Download 13. Investigation of an optocoupler

13. INVESTIGATION OF AN OPTOCOUPLER
13.1. Objective of the test
Knowledge of the simplest optoelectronic devices and thyristors,
their structures, principles of operation, characteristics, parameters,
properties and applications.
13.2. Short notes on optocouplers and thyristors
An optocoupler is a package that contains both a light source
(usually LED) and a photodetector (phototransistor, photodiode, pnpn
diode and so on). Optocouplers are used to couple signals between
two circuits that require total isolation. The control electrical signal is
applied to the LED.
The optocoupler to be investigated in this laboratory test contains
a photothyristor.
Thyristor is a semiconductor device that comprises three or more
junctions and can be switched from the OFF state to the ON state or
vice versa.
Many types of thyristors are used in electronic circuits. A pnpn
device with two electrodes is termed a pnpn diode, four-layer diode,
Shockley diode or reverse-blocking diode thyristor. If the device has
an n-gate or p-gate terminal, it is a triode thyristor. Reverse-blocking
triode thyristors frequently are called silicon-controlled rectifiers
(SCRs). Besides reverse-blocking thyristors, bidirectional thyristors
and reverse conducting thyristors exist. In practice the abbreviated
term thyristor is widely used for the reverse-blocking triode thyristor
or silicon controlled rectifier.
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13.3. Preparing for the test:
Using lecture-notes and referenced literature [2, p. 76–89], consider structures, principles of operation, characteristics, parameters
and properties of light-emitting diodes (LEDs), photodiodes,
phototransistors, pnpn photodiodes and optocouplers.
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Consider section “13.4. In laboratory” of this test.
Prepare squared millimetre paper for graphs.
Prepare to answer the questions:
Discuss structures, principles of operation, properties and applications of optocouplers.
Explain the structure and the principle of operation of a LED.
Explain the principle of operation of a photodiode. Draw and
explain its VAC.
Explain the structure and the principle of operation of a phototransistor.
Explain the principle of operation of a pnpn diode.
Explain the principle of operation of a pnpn photodiode.
Explain the principle of operation of silicon-controlled rectifiers
(SCR). Name their types.
Comment on VACs of an optocoupler.
13.4. In laboratory:
Answer the test question.
Familiarize with measurement devices and laboratory model.
Caution. Before any measurements the control knobs of all the
potentiometers of the laboratory model must be turned counterclockwise to the final position. During measurements voltages
and currents must not exceed the highest allowed values for used
devices. In the case of this laboratory test: maximal LED forward
current I1max=30 mA, maximal forward voltage U1max=2 V.
3. Connect the measurement circuit shown in Fig 13.1. Turn on the
current sources and set their output voltages (+9V and +150V) as
shown in Fig 13.1. After the teacher has checked the circuit
connect the measurement circuit to the current source.
After any change of the measurement circuit ask the teacher to
check it.
4. Measure the input forward VAC of the optocoupler (VAC of the
LED VD1) when the output voltage U2 of the optocoupler (the
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Fig 13.1. Measurement circuit
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forward voltage on the pnpn photodiode VS1) is 0 and 20 V,
respectively.
To this end turn the control knobs of the potentiometers R2, R4,
R5, and R6 counter-clockwise to the final position.
Note the values of the input voltage U1 while increasing the
current I1 . During this measurement the output voltage U2 must
be constant. Making measurements sketch the characteristics on
millimetre squared paper.
Measure the output characteristics I2 versus U2 of the optocoupler
for three values of the LED current I1 pointed out by the teacher.
There I2 is the forward current of the pnpn photodiode VS1.
To this end turn the control knobs of the potentiometers R2, R4,
R5 and R6 counter-clockwise to the final position.
Note the values of I2 and U2 while increasing the voltage U2.
Register the turn-on voltage U 2 of the pnpn photodiode VS1 and
continue the measurements while increasing the current I2. After
that note the values of I2 and U2 while decreasing the current I2.
Register the turn-off current I 2 of the pnpn photodiode VS1.
During this measurement the LED current I1 must be constant.
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5.
Making measurements sketch the characteristics on millimetre
squared paper.
Measure the control characteristics I1 versus U2 of the
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optocoupler. I1 is the value of the LED forward current when the
pnpn photodiode turns on.
To this end select a few fixed values of the voltage U2 (5 values
as minimum). Before each measurement turn the control knobs of
the potentiometers R2, R4, R5 and R6 counter-clockwise to the
final position.
Examine the results. Repeat the measurements if it is necessary.
Prepare the report.
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13.5. Contents of the report
Objectives.
Results of measurements (Tables).
Graphs of the measured characteristics.
Conclusions (explain the shapes of the input, output and control
characteristics of the optocoupler; discuss all possible means to
turn off the pnpn photodiode).
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