OPToCOUPTER

Learn'to ùie or¡ocouplers in círcuÍts that require highelectrical isolation between input and,,otrtput,

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RAY M. MARSTON

oFrroc ouPLEBs -oR OPTOISOLATORShave applications in many sit-uations where signals or datamust pass between two cir:cuits, but high eleciriòalrisola-tion must be maintained be.tween those circuits. Optocoup-ling devices are useful inchangi.ng logic levels betweenthe'circuits, blocking noisetransmission fiom one ciicuitto another, isolating logic levelsfrom ACline voltage, and elim-inating gfound loops.

DC level as well as signal in-formation can be transmittedbyan optocouplerwhile it main-tains the high electrical isola-tion between input and output.Optocouplers can also replacerelays and transformers inmany digital interfaces. More-oven the frequency response ofoptocouplers is excellent in ana-log circuits.

Optocoupler basics.An optocoupler consists ofan

infrared-emitting LED (typ-ically made from gallium arse-nide) optically coupled to asilicon photodetector (photo-transistor, pþotodiode or otherphotosensitive device) in anopaque light-shielding pack-age. Figure I is a cutaway viewof a popular single-channel, six-pin dual-in-line-(DIP) packagedoptocoupler. The IR-emittingLED or IRED emits infrared ra-diation in the 9OO- to g4O-nanometer region when for-ward biased current flowsthrough it. The photodetector isan NPN phototransistor sen-sitive in the same 9OO- to 940-nanometer region. Both IREDand phototransistor are in chipor die form.

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Most commercial opto-c_ouqlers are made by mountingthe IRED and phototransistoion adjacent arms of a lead-frame, as shown. The leadframeis a stamping made from thinconductive sheet metal withmany branch-like contours.The isolated substrates thatsupport the device chips areformed from the innerbranches, and the multiple pinsof the DIP are formed from theouter branches.

After the wire bonds are made

between the device dies and ap-propriate leadframe pins, theregion around both devices isencapsulated in an lR-trans-parelt resin that acts as af'lightpipe" or optical waveguide be-tween the devices. The assem-bly is then molded in opaqueepolcy resin to form the DIE andthe leadframe pins are bentdownward.

Figure 2 is a pin diagram ofthe most popular singlè-chan-nel, 6-pin pþototransistor op-tocoupler DIP It is called.an

irã.''¡-ioÉ' ùiÈù'èði;äiìò" "r "phototransistor-output optocoupler.

optocoupler because only in-frared energy or photons couplethe input IRED to the outputphototransistor. The device isalso an optoisolator because noelectric current passes betweenthe two chips; the emitter anddetector are electrically insu-lâted and isolated. These de-vices are also known as photo-cq-uplei rûr photon-couplèd iso-lators.,., ', , ,

, 'The,base terminal of the pho-

totransiqtor is available at pin 6on ,:thq, six.p-in, DIP ' but , in nor-maf ,ùôs,tt,iå left n¡..r.cireuíted.Alço,'no iconnectión {NC)',isrn4de. to,þin'Ê. The phototran-sistorcan be converted to apho-todiode by shorting togetherbasçr Þin'6 an{ emitte{: pin 4.That oþtion is,not, availablé, infour-pin optocoupler DIP's and

multi-channel optocouplers.There are, however, photodiode-output optocouplerg optimizedfor the wider bandwidth andhigher speeds needed in datacommunications, but they arefar less efficient as couplers.

Large-volume producers ofcommercial optocouplers in-clude Motorola, Sharp Elec-tronics Corp., and SiemensComponents, Inc. Optek Tech-nologr concentrates on optoin-terrupters and optoreflectorswhile Hewlett-Packard's op-tocouplers are focused on high-speed communications andspecial applications.

Optocoupler characteristicsOne of the most important

characteristics of the op-tocoupler is its light-couplingefficiency specified as c,urrenttransfer ratÍo. CTR. That ratiois maximized by matching theIRED's IR emission spectrumclosely with its detector/outputdevice's detection spectrum.CTR is the ratio of output cur-çnt to input current, at a spec-'ífledblas, of an optocoupler. It is.giieúqs a percent:,QTR.=,[Ià'Õ]/(IF) x lOO%r:: {.,Ç},ft sf TAOe/o provides anouþut cur,r-e-nt'of 1 :milliampere

for each milliampere of currentto the IRED. Minimum values ofCTR for a phototransistor-out-put optocoupler such as thatshown in Figs. I and 2 can be'expected to vary from 20 to lOO%. CTR depends on the inputand output operating currentsand on the phototransistor'ssupply voltage.

Figure 3 is a plot of phototran-sistor output current (l.r) vs.input current (lo) for a typicalphototransistor optocoupler ata collector-to-Lrase voltagie (V"")of 10 volts.

Other important optocouplerspecifications include:. Isolation voltage (V,ro). Themaximum permissible AC volt-age that can exist between theinput and output circuits with-out destruction of the device.Those values typically rangefrom 5OO volts to 5 kilovoltsRMS for a phototransistor-out-put coupler.¡ Vce. The maximum DC volt-age permitted across the pho-totransistor output. Tlpicalvalues for a phototransistor -

output coupler range from 3O to70 volts.o Ie The maximum continuousDC forward current permittedto flq¡y.-.l,.qJhe.IRED. Tþical val-ues fdr â'.phototrânsistor-out,put coupler range from 40 tolOO milliamperes.e Rise/fall time for a phototran-;sÉtor qqtput coupîer is typ-icalllr from 2 to 5 microsecondsfcr both rise and fall. Those de-termine'device bahdwidth.

Industry-standards',,4 widé varie$¡ of optocouplers

,is :produced, b5r man1¡ manufac.turers throughout the world.Some of thq suþÞlie¡s of corn-modity optocouplers includeMotorolà,'. Shâfp Electronics,;,Toshiba, and Siemens. In addi-tiôn:'to :thç. indust{y, stáldârdsi¡;pin DIP shown, in, FigÊ,,l.and2, some transistor-output op-

, lqeouplers arè,þ acka$-èd in,Qu.r.,pin DIPts, and surface-mountpackages.:,, Mtltiì-É.annel, eonfigurâtlonsof , the Þop,ula! :ojrJo çquÞlerl are

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Fig. 2 except that they lack ex-ternalbase pins. It is importantto note, however, that certainelectrical and thermal charac-teristics are derated in thosepackages because of the closerspacing of the semiconductordies.

The lowest cost industry-standard photot¡ansistor op-tocouplers with single g^ha_nnçlshave been desi$ñäted by theJÐDEC prefix "4N" and includethe 4N25 to 4N28 and 4N35 to4N37. However, many suppliershave developed their own pro-prietaryparts with unusual fea-tures which are sold under theirown designations. Popular pho-totransistor optocouplers arenow available in small quan-tities for less than a dollar each.

Because optocouplers areused in AC-line powered cir-cuits, they are subject to safetytests such as those of Under-writers Laboratories Inc. (UL)and Canadian Standards As-sosiation (CSA). Most suppliersare offering Ul-Recognized op-tocouplers and manymake cou-plers that conform to the tighterVerband Deutsch Electro-techniker (VDE) specifications.Compliance with those specifi-cations or the equivalent na-tional specifications is a man-datory requirement for theiruse in Europe.

Figure 4 illustrates a simpleoptocoupler circuit. The ç_o¡r_-ductlo,g current of thç phg-totransistor can be contri¡lledby the forward blas cuiiènf ofthe IRED although the two de-vices are separated. When Sl isopen no current flows in theIRÐD so no infrared energy fallson the phototransistor, makingit a virtual open-circuit withzero voltage developed across

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output resistor R2. When Sl isclosed, current flows throughthe IRED and Rl, and the result-ing IR emission on the.pho-totransistor causes it to con-

I duct and generate an outputvoltage across R2.

The simple optically-coupledcircuit shown in Fig. 4 will re-spond gqly to,- qq:off-s_ignals-, butit can be modified to accept ana-log input signals and provideanalog output signals as will beseen later. The pho-totra4qjstgrprovides output gain,

The schematics of six otheroptocouplers with differentcombinations of IRED and out-put photodetector are presentedas Figs. 5 thru 10. Figure 5 is aschematic for a bidirectional-input phototransistor-outputoptocoupler with two back-to-back gallium-arsenide IRED'sfor coupling AC signals or re-verse polarity input protection.A typical minimum CTRfor thisdevice is 2O"/".

Figure 6 illustrates an opto-coupler with a silicon pho-todarlington amplifier output.It provides a higher output cur-rent than tháf ávailable from aphototransistor coupler. Be-cause of their high curientgain, Photodarlington couplerstypically have minimum 5OO7oCTR's at a collector-to-emittervoltage of 3O to 35 volts. Thisvalue is about ten times that of aphototransistor optocoupler.

However, there is a speed-out-put current tradeoff whenusing a photodarlington cou-pler. Effective bqLndwit-h.- is r.e.Quced by about a factor of ten.Industry standard versions ofthose devices include the 4N29to 4N33 and 6N138 and 6N139.Dual- and quad-channel pho-todarlington couplers are alsoavailable.

The schematic of Fig. 7 illus-trates a bi-directional linear-output dÞtõCôïpiéi ðöns istingof an IRED and a MOSFET.Those couplers typically haveisolation voltages of 25OO voltsRMS, breakdown voltages of 15to 3O volts, and typical rise andfall times of 15 microsecondseach.

Figure 8 is the schematic forone of two basic types of -op_t_o-tþytþL_qr-sl+tp,¡¿t optocouplers,

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FlG. 8-PHOTOSCR-OUTPUT opto-coupler schematic.

FlG. 9-PHOTOTRIAC-OUTPUT opto-coupler sÇhemat¡c.

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FIG. 14_PHOTOTRANSISTOR-TO-photodiode conversion with base pinconnection.

one with an SCR output. Op-toSCR couplers have typical iso-lation voltages of 1O0O to 4OOOvolts RMS, minimum blockingvoltages of 2OO to 4OO volts, andmaximum turn-on currents(I.-.) of 1O milliamperes. Theschematic in Fig. 9 illustrates aphoJotriaç-ou tput coupler. Thy-ristor-output couplers typicallyhave forward blocking voltages(Vo*r) of 4OO volts.

Schmitt-trigger outputs areavailable from optocouplers.Figure 10 is the schematic foran optocoupler that includes aSchmitt,trigger IC capable ofp1'g-ducing a reçtangular ogtputfþm a sine-wave or þulsèd in-þut signal. The IC is'a form ofmultivibrator circuit. Isolationvoltages are from 25OO to 4OOOvolts, maximum, turn-on cur-rent is typically from I to tO mil-liamperes, the minimum andmaximum operating voltagesare 3 to 26 volts, and the max-imum data rate (NRZ) is l MHz.

Coupler applicationsOptocouplers function in cir-

cuits the same way as discreteemitters and detectors. The in-put current to the optocoupler'sIRED must be limited with a se-ries-connected external resistorwhich can be connected in oneof the two ways shown in Fig lO,

either on the anode sfde lal orcathode side lb) of the IRED.

Figure 5 showed an op-tocoupler optimized for AC op,eration, but a conventionalphototransistor coupler canalso be driven from an ACsource with the addition of anexternal conventional diode asshown in Fig. 12. That circuitalso provides protection for theIRED if there is a possibility

that a reveise voltage could beapplied accidentally across theIRED.

The operating p..1¿¡¡çnt of thecoupler's photoLransistor canbe conver-ted to avoltage byplac-ing àn éitérnáirèsiÈtoi in särieswith the transistor's collector or:mitter as shown in Fig. 13. Thecollector option is shown in laland the emitter option is shownin (b). The sensitivity of the cir,cuit will be directly proportionalto the value of either of the se-ries resistors.

A phototransistor_ output gp-tocouplêr in âsix-pin DIP canbeconverted to _4 phqto-{j-odq_oqt-put optocoupler by using thebase pin 6 as shown in Fig. 14and ignoring the emitter pin 4(or shorting it to the base). Thisconnection results in a greatlyincreased input signâl risetime, but it shâiply reducesCTR to a value of about O.zyo.

Digital interfacing.Optocouplers are ideally suit-

ed for interfacing digital signalcircuits that are driven at dif-ferent voltage levels. They caninterface digital IC's within thesame TTL, ECL or CMOS family,and they can interface digitalIC's between those families. Thedevices can also interface thedigital outputs of personal com-puters (or other mainfrarirecomputers, workstations andprogrammable controllers) tomotors, relays, solenoids andlamps.

Figure 15 shows how to inter-face two TTL circuits. The op-tocoupler IRED and current-limiting resistor Rl are con-nected between the 5-voltpositive supplybus and the out-put driving terminal of the TTLlogic gate. This conn'ection ismade rather than between theTTL gate's output and groundbecause TTL outputs can sinkfairly high current (typically 16milliamperes). However, TTLoutputs can only source a verylow current (typically 4OO mi-croamperes).

The open-circuit output volt-age of a TIL IC falls to less than4OO millivolts when in the logicO state, but it can rise to only 2.4volts in the logic I state if the ICdoes not have a suitable inter-

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nal pull-up resistor. In thatcase, the optocoupler's IREDcurrent will not fall to zero whenthe TTL output is at logic 1. Thisdrawback can be overcome withexternal pull-up resistor R3shown in Fig. 15.

The optocoupler's phototran-sistor should be connected be-tween the input and ground ofthe TTL IC as shown because aTTL input must be pulled downbelow 8OO millivolts at 1.6 milli-amperes to ensure correct logic0 operation. Note that the cir-cuit in Fig. l5 provides non-in-verting optocoupling.

CMOS IC outputs can sourceor sink currents up to severalmilliamperes with equal ease.Consequently, these IC's can beinterfaced with a sink config-uration similar to that of Fig 15,or they can be in the source con-figuration shown in Fig. 16. Ineither case, R2 must be largeenough to provide an outputvoltage swing that switchesfully between the CMOS logic Oand I states.

Figure l7 shows how a photo-transistor-output optocouplercan interface a computer's dig-ital output signal (5 volts, 5 mil-liamperes) to a l2-volt DC motorwhose operating current is lessthan I amp. With the computeroutput high, the optocouplerIRED and phototransistor are

by a phototransistor, output optocoupler.

Analog interfacingAn optocoupler can interface

analog signals from one circuitto another by setting up a"standing" current through itsIRED and then modulating thatcurrent with the analog signal.Fig f8 shows this method ap-plied to audio coupling. The op-erational amplifier IC2 is con-nected in a unity-gain voltage-follower mode. The oþ-tocoupler's IRED is wired intothe op-amp's negative feedbackloop so that the voltage äcrossR3 (and thus the currentthrough the IRED)precisely foì-lows the voltage applied to non-inverting input pin 3 of the op-amp. This pin is DC biased athalf-supply voltage with the Rl-R2 voltage divider. The op-ampcan be AC modulated with anaudio sisnal applied at Cl. Thequiescent IRÐD current is set at

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FlG. lSrAUDlO INTERFACE provided by a phototransistor-output optocoupler.

both ofl so the motor is turnedonbyQI andQ2. When the com-puter output goes low the IREDand phototransistor are drivenon, so 81, 92 and the motor areturned off. Note the l-amperecurrent limitation.

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pler a quiescent current is setup by its transistor. That cur-rent creates a voltage across po-tentiometer R4 which shouldhave its value adjusted to give a

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quiescent output equal to halfthe supply voltage. The audio-output signal appears acrosspotentiometer R4, and it is de-coupled by C2.

lliac interfacing.Interfacing the output of a

low-voltage control circuit to theinput of a Tiiac power-controlcircuit driven from the AC line

is an ideal application for theoptocoupler. (It is advisable thatone side.of its power supply begrounded.) That arranflementshown in Fig. lg can control thepower to lamps, heaters, motorsand other loads.

Figures 20 and 21 show prac-tical control circuits. The Thiacsshould be selected to match loadrequirements. The circuit inFig. l9 provides non-syn-chronous switching in whichthe Triac's initial switch-onpoint is not synchronized to the6O-Hz voltage waveform. Here;R2, Dl Zener diode D2 and Cldevelop a lO-volt DC supplyfrom the AC line. This voftágecan be fed to the Tiiac gate withQl, which turns the Tiiac on oroff. Thus, when Sl is open, theoptocoupler is off, so zero basedrive is applied to 81 (keepingTiiac and load off). When Sl isclosed, the optocoupler drivesQl on and connects the lo-voltDC supply to the Tiiac gate withR3, thus applying full line volt-age to the load.

The circuit in Fig. 20 includesa silicon monolithic zero-volt-age switch, the CA3O59/CA3O79, sourced by Motorolaand Harris Semiconductor.That IC with a phototransistor-output optocoupler providessynchronous power switching.The gate current is applied tothe Triac only when the in-stantaneous AC line voltage iswithin a few volts of the Zerocross-over value. This syn-chronous switching methodpermits power loads to beswitched on without generatingsudden power surges (and con-sequent radio frequency inter-ference (RFI) in the power lines).This scheme is used in manyfactory-made solid- state relaymodules.

PhotoSCR's and PhotoThiacsBoth photoSCR and photo-

Tliac-output optocouplers haverather limited output-currentratings. However, in commonwith other semiconductor de-vices, their sur$e-current rat-ings are far greater than theirRMS values. In the case of theSCR. the surge current rating is5 amps. but Lhis applies to a IOOmicrosecond pulse width and a

FlG. 23-INDUCTIVE LOAD CONTROL with Triac-driver output optocoupler and Triacslave.

FlG. 19-NON-SYNCHRONOUS TRIAC power switch with optocoupled input.

FlG. 2O-SYNCHRONOUS TRIAC power switch with optocoupled ínput.

FlG. 21-INCANDESCENT LAMP CONTROL with a Triac-driver output optocoalpler.

FlG. 22-HIGH-POWER LOAD CONTROL with Triac-driver output optocoupler.

dutycycle of less than l%. In thecase of the Tliac, the surge rat-ing is 1.2 amps, and this appliesto a lO microsecond pulse widthand a maximum duty cycle ofto"/".

The input IRED of opto-coupled SCR's and Tfiac's isdriven the same way as in aphototransistor-output op-tocouple¡ and the photoSCRand photoTliac perform thesame way as their conventionalcounterparts with limited cur-rent-handling capacity Figures21, 22, and 23 illustrate prac-tical applications for the photo-Ttiac-output optocoupler. In allcircuits Rl should be selected topermit an IRED forward cur-rent of at least 20 milliamperes.

In Fig. 21, t}re photoTliac di-rectly activates an AC-line-powered incandescent lamp,which should have an RMS rat-ing of less than lOO milli-amperes and a peak inrushcurrent rating of less than 1.2amps to work in this circuit.

Figure 22 shows how thephotoTriac optocoupler cantrigger a slave Thiac, thereby ac-tivating a load of any desiredpower rating. This circuit isonly suitable for use with non-inductive (i.e.resistive loads)such as incandescent lampsand heating elements.

Finally, Fig. 23 shows how thecircuit in Fig. 22 can be modi-fied for inductive loads such asmotors. The network made upof R2, Cl, and RS shifts thephase to the Ttiac gate-drivenetwork to ensure correct Tliactriggering action. Resistor R4and C2 form a snubber networkto suppress surge effects.

Figures 24 and 25 show twoother variations on the opto-coupler theme. A slotted cou-pler-interrupter module isshown in Fig. 23-a. T}le slot isan air gap between the IREDand the phototransistor. In-frared eners/ passes across theunobstructed slot without sig-nificant attenuation when theinterrupter is "on". Opto-coupling can, however, be com-pletely blocked by opaque ob-jects such spokes of a wheel orunpunched tape moving acrossthe slot.

Atypical slot width is about 3

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mm (0.12 inch) wide, and themodule has a phototransistoroutput that gives an "open"minimum CTR of about 107".The schematic for this device issimilar to that of Figure 2 exceptthat the IRED and photodetec-tor are enclosed in separateboxes.

Figure 24-b illustrates amethod for counting revolu-tions with the interrupter. Eachtime a tab on the wheel blocksthe optical path, a count ismade. Other interrupter usesinclude end-of-tape detection,limit switching, and liquidleveldetection.

A reflective optocoupler mod-ule is shown in Fig. 25-a. Directinfrared emission from theIRED is blocked from the pho-totransistor by a wall within themodule, but both IRED andphototransislor face a commonfocal point 5 mm (0.2-inch)away. Interrupters are used to

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detect the presence of movingobjects that cannot be easilypassed through a thin slot. In atþical application, a reflectormodule can count the passageof large objects on a conveyorbelt or sliding down a feed tube.

Figure 25-b illustrates a revo-lution counter based on reflect-ing IRfrom the IREDback to thephototransistor with reflectorsmounted on the face of a spin-ning disk. The module.diskseparation is equal to the 5-mmfocal length of the emitter-detec-tor pair. The reflective surfaces'can be metallic paint or tape.Other appìications for the re-flective module include tape-po-sition detection, engine-shaftrevolution counting, and en-gine-shaft speed detection.

Photointerrupters and photo-reflectors are also availablewith photodarlington, pho-toSCR, and photoTliac ouputstages. R-E