UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE
Department of Electrical and Computer Engineering
Experiment No. 2 - Semiconductor Diodes
Overview: In this lab session students will investigate I-V characteristics of regular and
zener diodes and use diodes in basic circuits.
The diode is the simplest and most fundamental nonlinear electronic element. In addition
to being an important electronic device, it forms the basis for nearly all other solid-state
devices.
Figure 1 shows the physical arrangement of the P-N junction diode and its electrical
symbol. When p-type and n-type materials are placed in contact with each other, current
will flow readily in one direction (forward biased) but not the other (reverse biased)
creating the basic diode, a one-way current valve. One of its many applications is
rectification as required in DC power supplies.
Forward biased diode
(a)
Reverse biased diode
(b)
(c)
Figure 1. The P-N junction diode showing (a) the diode in forward bias (b) the diode in
reverse bias and (c) the diode electrical symbol
The positive terminal of the diode is called the anode and the negative terminal the
cathode. Figure 2 below shows the typical current-voltage relationship for the diode.
Figure 2. The typical diode characteristic curve showing regions of forward bias, reverse
bias, and reverse breakdown
This curve can be divided into three regions:
1. The region in which VD > 0, called the forward -bias region,
2. The region in which VD < 0, called the reverse-bias region,
3. The region in which VD < -VZ, called the breakdown region,
Figures 3, 4 and 5 below show the typical I-V characteristic for the “regular” diode, and
the zener diode. The “regular” diode is used to convert AC to DC, perform digital logic,
provide circuit protection, etc. Some of the applications for the zener diode are DC
voltage regulation, clipping, wave-shaping, and piecewise-linear approximation for
function generating circuits.
It is very important that the maximum ratings of a diode not be exceeded. Some of these
ratings are maximum peak-inverse voltage, maximum forward current (peak and
continuous), and maximum wattage at which the device can operate for a particular
temperature. There may be other specified maximum ratings depending on the type of
diode.
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Figure 3. A typical I-V characteristic for the “regular” diode showing an expanded
current scale in the reverse bias region
Figure 4. A typical I-V characteristic for the zener diode showing an expanded current
scale in the reverse bias region
Part 5 of the Lab Session illustrates the application of a zener diode (Figure 4) in a
simple voltage regulator circuit. A well regulated voltage source is one in which, for a
specified range of load current, the voltage does not change significantly as the load
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current changes. In the circuit of Figure 10, a decrease in load current will tend to
increase the output voltage above the reverse breakdown voltage (VZ); however, a
resulting increase in zener current, equal to the decrease in load current, will maintain a
constant voltage drop across the series dropping resistor, and, therefore, a constant load
voltage. Likewise, an increase in load current will cause an equal decrease in zener
current, maintaining a constant output voltage. In the design of such a regulator there are
some limitations that must be considered. In the circuit shown, for a particular output
voltage and maximum load current, there will be a maximum series dropping resistance
given by RS = (12V – e o ) / I Lmax . Also, it should be noted that VZ must be equal to e o,
and that the power rating of the zener must be greater than the product of I Lmax and e o.
Part 6 of the Lab Session illustrates the use of zener diodes to limit or clip the output
voltage. In this case the output voltage can never be less than -VZ or greater than +VZ.
Similar circuits are often used to drive a circuit that can not tolerate being overdriven.
Also, a convenient method for generating a square wave is to drive this circuit with a
sinusoid of peak voltage much greater than VZ; the output then approximates a square
wave with a peak to peak value of 2VZ.
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Pre-Lab – Diodes
1. For the circuit shown below, sketch the output waveform assuming that
VDon = 0.7V.
D1
Vout
V1
VAMPL = 10Vp-p
R1
10k
FREQ = 60Hz
2. Draw a circuit utilizing regular diodes that will perform the full-wave
rectification of an AC signal. DO NOT INCLUDE A TRANSFORMER!
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3. Obtain the manufacturer’s specifications for the diodes listed below:
Diode 1N4002 (or its equivalent)
Forward Bias Voltage Drop VDon______________________V
Forward Bias Continuous Current ID___________________A
Peak Reverse Voltage PRV__________________________V
Zener Diode 1N5228 (or its equivalent)
Zener Voltage VZ_________________________________V
(INSTRUCTOR’S SIGNATURE_____________________________DATE
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)
Lab Session – Diodes
1. Connect the circuit shown in Figure 5. Set the output of the function generator
to 10Vp-p and the frequency to 60Hz, using the oscilloscope. Observe and
save the waveform across the 10kΩ resistor to your compact flash card.
Figure 5. The half-wave rectifier
2. Connect the circuit of Figure 6 and repeat Step 1. Then, set the oscilloscope to
AC coupling and adjust the volts/division and seconds/division manually to view
the ripple in the output waveform. The ripple should be in the range of 100 to
150mV. Save this waveform to you compact flash card. When you are finished,
return the oscilloscope to DC coupling.
Figure 6. The half-wave rectifier including a simple capacitor filter
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3. Connect the circuit of Figure 7. Record the output voltage for the connections given.
Figure 7. DRL (diode resistor logic) circuit
4. Connect the circuit of Figure 8. Record the output voltage for the connections given.
Figure 8. DRL (diode resistor logic) circuit
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5. Adjust the decade resistor to 1kΩ before connecting the circuit of Figure 9.
Take readings of the load voltage and resistance of the decade resistor as it is
decreased from 1kΩ to zero resistance.
Figure 9. Zener Diode voltage regulator
6.
Connect the circuit of Figure 10 setting the output of the signal generator to 20
V peak to peak at a frequency of 1kHz. Using the oscilloscope, observe and
save the waveform of the signal across the zener diodes to compact flash card.
Figure 10. Zener Diode clipper circuit
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Lab Session – Diodes (Data Sheet)
INSTRUCTOR'S INITIALS
DATE:
10
Post Lab – Diodes
1. Describe what function the diode is performing in each of the circuits of Figures 5
through 10.
2. What bias region are the diodes in Steps 3 and 4 operating in under each condition
(i.e. forward or reverse bias)?
3. Describe what is happening in the waveforms captured in Steps 1, 2, and 6, be
sure to include these waveforms in the report.
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