnetically,using a clamp-on milliammeter. odern Hall-e;ect sensors are sensiti(e and accurateenough to monitor the ea/ magnetic elds created by the passage of small !C currents inires. Ammeters using Hall-e;ect sensors ha(e are completely non-intrusi(e because theymerely clamp around the ire, ith no need to brea/1 the circuit. An e"ample of a such aclamp-on current meter is the 3lu/e model DD6, shon in this photograph)


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Iote ho this milliammeter not only registers loop current &9.8B mA as shon in thephotograph', but it also con(erts the milliamp (alue into a percentage of range, folloingthe 4 to 20 mA signal standard. ne disad(antage to be aare of for clamp-onmilliammeters is the susceptibility to errorfrom strong e"ternal magnetic elds. +teady
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magnetic elds &from permanent magnets or !C-poered electromagnets' may becompensated for by performing a =ero1 adGustment ith the instrument held in a similarorientation prior to measuring loop current through a ire.

Using test diodes to measure loop current

Another ay to measure a 4-20 mA signal ithout interrupting it in(ol(es the use of arectifying diode, originally installed in the loop circuit hen it as commissioned. A test1diode may be placed anyhere in series ithin the loop in such a ay that it ill be forard-biased. !uring normal operation, the diode ill drop appro"imately 0.D (olts, as is typical forany silicon rectifying diode hen forard biased. The folloing schematic diagram shossuch a diode installed in a 2-ire transmitter loop circuit)

#f someone connects a milliammeter in parallel ith this diode, hoe(er, the (ery lo inputresistance of the ammeters shorts past1 the diode and pre(ents any substantial (oltagedrop from forming across it. ?ithout the necessary forard (oltage drop, the diodee;ecti(ely turns o; and conducts 0 mA, lea(ing the entire loop current to pass through theammeter)

?hen the milliammeter is disconnected, the re


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The folloing photograph shos an e"ample of this on a >osemount model 906 di;erentialpressuretransmitter)

Iote the to test points labeled TE+T1 belo and to the right of the main scre terminalshere the loop iring attaches. Connecting an ammeter to these to test points allos fordirect measurement of the 4-20 mA current signal ithout ha(ing to un-do any ireconnections in the circuit.

Transmitters e


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#n electronics, such a precision resistor used for measuring current is often referred to as ashunt resistor. +hunt resistor (alues are commonly (ery small, for their purpose is to assistin current measurement ithout imposing undue (oltage drop ithin a circuit. #t is rare tond a 20 ohm resistor used strictly as a diagnostic shunt resistor, because the e"tra(oltage drop &6 to (olts, depending on the current signal le(el' may star(e1 loop-poeredinstruments of (oltage necessary to operate. +hunt resistor (alues as lo as 6 ohm may be

found installed in 4-20 mA current loops at strategic locations here technicians may needto measure loop current2.

Troubleshooting current loops with voltage measurements

#f neither component &diode nor shunt resistor' is pre-installed in the circuit, and if a Hall-e;ect &clamp-on' precision milliammeter is una(ailable, a technician may still perform usefultroubleshooting measurements using nothing but a !C (oltmeter. Here, hoe(er, one mustbe careful of ho to interpret these (oltage measurements, for they may not directlycorrespond to the loop current as as the case ith measurements ta/en in parallel ith theprecision resistor. Ta/e for e"ample this 4-20 mA loop here a controller sends a commandsignal to an #$% transducer)

There is no standardi=ed resistance (alue for #$% transducer coils, and so the amount of(oltage dropped across the #$% terminals for any gi(en amount of loop current ill be uni


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#f the #$% coil resistance is completely un/non, (oltage measurements become useless for


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reGection of poer supply (oltage changes means that the loop poer supply need not beregulated, and so in practice it rarely is.

This brings us to a common problem in loop-poered 4-20 mA transmitter circuits)maintaining suJcient operating (oltage at the transmitter terminals. >ecall that a loop-poered transmitter relies on the (oltage dropped across its terminals &combined ith acurrent of less than 4 mA' to poer its internal or/ings. This means the terminal (oltage

must not be alloed to dip belo a certain minimum (alue, or else the transmitter ill notha(e enough electrical poer to continue its normal operation. This ma/es it possible tostar(e1 the transmitter of (oltage if the loop poer supply (oltage is insuJcient, and$or ifthe loop resistance is e"cessi(e.

To illustrate ho this can be a problem, consider the folloing 4-20 mA measurement loop,here the controller supplies only 20 (olts !C to poer the loop, and an indicator is includedin the circuit to pro(ide operators ith eld-located indication of the transmitter:smeasurement)

The indicator contains its on 20 ohm resistor to pro(ide a 6- (olt signal for the metermechanism to sense. This means the total loop resistance is no 00 ohms &plus any ireresistance'. At full current &20 mA', this total resistance ill drop &at least' 60 (olts, lea(ing60 (olts or less at the transmitter terminals to poer the transmitter:s internal or/ings. 60(olts may not be enough for the transmitter to successfully operate, though. The >osemountmodel 906 pressure transmitter, for e"ample, re


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?hen a loop-poered transmitter is star(ed of (oltage, its beha(ior becomes erratic. This isespecially true of smart1 transmitters ith built-in microprocessor circuitry. #f the terminal(oltage dips belo the re


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Here, the loop iring is bro/en at the negati(e terminal of the loop-poered transmitter, and

the calibrator connected in series to measure current. #f this loop had a test diode installed,the calibrator could be connected in parallel ith the diode to achie(e the same function.Iote the polarity of the calibrator:s test leads in relation to the circuit being tested) thecalibrator is acting as an unpoered de(ice &a load rather than a source', ith the morepositi(e loop terminal connected to the calibrator:s red test lead and the more negati(eterminal connected to the blac/ test lead.

The same loop calibrator may be used to source &or dri(e' a 4-20 mA signal into anindicating instrument to test the function of that instrument independently. Here, e see theAlte/ calibrator used as a current source to send a 6.00 mA signal to the %* &process(ariable' input of the controller)


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Io transmitter need be included in this illustration, because the calibrator ta/es its place.Iote ho the calibrator is used here as an acti(e source of current rather than a passi(eload as it as in the last e"ample. The calibrator:s red test lead connects to the controller:spositi(e input terminal, hile the blac/ test lead connects to the negati(e terminal. The !Cpoer source inside the controller is not used for loop poer, because the calibrator insource1 mode pro(ides the necessary poer to dri(e current through the 20 ohm resistor.

An alternati(e method of sourcing a /non current signal into an indicating instrument thatpro(ides loop poer is to set the loop calibrator to a mode here it mimics the electricalbeha(ior of a loop-poered 2-ire transmitter. #n this mode, the calibrator ser(es to regulateloop current at a user-determined (alue, but it pro(ides no moti(ating (oltage to dri(e this

current. #nstead, it passi(ely relies on some e"ternal (oltage source in the loop circuit topro(ide the necessary electromoti(e force)


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Iote the polarity of the calibrator:s test leads in relation to the controller) the red test leadconnects to the positi(e loop poer terminal hile the blac/ lead connects to the positi(einput terminal. Here, the calibrator acts as a load, Gust as a loop-poered transmitter acts asan electrical load. The only source of electrical poer in this test circuit is the 24 *!C sourceinside the controller) the same one normally pro(iding energy to the circuit hen a loop-poered transmitter is connected.

This simulate transmitter mode is especially useful for testing a 4-20 mA loop at the end ofthe cable here the transmitter is physically located. After disconnecting the cable iresfrom the transmitter and re-connecting them to the loop calibrator &set to simulate1 mode',the calibrator may be used to simulate a transmitter measuring any (alue ithin itscalibrated range.

A legacy loop calibrator still familiar to many instrument technicians at the time of thisriting is the classic Transmation model 6040)


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ther e"amples of (intage loop calibrator technology include the Iassau model B00 &left'and the Fiddle *ersa-Cal &right')

A modern loop calibrator manufactured by 3lu/e is the model D0)


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?ith this calibrator, the measure, source, and simulate modes are accessed by repeatedlypushing a button, ith the current mode displayed on the screen)

NAMUR Signal Levels

Signal level Fault condition

utput L 9. mA +ensing transducer failed lo

9. mA M utput M 9.B mA +ensing transducer failed &detected' lo

9.B mA L utput M 4.0 mA easurement under-range

26.0 N utput O 20. mA easurement o(er-range

utput O 26.0 mA +ensing transducer failed high

4-20 mA process transmitters compliant ith the IA> recommendations for fault signalle(els ill limit their output signals beteen 9.B mA and less than 26 mA hen functioningproperly. +ignals lying outside this range indicate some form of failure has occurred ithinthe transmitter4. Any control system programmed to respond to these specic fault-induced


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current le(els may act upon them by forcing controllers into manual mode, initiatingshutdon procedures, or ta/ing some other form of safe action appropriate to the/noledge of a failed process transmitter.

Current Loop

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