Diode Characteristics EELE101 Laboratory

Diode Characteristics
EELE101 Laboratory
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FVCC EELE 101
1
Diodes
1
Objective
In this lab we will measure the basic characteristics of diodes and explore how they operate. We will
use the power of LabVIEW in this laboratory. Specifically, LabVIEW will take large quantities of data
quickly so that we might focus our attention on what is happening in the circuit.
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8
2
Build basic test circuit
Load and run LabVIEW VI for voltage verses current measurement.
Test a standard diode
Test an Light Emitting Diode (LED)
Test a Zener Diode
Build a half-wave rectifier
Build and test a full-wave rectifier
Examine a practical circuit application
Theory
Diodes are the first non-linear component we work with in this class. They are a building block component in electronics and understanding them allows us to use them. Diodes are created by joining a
’P-type’ semiconductor with an ’N-type’ semiconductor creating a PN junction. This PN junction holds
the key to understanding diodes.
Semiconductors are actually made of a base material like silicon (Si) which is a poor conductor. The
reason for being a poor conductor is because all 4 valence electrons in Si are involved in covalent bonding
with a neighboring silicon atom. Consequently there are no electrons available to transport charge. This
is illustrated in Figure 1.
In a brilliant insight, we discovered that Si could be turned into a conductor by adding impurities to the
crystal structure. This is called doping the Si. In covalent bonding, electrons are shared between atoms.
When the Si crystal is doped, an element other than Si is added to the crystal. Specifically, an element
with 5 valence electrons is added to silicon to create a ’N-type’ semiconductor. This is illustrated in
Figure 2 on the right side, where Phosphorus (P) is the added impurity.
So adding the impurity P to Si creates an excess negative electric charge which becomes available to
carry current as indicated in the figure. The N-type semiconductor gets its label from the fact the
negative charges, electrons, are the charge carriers. A ’P-type’ semiconductor is created by doping the
Si with an element that has 3 valence electrons, which means the new material lacks an electron creating
FVCC EELE 101
Diodes
2
Figure 1: Covalent bonding of pure silicon
Figure 2: Covalent bonding of pure silicon with Boron impurity for the P-type, and Phosphorous impurity
for the N-type
FVCC EELE 101
Diodes
3
what is called a hole. These semiconductors are labeled P-type because the holes appear to be excess
positive charge carrying current in the material. It is important to realize that an N-type or P-type by
themselves are electrically neutral. This may seem to contradict the expression of ’excess charges’, but
just like any atom, there are an equal number of electrons and protons in either material. The key to
understanding semiconductors is to realize that the valence electrons are either being used to create a
bond, or they are free to carry charge. Since only 4 of the 5 valence electrons in P are used to create
the bond with neighboring atoms, one electron is free to carry charge. So the N-type semiconductor
has negative charge carriers available for charge transport, and the P-type semiconductor has positive
charge carriers available for transport. (Positive charges don’t really move in the material, they only
appear to since the electrons tend to fall into the holes and consequently move backwards relative to
the direction most electrons move.) So what? To this point I have just told you about two ways to
create a conductor. But we already had really good conductors like aluminum and copper. What is
so special about manufacturing another conductor? Nothing if we did nothing else, but it turns out
these two new conductors have an asymmetry that produces a remarkable effect when the two are joined.
Now I construct a PN junction by joining a P-type semiconductor with an N-type semiconductor. When
I do this, a very curious thing happens: the excess electrons in the N-type ’fall’ into the holes of the
P-type. This is curious because as stated in the previous paragraph, both of these materials start out
electrically neutral. So it could not have been a positive charge that pulled the excess electrons from
the N-type to the P-type. Rather the electrons responded to the need for the crystal to complete its
symmetry with 4 covalent bonds. This is indeed quite remarkable. The crystal structure created a
potential hole in which the excess electrons fell into. They call this the diffusion force as opposed to the
electrical force. So when the P and N materials are joined, free electrons diffuse from the N-type to the
P-type. From an electric charge point of view, the P-type material started out neutral and the arrival of
new electrons from the N-type material means that the P-type material builds up an excess net negative
charge. Conversely, the N-type material is loosing its electrons and builds up an excess positive charge.
That’s right; in a PN junction the P-type has excess negative charge and the N-type has excess positive
charge!
So how long does this diffusion go on for? Do all of the excess electrons in the N-type fall into
holes on the P-type side? No. It turns out that as this diffusion happens, an electric field develops
opposing migration. This electric field increases in strength as each new electron moves from one side
to the other. Eventually the electric field becomes too strong and the diffusion of excess electrons stops.
Figure 3 illustrates the region where electrons have moved from one side to the other. We call this region
the depletion region since the charge carriers of the doped semiconductors have now been occupied for
the crystal bonding task. The name depletion region fits well since the charge carries in this engineered
material have been removed or depleted. Thus the PN junction cannot conduct electricity. Brilliant! I
first described a way to turn an insulator like Si into a conductor which I didn’t really need, and now
FVCC EELE 101
Diodes
4
I’ve ruined the ability of the new conductor to conduct! This seems like a lot of work for nothing.
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This PN junction now has a very useful property. That is, it conducts electricity in one direction, but
blocks it in the reverse direction. I have described an electronic check valve. You might be familiar
with check valves in plumbing, hydraulics or pneumatics. A simple schematic of a pipeline check valve
is shown in Figure 4.
In this figure the oil flows when the direction of flow opens the valve clapper. The oil stops flowing if
the flow is reversed since this would close the clapper valve. The diode works in a completely analogous
fashion. When a battery is connected so that the electric field from the battery is opposed to the intrinsic
electric field in the PN junction, then the two fields will cancel each other and the depletion region that
was stopping current, is gone and current can flow. Thus to get current flowing in a forward direction,
you need a certain amount of voltage first. This is called forward biasing the diode. A typical Si PN
junction requires 0.7 [V] to overcome the intrinsic field. After 0.7 [V], the battery voltage is used to
push current in the normal fashion. On the other hand, if the battery is connected in a manor such that
the electric field enhances the intrinsic PN junction field, the depletion region grows and no current can
flow. Figure 5 illustrates these two biasing conditions.
Figure 6 shows the various symbols used to discuss diodes. Generally the schematic symbol is the most
useful because the arrow shows you which way current will flow. (Conventional current) The component
generally has a single stripe to indicate the cathode which is the N-type side.
FVCC EELE 101
Diodes
5
Figure 4: A mechanical check valve
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(c) Reverse bias
Figure 5: Illustration of forward and reverse bias
3
Procedure
To explore the diode, you will build several circuits and analyze those circuits with NI ELVIS and
accompanying virtual instruments. The first part of the lab focuses on diode characteristics. This is
done by building a simple diode circuit, measuring the voltage and current response and then plotting
the data on a V verses I curve. This is identical to the procedure to characterize a resistor and because
of this you’ll start by plotting out the response of a purely resistive circuit. The basic procedure for this
the lab is the same for all sections and is as follows:
1. Build the circuit described in each of the following subsections
FVCC EELE 101
Diodes
6
Figure 6: Representations of a diode
1
2. Take data numerous times, tweaking various parameters until you get the data you are looking for
3. Record the plots digitally or by sketch
4. Write a brief description explaining the characteristic plots you obtain.
The second part of the lab is to use the diodes to rectify an AC signal. Both half- and full-wave rectifiers
will be built and demonstrated.
4
Equipment
The basic equipment for this experiment is as follows:
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2
3
4
5
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7
Resistor of about 2[kΩ]
Resistor of about 10[kΩ]
Resistor of about 500[Ω]
NI ELVIS workstation
Computer with LabVIEW installed
Virtual Instrument: OhmsLaw.vi
Virtual Instrument: RectifierAnalysis.vi
FVCC EELE 101
8
9
10
11
12
13
4.1
Diodes
7
Jumper wire set
Multimeter for measuring resistance
4 diodes
1 Zener diode
1 Red LED
Banana end wires
V-I plot for a resistor
The first exercise is to gain familiarity with the virtual instrument you will work with to characterize
diodes. This VI, called OhmsLaw.vi has a front panel shown in Figure 7. The key to using this virtual
instrument is to first build the circuit you’re analyzing and then hook-up the ports of the instrument
such as the variable power supply (VPS) and the current meter. The VI will input a sequence of voltages
and then measure the current and voltage drop in the component under test. For resistors, they respond
linearly as predicted by Ohm’s law, the slope of the line is the resistance in the component.
Figure 7: The VI front panel used in the plotting of V verses I
FVCC EELE 101
Diodes
8
1. Select and measure with the external DMM (Hand held) the exact resistors you will be using in
the circuit. Record these values.
2. On the NI ELVIS prototyping board, build the circuit of Figure 8. Note that the schematic shown
on the front panel of the VI is not the circuit you want to build in this first exercise. The next
few instructions are more of a step-by-step guide to getting your test up and running.
Banana A
Banana B
+
0.000
A
Current DMM
R1
VPS
R2
Figure 8: Series circuit to analyze in first experiment of lab
3. We are using the built in current meter on the ELVIS unit. So, we need to connect the Banana
A port to the input for the DMM on the chasis of the ELVIS unit. From the VPS pin on the
protoboard, go to the pin-out labeled Banana A. Then run a banana wire from the banana port
connection to the ”A” input on the ELVIS chassis.
4. Connect the COM port to the Banana B port on the prototyping board. Figure 9 is a photo
showing the connections. Note that the ground wire is missing on the circuit in this photo.
5. Run the signal coming out of the banana port B through one (or two) resistors.
6. Then complete the circuit by running the output from the resistor(s) to ground.
FVCC EELE 101
Diodes
Figure 9: Photo of 2 resistors in series being analyzed by the Ohm’s Law VI
9
FVCC EELE 101
Diodes
10
7. Now connect the Voltmeter. The voltmeter for this VI is connected to one of the analog input
channels, specifically AI 0. So the ”+” side is connected to the positive side of the resistor(s) and
”-” side is connected to the lower voltage point in the circuit. These are the green wires in the
photo.
8. On the front panel of OhmsLaw, the data parameter inputs should be changed to the values you
will use. Here is some guidance on the input.
N data points
Number of data points to take. Generally 100 is more than enough.
Start Volts
First voltage to apply to the circuit. Almost always 0 [V].
End Volts
Last voltage to apply to the circuit. Usually 4 to 6 [V] is good.
Write Data
Leave this off until you are sure you are getting the data you want. You only need to save
the data once.
9. Take data, making small adjustments as needed to get the results you expect.
10. Measure resistor 1. Save the data, in Excel, in a photo or even a notebook sketch. Explain why
you get 2 slopes, and what do these lines tell you. This should be explained in your lab report in
a brief written description.
11. Measure resistor 2. Again find a way to save the data for your report. Explain the graph much as
you did in the previous step.
12. Estimate the resistances from the slopes of the lines in your data and compare these values to
those measured with the DMM.
4.2
V-I plot for a diode
1. Using the ELVIS protoboard, build the circuit where one resistor is in series with a diode. The
connection of this circuit will be the same as that in the first experiment with a diode replacing
one of the resistors.
2. Take data using approximately the same voltage range as before: 0-4 [V]
FVCC EELE 101
Diodes
Banana A
Banana B
+
0.000
11
Banana A
Banana B
-­
A
+
0.000
Current DMM
-­
A
Current DMM
D1
VPS
D1
AI 0
VPS
AI 0
R1
Forward R1
Reverse 3. Measure resistor 1. Save the data, in Excel, in a photo or even a notebook sketch. Explain why
you get 2 slopes, and what do these lines tell you. This should be explained in your lab report in
a brief written description.
4. Measure the diode. Again find a way to save the data for your report. Explain the graph much as
you did in the previous step.
5. Briefly explain in your report why the to V verse I plots look so different in the previous two steps.
6. Explain the what is different between the plots for the resistor with and without a diode present.
7. Reverse the diode in the circuit and again measure the V verse I curves for both the resistor and
the diode.
8. Briefly explain the plots.
9. Be sure to report the forward voltage needed to operate the circuit with a diode present.
4.3
V-I plot for an LED
1. Using the ELVIS protoboard, build the circuit where one resistor is in series with a light emitting
diode (LED). The connection of this circuit will be the same as that in the first experiment with
an LED replacing one of the resistors.
FVCC EELE 101
Diodes
Banana A
12
Banana B
+
0.000
-­
A
Current DMM
LED1
VPS
AI 0
R1
2. Take data using approximately the same voltage range as before: 0-4 [V]. Note: the LEDs will
typically run on 5 to 20 [mA]. You should be very careful to choose the value of the resistor in
series with the LED so that the chosen voltage range will produce the amperage needed. Put this
small calculation into your report.
3. Measure resistor 1. Save the data, in Excel, in a photo or even a notebook sketch. Explain why
you get 2 slopes, and what do these lines tell you. This should be explained in your lab report in
a brief written description.
4. Measure the LED. Again find a way to save the data for your report. Explain the graph much as
you did in the previous step.
5. Reverse the LED in the circuit and again measure the V verse I curves for both the resistor and
the LED.
6. Briefly explain the plots.
7. Be sure to report the forward voltage needed to operate the circuit with a LED present.
4.4
V-I plot for a Zener diode
1. Using the ELVIS protoboard, build the circuit on the right side of Figure ?? (forward bias) where
one resistor is in series with a Zener diode. The connection of this circuit will be the same as that
in the first experiment with a Zener diode replacing one of the resistors.
FVCC EELE 101
Diodes
Banana A
Banana B
+
0.000
13
Banana A
Banana B
-­
A
Current DMM
+
0.000
R1
VPS
-­
A
Current DMM
VPS
D1
1N4681
Reverse Bias R1
D1
1N4681
Forward Bias 2. Take data using approximately the same voltage range as before: 0-4 [V]
3. Measure resistor 1. Save the data, in Excel, in a photo or even a notebook sketch. Explain
anything odd.
4. Measure the Zener diode. Again find a way to save the data for your report. Explain the graph
much as you did in the previous step.
5. Explain the what is different between the plots for the resistor with and without a diode present.
6. Reverse the Zener diode in the circuit and again measure the V verse I curves for both the resistor
and the Zener diode.
7. Take data using approximately the same voltage range as before: 0-4 [V]. Assuming your Zener is
Part # BZX79-C2V7,133, (see Mouser Electronics) the breakdown voltage should be about 2.7
[V].
8. Briefly explain the plots.
9. Be sure to report the forward voltage needed to operate the circuit with a Zener diode present as
well as the breakdown voltage of the Zener.
FVCC EELE 101
4.5
Diodes
14
Build a half-wave rectifier
Figure 10: The virtual instrument which sends an AC signal to the ELVIS unit and is then picked up by
an oscilloscope built into the VI.
In this part of the lab you will input an AC signal into a diode circuit and use the diodes to block the
negative cycle on the AC signal. The VI used is RectifyVI and the front panel is shown in Figure 10
1. Build the diode circuit given in Figure 11. In this case, the function generator on the ELVIS
protoboard is use for the input and the AI 0 and AI 1 channels are used to measure response. R1
is the load resistor and the 2 [kW] resistor will work fine for this exercise. The following guidelines
are suggested for the settings on the VI.
Frequency
AC frequency, 120 [Hz] is convenient.
Amplitude
More than 0 [V], less than 8 [V].
device
dev 1 (drop down list will have this if the ELVIS unit is on)
FVCC EELE 101
Diodes
15
XSC1
Ext T rig
+
_
B
A
+
XFG1
D1
_
+
_
R1
D2
Figure 11: Halfwave rectifier circuit
Channels
2 (this should not need to be adjusted. If it says 1, leave it alone.)
Minimum Value
Minimum voltage the analog pins can measure. Leave this at -10 [V].
Maximum Value
Maximum voltage the analog pins can measure. Leave this at +10 [V].
Sample Rate (Hz)
Leave this at 10000.
Samples per Channel
Leave this at 250.
2. The input parameters on the VI are fairly simple. You should only have to adjust the frequency
and amplitude. Use the stop button on the VI to stop the data collection.
3. Sketch the data curves in you notebook and explain what is happening. Be sure to explain the
difference in peak voltage between the load and the reference signals.
4. In Figure 11 two diodes are shown. Only one is needed. Remove one of the diodes and repeat the
data. Explain what happens to the rectified signal when only one diode is used. Explain why two
diodes are used.
4.6
Build a full-wave rectifier
This part of the lab passes both phases of an AC signal as shown in Figure ??
FVCC EELE 101
Diodes
16
XSC1
Ext T rig
+
_
B
A
+
D1
D2
D3
D4
_
+
_
XFG1
R1
Figure 12: Full wave rectifier
1. Build the diode circuit given in Figure 12. R1 is the load resistor and the 2 [kW] resistor will work
fine for this exercise. The guidelines given in the previous exercise will also work for this exercise.
2. Sketch the data curves in you notebook and explain what is happening. Be sure to explain the
difference in peak voltage between the load and the reference signals.
3. The input amplitude can be changed. Do this and sketch the results in your report. Explain how
things change. It is probably more clear to change the voltage to several different levels so that
you can get an idea of the trends. (NOTE: the resistor used can dissipate a maximum of 14 [W].
Calculate how big the voltage can be and stay well below that maximum.)
4. Change the frequency and note any changes when you do this.
5
Beyond the Lab
A useful circuit can now be built with the concepts you have learned in this lab. Figure 13 takes AC
line voltage, rectifies it then breaks it into regulated ±13[V ] DC sources.
FVCC EELE 101
Diodes
Figure 13: Rectifying AC voltage and regulating it with a Zener diode
17