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Optocouplers

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Optocouplers

• LED for emitter 

•  Air as barrier for isolation

• Phototransistor for detector 

• Transformer is similar, but only for AC

• Optocoupler can be used for DC

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Emitter 

• !ncandescent lamp

 " #uch slo$er response time than LED

 " Can filter out hi%h fre&uency noise

 " Lo$er lifespan than LED ho$e'er 

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)hen to *se

There are many situations $here si%nals and data need to

be transferred from one subsystem to another $ithin a

piece of electronics e&uipment, or from one piece of 

e&uipment to another, $ithout ma+in% a direct electrical connection

Often this is because the source and destination are at 'ery different

'olta%e le'els, li+e a microprocessor $hich is operatin% from -. DC but

bein% used to control a triac $hich is s$itchin% 2(/. AC

!n such situations the lin+ bet$een the t$o must be an isolated one, to

protect the microprocessor from o'er'olta%e dama%e

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Other De'ices

#echanical 0elays can also pro'ide isolation, but

e'en small relays tend to be fairly bul+y compared $ith !Cs

ecause relays are electromechanical, they are not as reliable and

are only capable of relati'ely lo$ speed operation

)here small si3e, hi%her speed and %reater reliability are important, a

much better alternati'e is to use an optocoupler

These use a beam of li%ht to transmit the si%nals or data across an

electrical barrier, and achie'e e4cellent isolation

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Parameters

The most important parameter for optocouplers is their isolation

The second most important parameter is transfer efficiency,

measured as the current transfer ratio or CT0

CT0 is simply the ratio bet$een a current chan%e in

the output transistor and the current chan%e in the

input LED $hich produced it

Typical 'alues for CT0 ran%e from 1/6 to -/6 for

de'ices $ith an output phototransistor and up to 2///6 or so

for those $ith a Darlin%ton transistor pair in the output

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Parameters

• Optocoupler8s band$idth determines the

hi%hest si%nal fre&uency that can be

transferred throu%h it

• Typical optocouplers $ith a sin%le output

phototransistor may ha'e a band$idth of

2// //+93, $hile those $ith a

Darlin%ton pair are usually about 1/ timeslo$er, at around 2/ /+93

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Darlin%ton Pair 

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The other main type of optocoupler is the type ha'in% anoutput Diac or bilateral s$itch, and intended for use in

dri'in% a Triac or <C0

E4amples of these are the #OC/2/ and #OC/219ere the output side of the optocoupler is desi%ned to be

connected directly into the tri%%erin% circuit of the Triac

$here it8s operatin% from and floatin% at full 12/=2(/ .AC

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•  A simple circuit $ith an optoisolator

• )hen s$itch <1 is open, LED D1 is off, so >1 is offand no current flo$s throu%h 02, so .out ? .cc

• )hen s$itch <1 is closed, LED D1 li%hts

• Phototransistor >1 is no$ tri%%ered, so current flo$sthrou%h 02

• .out is then pulled do$n to lo$ state

• This circuit, thus, acts as a @OT %ate

.cc

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