Opto Isolators Lesson 07-17-2012

8/12/2019 Opto Isolators Lesson 07-17-2012

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Lesson 1452,Optoelectronics

Experiment 7, Opto-isolator

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Objective

To demonstrate the operatingcharacteristics of a typical optical isolator


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Introduction

We have been concentrating on simpledevices where only one electrical tooptical or optical to electrical conversionwas involved.

This experiment will demonstrate how

two such devices can be combined intoone package to make a different type ofdevice.


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This device is called an opto-isolator; which

is short for optical isolatorThis device combines a LED with aphototransistor in one package

Opto-isolators are often used in digitalcircuits and have two important functions:

1) The ability to electrically isolate twocircuits from one another


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a) For that reason, optical isolators are often

used at the inputs and outputs of digitalequipment where electrical isolation isimportant

2) A second advantage of opto-isolators isthey reduce the noise in the signal.

a) This noise reduction is especially important

in applications where long signal pathsoccur


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The schematic symbol for a typical

opto=isolator is shown after we finish thecomponent description.

There are two important parts to thedevice

1) At the input, an LED turns an electrical into

an optical equivalent2) The phototransistor at the output turns the

optical signal back into an electrical signal


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Ideally there should not be a change

through this two-process conversionSome signal degradation is inevitable athigher frequencies


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It is easy to see there is not a direct

electrical connection between the LED andthe Phototransistor

Another way of saying the same this is;there is not a direct electrical connectionbetween the input and output terminals

The electrical isolation of a typical opto-isolator is rather high, generally greaterthan 1000V


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This does not mean however, that a 1000V

potential can be connected between thetwo input terminals or the two outputterminals

Doing so would result in catastrophic failure!

However, you could hook up 1000V betweenon of the input terminals and ground or oneof the output terminals and ground and nodamage would occur since isolation is thepurpose of the device


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Practical isolators usually have a fifth

terminal which was shown in the Opto-isolator symbol

This is the base connection to thephototransistor, which is used to bias it

This method of biasing the phototransistorallows the device to be used in a wide varietyof circuits by simply adjusting the operatingpoint of the phototransistor


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Spec Info for the 4N28 Opto-isolator

LED Max Input current (I in) 80mA

Phototransistor Max I C 3mA

IC = Collector currentR1 is used to adjust the input current

R2 limits the LED current to a safe value

Iout measured at collector of transistorIout = Output current


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Exp 7 Opto-isolator circuit


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Procedure continued

1. Turn on the trainer and set positivevoltage to + 15Va) Be sure not to disturb the setting

throughout the remainder of experiment

2. With the power off, construct the circuit

on the following slidea) Be sure to choose the correct part since the

4N28 looks similar to the 741 Op Amp


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b) Pay attention to the lead configuration of

the opt-isolator as you build the circuit.1. The pinout of the IC is at the top of the

diagram

c) Also note there are two places on thediagram where you are to insert your meter

1. Make sure the jumper is installed, when themeter is removed from the circuit

2. Obviously, the jumper is removed when themeter is in the circuit


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Construct This Circuit


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3. Switch the meter to the 25mA range and

turn on the powera) Adjust R 1 through its range, to verify that the

LED current varies. (If not, turn power off andcheck the circuit connections)

4. Set R 1, for zero LED current and removethe meter from the circuit and connect the

jumper in its place.a) Turn off power when making all circuit

changes


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b) Remove the jumper from the output circuitand connect the meter in its place

5. Switch the meter to the 2.5mA range andturn on the power

a) Note the reading and record its value in theData table for this experiment

6. Change the connections back so you can

again read the input currenta) Remember to place the jumper back in the

output circuit and remove the jumper in theinput circuit


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Data Table for Experiment 7IIN IOUT

0 mA

1 mA

2 mA

3 mA4 mA

5 mA

6 mA

7 mA

8 mA

9 mA

10 mA


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7. Adjust R 1 so that the input current is

exactly 1 mA. Then change yourconnections so you can read the outputcurrent and record value in the data table

8. Continue the process for all the inputcurrent values in the data table, until youhave measured and recorded all theoutput current values

a) Remember to replace the jumper whenremoving the meter


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9. Turn off the power when all current

measurements have been made.a) Graph your results in the following

Experiment 7 Graph

1. The horizontal axis shows the various inputcurrent values

2. The vertical axis shows the various outputcurrent values

3. Use the data from the table to plot thepoints and connect them to form a smoothcurve


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Experiment Data Graph


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CIE Results

While the following results are typical for a4N28 Optical Isolator, your results could besomewhat different due to component

variationsYou should note from the results, theoutput current increased up to a point

The maximum current is limited by thesaturation point of the transistor


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Experiment Data Graph Plotted


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Final Discussion

An opto-isolator is a special device whichcontains a LED and a phototransistor

It provides a high degree of electricalisolation due to its ability convert anelectrical signal to optical and the opticalsignal back to an electrical signal

The particular opto-isolator we used is rated at7000V isolation


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We did not use the additional terminal onthe IC, which is often used to bias the baseof the phototransistor.

A negative voltage can be applied to thebase of the transistor to reduce/control thegain of the transistorThis would mean more LED current would berequired to drive the transistor into

saturationConversely, a positive base voltage could beapplied to decrease the LED current required

to drive the transistor into saturation


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Your graph should the opto-isolator was

fairly linear up to a point.This means the output current should havechanged by a corresponding amount when the

input current changedOpto-isolators of this type are restricted to usein digital circuits, (circuits which have an on/off

nature) since the conversion process is notperfectly linear.


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The isolator you used has a rated isolation

factor of 7000V; therefore, you should beable to measure an infinite resistancebetween the input and output terminals of

the device.While some leakage current may be present, itwould be much too small for your meter tomeasure


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Questions?


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ResourcesRosenow. (2001). Lesson 1452:Optoelectronics . Cleveland: ClevelandInstitute of Electronics.


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Opto Isolators Lesson 07-17-2012

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