EECE 280 - Lab #3
Diodes, LEDs & BJTs
In this lab you will gain experience with:
1.
2.
A variety of diodes and light emitting diodes (LEDs).
Bipolar junction transistors.
Requirements
1.
2.
3.
4.
5.
6.
Tools & component kit
1N4148 rectifier (diode)
MBR150 Schottky diode
1N5221B & 1N5226B Zener diodes
5 identical LEDs from your parts kit
2N3904BU (NPN) & 2N3906BU (PNP) BJTs
Lab Preparation
1.
2.
Obtain datasheets for all of the above components.
Review the difference between Peak voltage, Peak-to-peak voltage and offset.
Lab Procedure
A) Diodes
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Build the circuit in Fig. A on your breadboard with
Vin R30
Vout
R30 = 1K. Use your function generator to create a
1KHz, 10Vp-p triangle wave with 0V offset. Display
the input and output voltages on your scope.
10Vp-p
1N4148
Sketch the two waveforms in your lab book.
Compute the current through the diode at the
peak input voltage (5V). Predict and sketch the
Fig. A
GND
GND
current in the diode (as a function of time).
How can you verify your prediction using only your scope? Explain how, try it, and
sketch the result. Hint: refer to the continuity table you completed in Lab #1.
Reverse the direction of the diode.
Measure and sketch the input voltage, output voltage and current.
Replace the 1N4148 with the MBR150 diode (original direction shown in figure).
Measure and sketch the input voltage, output voltage and current.
Replace the MBR150 with the 1N5221B diode (original direction shown in figure).
Measure and sketch the input voltage, output voltage and current.
Replace the 1N5221B with the 1N5226B diode (original direction shown in figure).
Measure and sketch the input voltage, output voltage and current.
Describe the difference between a rectifier, Schottky and Zener diode.
ECE 280 - ECE Laboratory I
Department of Electrical & Computer Engineering, UBC
B) LEDs
1.
2.
3.
4.
5.
6.
7.
Select 4 identical LEDs from your
kit. Use the previous circuit (Fig.
R31
R32
R33
R34
A) to determine the turn-on voltage
5*Von
(Von). Record it in your lab book.
peak
Build the following circuit (Fig.
B1) on your breadboard. Compute
resistor values for R31-34 so that
Fig. B1
the 4 LEDs draw a maximum
current of 1uA, 1mA, 5mA and
GND
GND
GND
GND
50mA. Use real resistor values that GND
are available in your component kit.
Set your function generator to a 1/4Hz triangle
wave with 0V offset and a peak voltage equal to
1K
1K
1K
5x the turn-on voltage you determined in step 1.
The triangle wave should range from -Peak to
+Peak (V).
Describe the effect that the resistor value has
5*Von
on the way the LEDs operate.
peak
Modify the circuit so that the first two LEDs and
Fig. B2
the second two LEDs turn on and off in an alternating fashion. Describe how you did it.
GND
GND
GND
GND
Modify the circuit so it uses five identical LEDs
as shown in Fig. B2.
Describe the differences in behaviour between the single, series and parallel LEDs.
Explain why these differences exist even though the biasing resistors are all the
same value.
C) BJTs
1.
2.
3.
4.
5.
6.
7.
8.
Build the following circuits in Fig. C1 on your breadboard using a 1KHz triangle wave.
Note that one terminal of the BJT is not connected (OPEN circuit).
Measure and sketch the input voltage and output voltages.
On both circuits, interchange the emitter
Vin 1K Vout
Vin 1K Vout
and collector. The emitter is the terminal
with the arrow on it and the collector is the
10Vp-p
10Vp-p
terminal that is not connected.
2N3904
2N3906
Measure and sketch the input voltage
and output voltages.
Fig. C1 GND
GND
GND
GND
What do the Base/Collector and Base/
Emitter junctions of an NPN and PNP BJT behave similar to?
What is the difference between the behaviour of an NPN and a PNP BJT?
Build the following circuit in Fig. C2 on your breadVCC
board with Vcc=10VDC, R38=10K and
R39=100..
R38
Use your scope to measure and record the cur2N3904
10K
rent through the 100 resistor. Use the Base/
R39
5V DC
Emitter junction voltage and LED voltage that
100
you measured earlier to calculate the  (Beta)
value of your transistor.
Fig. C2
GND
GND
ECE 280 - ECE Laboratory I
Department of Electrical & Computer Engineering, UBC
Look up the  (Beta) value of the transistor. How does the listed value compare to
your measured value. Explain any differences.
10. In part B, you lit an LED directly from the signal generator. In the previous step, you
used a transistor. If you wanted to turn an LED on and off using a digital circuit,
which method is preferrable? Why?
9.
Demonstration (1 circuit per PAIR of students)
1.
2.
3.
4.
Display the input and output voltage for a randomly selected diode (A4-13).
Demonstrate your 4 LED alternating circuit (B5).
Demonstrate your 5 LED circuit (B6).
Demonstrate your BJT/LED circuit and current measurement (C7/8).
ECE 280 - ECE Laboratory I
Department of Electrical & Computer Engineering, UBC