Zener Diodes and LED's Rev. 6/11/09
ELEC 2020 EXPERIMENT 6
Zener Diodes and LED's
Objectives:
The experiments in this laboratory exercise will provide an introduction to diodes. You
will use the Bit Bucket breadboarding system to build and test several DC and AC diode
circuits. The objectives of this experiment include:
• Review basic principles of diodes from ELEC 2210
• An understanding of diode rectifier circuits
• More experience with the Bit Bucket breadboarding system and the oscilloscope
• Continue to develop professional lab skills and written communication skills.
Introduction
A thorough treatment of diodes can be found in Chapter 3 of the ELEC 2210 textbook,
Microelectronics Circuit Design by R.C. Jaeger.
Diodes are circuit elements that allow current to pass in one direction, while blocking it in
the other direction. They are used primarily in rectifier circuits, which convert AC to DC,
and in voltage regulation and voltage limiting circuits. They are also sometimes used to
produce a fixed DC voltage drop (approx. 0.7 volts for Si p-n junction diodes). In the latter
case, several diodes may be connected in series to achieve the desired voltage drop.
The mathematical model of the diode (not including reverse breakdown) is given in
Equation 1:
iD= I S (−1 + e
vD
nVT
)
[1]
The diode current iD, and voltage, vD are defined in Figure 1.
+
vD
–
cathode
anode
iD
Figure 1. Circuit symbol and nomenclature for the diode.
Table 1 explains the parameters found in Equation 1.
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Zener Diodes and LED's Rev. 6/11/09
Table1. Diode equation parameters.
Symbol
Name
IS
Saturation
current
n
Nonideality
factor
VT
Thermal
voltage
Meaning
Typical
value(s)
PSPICE
Default
Ideal maximum reverse current. Also
the scale factor for forward current.
Proportional to junction area.
Corrects for manufacturing variations
and temperature uncertainty.
10–18 to
10–9 A
10–14A
1.0 to 2.0
1.0
This parameter has units of voltage
and is proportional to absolute
temperature.
25 mV “at
room
temp.”
25.86 mV
(27 oC)
From equation 1, we can see that the diode current increases nearly exponentially as the
diode voltage becomes more positive (forward bias), but the current is asymptotically
limited to –IS when vD is negative (reverse bias). If the diode is forward biased
substantially, Equation 1 can be simplified to a pure exponential, and then near room
temperature we can deduce the “60 mV per decade” rule, which says that vD increases
approximately 60 mV for every factor of 10 increase in iD.
In addition to the behavior described by Equation 1, all diodes have a reverse breakdown
voltage, VBR, at which significant reverse current will begin to flow. For many
applications, it is desirable to have VBR be as large as possible. Values of VBR = 1000 V are
not uncommon for small-signal rectifier diodes. For other applications, such as voltage
regulation, we require relatively low reverse breakdown voltages, such as 5 V or 9 V. For
these applications, we use Zener diodes. The reverse characteristics of a typical Zener
diode are shown in Figure 2.
iD
VZ
VZK
vD
IZK
Slope 1
Slope 2
IZ
Figure 2. Piecewise linear approximation of the reverse characteristics of a Zener
diode.
The Zener knee resistance is
R ZK =
1
Slope 1
2
Zener Diodes and LED's Rev. 6/11/09
and the Zener resistance is
RZ =
1
Slope 2
Typcial values of RZ are on the order of 10 ohms.
Real diodes have series resistance and junction capacitance which affect their behavior in
circuits at higher currents and higher frequencies. Furthermore, IS is a strong function of
temperature. These effects will not be considered in this lab experiment.
An important simple model for the diode is the piecewise linear (PWL) model, illustrated
in Figure 3.
iD
–VZ
VON
vD
Figure 3. Piecewise linear (PWL) diode model.
Figure 4 shows a series DC circuit with a diode.
+VD I
+
VR
-
Figure 4. Series circuit including a diode.
When the circuit is connected, the diode will operate at a Q-point (I, VD) determined by the
intersection of the diode characteristic curve and the load line. The load line is established
by the values of VDC and R. The circuit must satisfy Kirchhoff’s Voltage Law (KVL),
such that VD + VR = VDC.
If the source is changed to an AC voltage source, current will flow in only the forward
direction through the diode, as long as the peak negative value of the voltage does not
exceed the reverse breakdown voltage of the diode. In this case, vR(t) will never be
negative, thus the AC voltage is said to be rectified.
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Zener Diodes and LED's Rev. 6/11/09
Pre-Lab:
(1) Obtain the data sheet for the 1N4733 Zener diode from the class web site, or from
http://www.fairchildsemi.com/pf/1N/1N4733A.html
Use this to determine the values of VZ, IZ, IZK, RZ, and RZK. Note that the datasheet uses the
letter Z (impedance) instead of R (resistance).
(2) Determine an expression for the RMS value of (a) a sine wave with peak value VP, and
(b) a half-wave rectified sine wave with peak value VP. You may obtain these either by
direct calculation, or by looking them up in any resource you choose.
(3) Look up the “turn-on” voltage for red LED’s and green LED’s.
Lab Exercise:
There are four parts. Have your GTA sign off on each part before proceeding to the
next part.
(1) Fuse check, current measurement with the DMM, and verifying Ohm's law.
In this part, you will learn how to measure current with the digital multimeter (DMM).
You will also check to make sure the DMM fuse is not blown.
(a) Identify the variable +15 VDC supply on the Bit Bucket.
The output terminal is labeled 0~+15 as shown in the photo.
Set the knob to Min. (fully CCW).
(b) Connect this output to the + input on the Bit Bucket's DVM. Connect the – input on the
DVM to ground. Turn the 15V supply knob and verify that the voltage changes as
expected. Record the minimum and maximum values.
(c) Turn off the Bit Bucket Power, and adjust the variable voltage knob back to Min.
(d) Obtain and measure a 100 ohm, ½-Watt resistor. Record your measurement. Leaving
the DVM connected as above, connect the circuit shown below in Fig. 5. Set the DMM
knob to the DC current position, and connect the positive lead (from the resistor) into the
jack labeled 300mA. Connect the negative lead (returning to ground) to the jack labeled
COM. (the labels may be slightly different if you are using a different model DMM).
(e) Re-check to make sure the variable supply knob is set to Min. Turn on the Bit Bucket
Power.
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Zener Diodes and LED's Rev. 6/11/09
(f) Slowly increase the variable DC. The voltage shown on the DVM should increase, and
the current shown on the DMM should increase proportionally. If you get to 1 V and your
DMM shows no current, the DMM most likely has a blown fuse. Check with your GTA
for repair procedures.
0 to 15 V
Variable
DC
Figure 5. Measuring current with the DMM
(g) Record the the DVM voltage and the DMM current at three different voltages: roughly
1 V, 3 V, and 5 V. DO NOT EXCEED 60 mA OF CURRENT. Compute the resistance
using Ohm's law, and compare with what you measured in part (d).
(2) Diode I-V characteristics.
Carefully measure both the forward and reverse characteristics of the 1N4733 diode, using
the series circuits shown in Fig. 6. Use the variable 0 to +15 V DC supply on the bit
bucket, and use a 100 ohm, ½ Watt resistor. Use a DMM to measure the current, and the
DVM on the Bit Bucket to measure the diode voltage. In order to measure the forward
characteristics, insert the diode so that the cathode is grounded, as shown in Fig. 6a. To
measure the reverse characteristics, switch the diode end-for-end, so that the anode is
grounded, as shown in Fig. 6b.
For each part (forward and reverse) start with the variable DC voltage set to zero, and
increase it carefully!! DO NOT EXCEED 60 mA OF CURRENT IN EITHER
DIRECTION.
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Zener Diodes and LED's Rev. 6/11/09
1/2 Watt
0 to 15 V
Variable DC
1N4733
DMM
DC current
1/2 Watt
1N4733
DVM
DC
voltage
DMM
DC current
DVM
DC
voltage
(b)
(a)
Figure 6. (a) Measuring the forward diode characteristics (cathode grounded). (b)
Measuring the reverse diode characteristics (anode grounded). Black stripe on part
indicates cathode.
Measuring procedure. With the circuit connected as above, slowly increase the variable DC
voltage until you have about 50 mA of current flowing through the diode. Record the diode
voltage. Incrementally reduce the supply voltage, and continue recording I and VD pairs
until the current is reduced to zero. You should let the meter readings stabilize for about 10
seconds at each step.
You can use the example tables below as a rough guide for the number of values to record
and the expected values.
Table 2a. Example forward characteristics of the 1N4733.
VD (volts) ID (mA)
0.826
50.6
0.822
45
0.82
41.7
0.819
39.7
0.816
35
0.812
29.66
0.806
24.31
0.8
18.96
0.793
14.87
0.783
9.93
0.766
5.13
0.729
1.2
0.708
0.54
0.672
0.15
0.604
0.02
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Zener Diodes and LED's Rev. 6/11/09
Table 2b. Example reverse
characteristics of the 1N4733.
VD (volts) ID (mA)
-51.5
-43.6
-40
-34.9
-30.5
-27.52
-24.32
-23.06
-20.97
-18.12
-16.81
-15.53
-12.99
-8.24
-6.23
-3.23
-1.76
-1.01
-0.42
-0.1
-0.01
1N4733 Measured I-V Characteristics
60
40
Diode Current (mA) _
-5.13
-5.12
-5.11
-5.1
-5.09
-5.08
-5.07
-5.06
-5.05
-5.04
-5.03
-5.02
-5
-4.94
-4.9
-4.76
-4.61
-4.44
-4.15
-3.58
-2.25
20
0
-6
-5
-4
-3
-2
-1
0
1
2
-20
-40
-60
Diode Voltage (V)
Example plot of the 1N4733 forward and
reverse characteristics combined.
In your lab report, plot the I-V characteristics that you measured. See the example plot
above.
From the data you obtained, estimate the values of RZK and RZ. Compare your values with
those given in the data sheet. Are they within the stated limit(s) ?
In your lab report, calculate the value of IS from your data, at three different data points.
Assume n = 1 and VT = 25 mV. Are the values consistent with each other? Do they fall
within the expected range (see Table 1) ?
Compare your measured data with the 60 mV per decade rule at two different current
values. How well does the rule match your data? Use approximate current values of (50
mA, 5 mA) and (30 mA, 3 mA) to test the rule. Compare the corresponding voltage
differences with 60 mV, and express your comparison as a percent difference.
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Zener Diodes and LED's Rev. 6/11/09
(3) Half-wave rectifier.
Connect the Function Generator (FG) on the Bit Bucket to the oscilloscope and adjust the
FG to sine, 2 V peak (4 V peak-to-peak), 1 kHz. Use the MEASURE feature of the scope
to determine the RMS value of the sine wave and compare with the value you expect from
the prelab.
Connect a series circuit as shown in Figure 7. Use the 1N4733 diode, a 1000 ohm, ¼ Watt
resistor, and the Function Generator (FG) on the Bit Bucket.
Oscilloscope
Figure 7. Half-wave rectifier.
If your circuit is connected properly, one end of the resistor should be grounded, and the
other should be connected to the cathode of the diode. Use the oscilloscope to display and
measure the voltage across the resistor. Be sure the oscilloscope ground is connected to
the Bit Bucket ground. You should observe a half-wave rectified voltage across the
resistor. Sketch your observation. Use the MEASURE feature of the scope to determine
the RMS value of the voltage, and compare with the value you expect based on your
expression in the prelab.
Use the MEASURE feature of the scope to determine the MAXIMUM value of the
voltage. Explain this value, considering KVL and the forward voltage drop you observed
in Part (2).
Increase the amplitude of the FG to its maximum value. Sketch and explain what you
observe on the oscilloscope. It will help to take a moment to measure the FG output on the
scope, and note that the peak negative output exceeds the Zener voltage of the diode.
Now exchange the positions of the resistor and the diode, so that the anode is grounded, as
shown in Figure 8. Reduce the FG voltage to 2 V peak. Use the scope to observe the diode
voltage. Measure the most negative voltage. Increase the FG voltage to maximum. Explain
your observation. Switch the FG to a square wave. What is the peak value of the output?
Why?
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Zener Diodes and LED's Rev. 6/11/09
Oscilloscope
Figure 8. Measuring the forward turn-on and reverse breakdown.
(4) Forward turn-on voltage of LED's.
Replace the 1N4733 shown in Fig. 8 with a Red LED, and replace the 1kΩ resistor with a
330 Ω, ¼ W resistor. It does not matter which way the LED is inserted, since we are using
an AC source. If your circuit is properly connected, one leg of the LED will be grounded.
With the FG set to sine wave of maximum amplitude, confirm that the diode lights up, and
observe the diode voltage with the scope. Sketch your display. What is your best estimate
of the “ON” voltage? Compare with your expectations from the prelab.
Replace the Red LED with a Green LED, and repeat.
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