Electronics I - Laboratory 2 Rev A
LED Interfacing and Rectification
I. Objectives
1.
2.
3.
4.
Interface an LED with various voltage sources.
Build a half wave rectifier
Build a full wave rectifier
Build a full wave rectifier bridge
II. Pre-Lab Requirements
1. LED Interfacing with Power
In Experiment 3 of Lab one you determined an I/V curve for a LED . This curve showed
that the LED illuminated when it was forward biased. From your I/V curve, select an
operating point where the diode has bright illumination and then pick a current limiting
resistor that will allow the LED to operate at that point with a 3.3VDC, 5VDC, and
9VDC power source. Submit values for the current limiting resistors for the three input
power forms. Show all your work and calculations.
Chosen LED operating point: 0.89V and 0.065A
Figure 1. Notional Diode I/V Characteristics
A. Methodolgy for selecting a current limiting resistor
The objective for selecting a current limiting resistor is to allow the LED to operate
with an available circuit power form. For this pre-lab assignment three different
power forms will be used to allow the student understand and practice the technique
for three common circuit voltages.
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The graph of the LED I/V characteristics is really a collection of operating points of
the LED. Some of these points cause the LED to illuminate. Your task is to cause the
selected operating point current to flow through the LED and the selected operating
point voltage to drop across the diode. Two methods will be shown here:
a. Computational Method – On the graph in Figure 1, a notional LED’s
operating point of 0.89V and 0.065A was chosen. To achieve this diode
operating point 0.065A must flow through the circuit (i.e through RS and the
LED) and 0.89V must drop across the diode (which it will with 0.065A
flowing through the diode) and therefore VRS = (Vin – 0.89V) must drop
across RS.
VRS = (Vin – 0.89V)
Rs
Vin
ID = 0.065A
LED
VD = 0.89V
Figure 2. Computational Method for Selecting LED Current Limiting Resistor (RS)
b. PSpice Method - Treat the LED as a resistor with its operating point (i.e. V/I)
as the resistance of that resistor. Perform a parameter sweep of the current
limiting resistor (RS) monitoring current through the LED (simulated by a
resistor). On the simulation output trace locate the desired LED current: the
corresponding location of this point on the X-axis is the desired value for RS.
c. III. Laboratory Requirements
1. Required Parts and Equipment
A.
B.
C.
D.
E.
F.
G.
H.
DC power supply
Center-tapped transformer
One bench DMM
1 - Fluke hand-held DMMs
1 - Proto-Board (PB-103)
4 – 1N4148 silicon diode
1- Red LED
Resistors of the following values:
#
1
Value
3.9KΩ
I. Wires and leads for circuit connections.
2. Required Information
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A. Diode Pin Out Orientation
The diodes used for this experiment and their respective pin-out orientations are
shown in Figure 3 below. Generally diodes are marked with a colored band that
denotes the negative pin. For LEDs the pins are usually different lengths with the
short pin being negative; however, on the LEDs that you will be using, the pins are
the same length. The negative pin on the LEDs is connected to the larger internal
structure.
Figure 3. Diode Pin Orientation
B. Diode Data Sheets
The diode data sheet may be found at the following web site:
IN4148 – http://www.nxp.com/documents/data_sheet/1N4148_1N4448.pdf
C. Transformer
For experiments 2, 3, and 4 the AC power will be supplied from a step-down,
center-tapped transformer. The one you will be using is provided in the laboratory
and is shown in Figure 4 with its corresponding schematic in Figure 5.
Secondary Winding Center-Tap
Figure 5. Step-Down Transformer
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Figure 5. Step-Down Transformer
3. Laboratory Procedure
A. Interfacing a LED with power
Using your LED I/V data from Lab 1 and your pre-lab design values, you will build
a LED circuit powered with 1 of the 3 pre-lab input voltages and verify your design.
Experiment 1 – Interfacing a LED with power.
1. From your pre-lab calculations for a current limiting resistor, construct a test
circuit and verify your design for one current limiting resistor values you
determined and the corresponding input voltage. Verify the following
parameters:
Circuit input voltage (Vin)

Voltage drop across the LED (Vd)

Voltage drop across RS

 LED current (ID)
B. Half Wave Rectifier
In Experiment 2, a half wave rectifier will be built using a 1N4148 silicon diode.
The alternating current input will be supplied from a step down transformer
supplied by your instructor. Output waveform measurements will be taken across a
load resistor.
Experiment 2 – A Half Wave Rectifier
1. Construct the circuit shown in Figure 6. On the oscilloscope, capture the
input and output waveforms. Using the measure function capture and display
the following parameters:
 Input voltage amplitude (use the “Max” measurement feature)
 Input voltage RMS (use the “Cycle RMS” feature )
 Output voltage amplitude (use the “Max” measurement feature)
 Output voltage RMS (use the “Cycle RMS” feature )
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120 Vac
Transformer
D1
D1N4148
Ls1
Lp
RL
3.9k
Ls2
Figure 6. Half-Wave Rectifier
B. Full Wave Rectifier, Center-Tapped Configuration
In Experiment 2, a full wave rectifier will be built using two 1N4148 silicon diodes.
The alternating current input will be supplied from a step down transformer using
the center-tapped configuration. Output waveform measurements will be taken
across a load resistor.
Experiment 2 – Full Wave Rectifier, Center-Tapped Configuration
1. Construct the circuit shown in Figure 7. On the oscilloscope, capture the input
(between outer transformer leads, not outer and center-tap) and output
waveforms. Using the measure function capture and display the following
parameters:
 Input amplitude (use the “Max” measurement feature)
 Input RMS (use the “Cycle RMS” feature )
 Output amplitude (use the “Max” measurement feature)
 Output RMS (use the “Cycle RMS” feature)
120 Vac
Transformer
D1
D1N4148
Ls1
Lp
D2
Ls2
RL
3.9k
D1N4148
Figure 7. Full-Wave Rectifier, Center Tapped Configuration
C. Full Wave Rectifier Bridge
In Experiment 3, a full wave rectifier bridge will be built using four 1N4148 silicon
diodes. The alternating current input will be supplied from a step down transformer
using a center-tapped secondary winding. Output measurements will be taken across
a load resistor with an oscilloscope.
Note: Due to grounding issues with the full wave rectifier bridge configuration
when used in conjunction with the oscilloscope, capturing of the input and
output waveforms must be done separately.
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Experiment 3 – Full Wave Rectifier Bridge
1. Construct the circuit shown in Figure 8. On the oscilloscope, first capture the
input waveform displaying the following measurements:
 Input amplitude (use the “Max” measurement feature)
 Input RMS (use the “Cycle RMS” measurement feature)
2 On the oscilloscope, capture the output waveform displaying the following
measurements:
 Output amplitude (use the “Max” measurement feature)
 Output RMS (use the “Cycle RMS” measurement feature)
120 Vac
Transformer
D2
D1N4148
D1
D1N4148
D4
D1N4148
D3
D1N4148
Ls1
Lp
Ls2
RL
3.9k
Figure 8. Full-Wave Rectifier, Center Tapped Configuration
4. Data Reduction and Lab Report
This Lab submittal will be an informal report. Your report should be in Word with
graphics pasted in.
The items listed below are the minimum that should be included in your report.
A. Experiment 1 Data: Interfacing a LED with power
1. A schematic of your circuit.
2. Measured value for VD and ID.
3. Answer to the following question:
a. Did the LED light up?
B. Experiment 2 Data: A Half Wave Rectifier
1. A schematic of your circuit.
2. A oscilloscope trace of the input and output wave with the following
measurements shown:
 Input amplitude
 Input RMS
 Output amplitude
 Output RMS
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C. Experiment 3 Data: Full Wave Rectifier, Center-Tapped Configuration
1. A schematic of your circuit.
2. A oscilloscope trace of the input and output wave with the following
measurements shown:
 Input amplitude
 Input RMS
 Output amplitude
 Output RMS
D. Experiment 4 Data: Full Wave Rectifier Bridge
1. A schematic of your circuit.
2. A oscilloscope trace of the input wave with the following measurements shown:
 Input amplitude
 Input RMS
 Output amplitude
 Output RMS
3. A oscilloscope trace of the output wave with the following measurements
shown:
 Input amplitude
 Input RMS
 Output amplitude
 Output RMS

E. Summary Table
1. A table comparing the input and output RMS values for each of the rectifiers
built and tested.
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