Op to Isolators Theory

8/13/2019 Op to Isolators Theory

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Understanding and testing opto-isolators

Understanding opto-isolators:

The opto-isolator is simply a package that contains both a light emitting diode (LED) and a photo-

detector such as a photosensitive silicon diode, transistor, Darlington pair, or silicon controlled rectifier(S!)" The most common type uses an infrared LED and a phototransistor" #ther circuitry, for e$ample

an output amplifier, may be integrated into the package" %n opto-isolator is usually thought of as a

single integrated package, but opto-isolation can also be achieved by using separate devices as &ell"#pto-isolators are very common in many devices especially in the feedback section of S'S circuits

found in LD and lasma Ts and most other consumer electronics as &ell" *ou &ill typically see +

and pin opto-isolators, + pin definitely being the most common" Some opto-isolator s &ill also havemultiple opto-isolators in one package"

Symbol for 4 pin LED/Phototransistor type opto-isolator IC.


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Symbol for pin opto-isolator IC.

Pi!t"re sho#ing m"ltiple opto-isolators in one pa!$age.

%n opto-isolator also called an opto-coupler or photo-coupler allo&s a signal to pass from one circuit

to another but allo&s the t&o circuits to remain electrically isolated" The most common opto-isolator&hich comes in package consists of an LED &hich shines onto the base of a photo-transistor

(usually an npn transistor) and allo&s current to flo& bet&een collector and emitter until the LED is

turned off" .hen a signal is applied to the LED it then shines light that is varied in brightness &ith thesame amplitude as the input signal, this light lands upon the photo-transistor (the resistance of the

collector/emitter 0unction no& changes &ith the varying light) &hich passes the signal onto the ne$t

circuit"


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% methods for testing opto-isolators:These methods &ill cover testing the most common opto-isolators found in electronics e1uipment, +

pin LED/phototransistor opto-isolators"

&ethod ':

!emove the opto-isolator from circuit or at least de-solder and lift the emitter and cathode pins from

the circuit to get accurate readings" f you completely remove the opto-isolator from the circuit you can

use a piece of 2 prototype board to hold it in place, do not solder it to the board only stick the leadsthrough the holes" 3sing your analogue meter set to the $4 ohm range test the LED side of the opto-

isolator" .hich &ill be found be looking up the part number online or referring to the circuits schematic

diagram" % common opto-isolator found in LD Ts is the 546 type for instance the 546"

lace the red test lead on the anode pin and the black test lead on the cathode pin, the reading should be

infinite"


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Infinite reading #ith red test lead on anode and bla!$ test lead on !athode.

7e$t place the black test lead on the anode and the red test lead on the cathode, no& you should get a

resistance reading that is lo&, typically around 89 ohms or so"

&eter indi!ates a lo# resistan!e reading of '( ohms #ith bla!$ test lead on anode and red test

lead on !athode.

f you get a lo& resistance or 9 ohms reading in both directions the the LED is considered shorted and

the opto-isolator must be replaced" f you get a reading of infinity in both directions the LED is

considered open and again the opto-isolator must be replaced"


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7o& to test the phototransistor side of the opto-isolator" Set your analogue meter to the $49k ohmrange" lace the black test lead on the collector pin and the red test lead on the emitter pin of the opto-

isolator, you should get a resistance reading of infinity"

Infinite reading #ith bla!$ test lead on !olle!tor and red test lead on emitter.

7e$t place the red test lead on the collector and the black test lead on the emitter" *ou should get a

large resistance reading, typically around :99k ohms or so" f you get a 9 ohms or lo& resistance

reading in both directions the collector/emitter 0unction of the phototransitor is shorted and the opto-isolator must be replaced"

7ote; Testing sho&n is on a + pin opto-isolator , &hen testing a pin opto-isolator use the same

procedure, but double check the pin configurations for the particular opto-isolator"


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&ethod t#o:

De-solder and remove the opto-isolator you &ant to test from the circuit it is in" 3sing alligator clips or

some other method attach a D'' (digital multimeter) test leads to the collector and emitter pins of the

opto-isolator, polarity does not matter" Set the D'' to the resistance or ohms setting" The meter

should read #"L (#ver Limit), infinity or something that indicates the resistance is beyond the range ofthe meter"

)tta!h test leads to the emitter and !olle!tor pins* polarity does not matter.

Set the D&& to the resistan!e setting* reading sho"ld be +.L.


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7o& using you analogue meter set to $4 ohm range, place the red test lead on the cathode pin and the

black test lead on the anode pin, you should get a lo& resistance reading on the analogue meter and alsoif you look at the digital multimeter it &ill no& also sho& a lo& resistance reading"

,ith analog"e meters bla!$ test lead on the anode pin and the red test lead on the !athode pin

the D&& no# sho#s a lo# resistan!e of '.% ohms bet#een the !olle!tor and emitter.

This is the characteristics of a good opto-isolator"


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&ethod three:

The third method re1uires assembling an easy to make opto-isolator tester"

4 pin* LED/phototransistor opto-isolator tester

The idea is simple, &ith the po&er on and an opto-isolator placed in the proper section of the socket,

pressing the push to test button &ill cause the red indicator LED to light other&ise the opto-isolator isno good and must be replaced"


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Pla!e opto-isolator into proper se!tion of the IC so!$et.

"rn tester po#er on* the po#er on LED is no# ill"minated.

"


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If the opto-isolator is good #hen the p"sh to test b"tton is pressed the red indi!ator LED #ill

light.

This tester is good for testing many different + pin LED/phototransistor type opto-isolators like the

546, 48


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Parts list for opto-isolator tester "sed in this arti!le:

S.4, SST slide s&itch"

S.8, SST momentary contact push to make tactile s&itch"

24, = battery"

D4, 47+994 7 diode"!4,!< and !+,

Op to Isolators Theory

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