Download experiment no 4

Esc 102 Introduction to Electronics, 2004-2005/I
EXPERIMENT NO 5
SOME ELECTRONIC COMPONENTS AND THEIR CHARACTERISTICS:
DIODE, ZENER DIODE, LED, PHOTODETECTOR, MICROPHONE
Introduction
The aim of this experiment is to verify the V-I characteristics of diodes and Zener diodes, and also to study the
functions of LED, photodetector and microphone.
Difference Amplifier
A Difference Amplifier gives an output VO = A(V2 – V1), where V2 and V1 are the input voltages. See Fig.1.2
for a typical circuit using an Operational amplifier, which will be explained in detail in a later experiment.
When all the resistors in the circuit are equal, A=1. We will use the difference amplifier circuit in this mode.
This circuit is already made on a PCB (Printed Circuit Board) with P and Q as its inputs and R as its output.
Since 1V voltage drop across the 1K resistor corresponds to 1mA of current through the diode, the potential
drop across this resistor is used to give us the current through the diode in mA.
1. Diode
The approximate V-I characteristics of a diode is given by
I = IS [exp(V/nVT) –1]
where IS is called the saturation current, VT (=kT/q) is the thermal voltage, and n is a constant with value 1 to
2 depending on the material and the physical structure of the diode. IS ranges from a few nA to A.VT is 25mV
at 20C. In a silicon diode till about 0.5V, there is very little current. But when V is increased above 0.6V the
current shoots up exponentially. When the diode is reverse-biased, the current is negligible and remains
constant at IS. The reverse-bias should not exceed the PIV (peak-inverse voltage) of the device, as it would
result in a destructive breakdown. PIV of the diode (IN4007) used in your experiment is 1000V.
Experiment
(i).Wire the circuit of Fig.1.3. Connect the inputs to the Difference Amplifier as indicated. (Be sure to
connect the +12V, -12V and GND connections also to the PCB). Apply an 8V peak Triangular wave
(200 Hz) as input to the circuit. Connect the diode voltage VD to CH-2 and the Difference amplifier output
VA to CH-1. Make sure that you are getting waveforms on these channels.
(ii) Put the CRO into XY mode and adjust the beam such that the origin is at the centre of the CRO display.
(iii) See the XY plot on the CRO for the diode waveforms and verify that you are getting the diode
characteristics correctly.
(iv) Measure the knee voltage (VKnee)of the given diode. Choose CH-2 voltage setting as 0.5V/div or
0.2V/div for this purpose.
(v) Sketch the diode characteristics and note down the salient points. (You will see that a practical diode is
quite different from an ideal one).
(vi) Estimate the diode forward resistance by measuring the slope of the diode in the conducting region
(choose a nearly linear region). For this purpose measure accurately the diode voltages and currents at two
points in the displayed characteristics.
2. Zener Diode
Zener diodes are special diodes with lower reverse breakdown voltages. Zener diodes are used as reference
voltage sources in Voltage regulator applications.
Experiment
Repeat the steps (i) to (vi) of Section 1 and obtain the characteristics of the given Zener diode. In step (iv)
measure both the Zener breakdown voltage (VZ) and the forward knee voltage (VKnee). Similarly, in step (vi)
measure the resistances in both the forward and Zener regions.
3. Clipper Circuits using Diodes and Zener Diodes
A. Wire the diode Clipper circuit of Fig.3.1. Apply an 8V peak Triangular wave (200 Hz) as input to the
circuit. Observe and sketch the input and output waveforms. Explain why the actual output waveform
is different from the ideal one.
B. Wire the Clipper circuit of Fig.3.2, using the Zener diode. Apply an 8V peak Triangular wave (200 Hz)
as input to the circuit. Observe and sketch the input and output waveforms. Explain why the actual
output waveform is different from the ideal one.
4. A Simple Optocoupler Circuit using LED and Photodetector
It is often required to connect signals from one system to another system without any physical connection.
This is required in applications where the two systems have different Ground voltages or when one desires
isolation between the two systems. For example, you might like to take the signal from a certain process to a
PC for monitoring and control. The process instrument might be using higher voltages etc and hence it may
not be desirable to directly interconnect the two systems. An optocoupler is an ideal solution for such cases. It
accomplishes this task using electrical-to-optical (EO) and optical-to-electrical (OE) converters. A Light
Emitting Diode (LED) is an example of an EO converter, while a photodetector (PD) is an OE converter. You
can use an optocoupler to couple either an analog (continuous) signal or a digital (abruptly changing) signal.
Visible LEDs are made of materials such as AlGaAs, GaAsP, GaP, AlGaInP. An LED emits light when it is
forward biased. The light emitted in a LED is directly proportional to the current flowing through it (a linear
response). PDs are specially fabricated pn junctions where the external light is allowed to fall on the junction.
PDs are always reverse-biased. A current (called the photo current) proportional to the light intensity is
generated when light shines on the junction (again a linear response for a given wavelength).
Experiment
(i) Wire the circuit of Fig.4. You need to connect the LED such that it is forward-biased. Apply a 0 to 5V
square wave (TTL output of FG) of 5Hz or 10 Hz and check whether the LED is going ON and OFF as per the
input square wave.
(ii) Keep the PD glass window in close proximity to the LED, such that maximum light from the LED is
coupled to the PD.
(iii) Connect VO to CH-2 of the CRO and verify that you are getting some reasonable signal.
(iv) Cover the optocoupler with your hand or some dark object and observe the effect of ambient light (both dc
and ac component) at the VO output.
(v) Increase the square wave input frequency to about 500Hz and compare the input square wave (connected to
CH-1) with the VO output (on CH-2). Sketch the two waveforms.
(vi) Increase the frequency further and observe that VO output is getting more and more distorted. The shape of
the waveform is familiar to you! Give an explanation for the distortion. Comment on the maximum frequency
up to which your optocoupler can be used.
Note: In an actual optocoupler there will be signal processing circuits (amplifier, comparator, etc) after the PD
so that you get the desired VO (analog or digital). Commercial optocouplers work up to about 1 MHz.
5. Microphone
The microphone is a device that converts the vibrations in air caused by sound into an electrical signal. There
are different types of microphones, the most popular ones being the Condenser and dynamic microphones.
The one given to you in the lab is a dynamic type where a tiny low-inertia coil is suspended inside the air gap
of a magnet. As the coil vibrates due to the air vibrations, a corresponding time varying voltage is generated.
Experiment
Connect the microphone to the CH-1 of the CRO. Set the sweep rate at 2ms/div. Sketch the waveforms seen
for the following sounds: (i) Blowing air, (ii) whistling, (iii) steady vowels aaa, eee, ooo, etc, (iv) consonants
ka, pa, tha, etc. Note down typical amplitudes of various signals and comment on the different waveshapes.