PHYSICS 5620
LAB 3: DIODE CIRCUITS AND POWER SUPPLIES
Objective
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To measure the current-voltage characteristics of a p-n junction diode (standard
silicon diode) and a light emitting diode (LED).
To observe the features of several circuits which utilize diodes.
To build simple unregulated power supplies.
Introduction
The large reverse resistance and low forward resistance characteristics of the standard
diode find extensive use in electronic circuitry. In order for the diode to begin
conducting, a positive voltage bias greater than the forward voltage drop of the diode
(about 0.3 V for Ge and 0.7 V for Si) must be applied. In this laboratory, a standard
series of silicon diodes for low-current applications will be used: the 1N4001 through
1N4007 diodes. The maximum ratings for these diodes are a forward current of 1000 mA
and a reverse voltage varying from 50 to 1000 V.
Experiments
3.1.
Current-Voltage Characteristics of a p-n Junction Diode
0-12 V
R
Figure 3.1: Circuit to measure the I-V characteristics of a p-n junction diode.
Construct the circuit shown in Figure 3.1. The resistor in the circuit should be chosen so
that it limits the current through the diode to less than 100 mA. Ask your instructor if
you are unsure of an appropriate value for R. HINT: Consider the maximum voltage
which can be applied across the diode and resistor. The 12 V variable DC voltage
source on the NI ELVIS board is used in this circuit.
Measurement 1: Using the DMM, measure the voltage drop across the diode and the
current flowing through the diode. Note: It is almost always more convenient to
measure a voltage than a current. The current through the diode is equal to the
current through the 100 Ω resistor, which is proportional to the voltage across the
resistor. Measure a sufficient number of points in both the forward-biased and the
reverse-biased directions so that you can accurately plot the I-V curve. Present your
data in a table and an I-V graph.
Question 1: Compare your results to the Ebers-Moll prediction:
I = I s ⎡⎣exp (V VT ) − 1⎤⎦ ,
(1)
where Is ∼ 10-9 A is the reverse bias saturation current, and VT ∼ 0.045 V for these
diodes. (VT = ηkBT/q = η15.3 mV at room temperature, η, emission coefficient, = 1 to
3.) By varying Is and VT around these starting values, find the values that gives the best
fit of the Ebers-Moll formula to your data.
3.2.
I-V Characteristics of a Light Emitting Diode (LED)
LED
0-12 V
R
Figure 3.2: LED Circuit.
An LED is a diode that emits light when current flows through it. Here you will study
the I-V characteristics of an LED to understand how an LED functions. Connect the
circuit shown in Figure 3.2 using the NI ELVIS 12 V variable DC voltage source and a
100 Ω current limiting resistor.
Measurement 2: Measure the current through the LED as a function of voltage across
the LED. Also note the relative brightness of the LED as a function of the current (for
consistent results, look at the LED from directly above). Present your data in a table and
plot a graph of I vs. V.
Question 2: What is the minimum current needed to make the LED easily visible?
3.3.
Half Wave Rectifier
Construct the half-wave rectifier circuit shown in Figure 3.3.
10k
Figure 3.3: Half Wave Rectifier Circuit.
Measurement 3: Using a 1 kHz sine wave from the function generator, observe the
output voltage across the resistor for input voltages (peak to peak) of 0.5V, 1V, 2V and
5V. Measure the peak voltage of the output in each case, and sketch the input and output
waveforms.
Question 3: Why is there a difference between the peak voltage of the input and
output signals?
Next observe the effect of connecting a 0.47 μF capacitor and then a 4.7 μF capacitor in
parallel with the resistor.
Measurement 4: Measure the ripple (Vpp) in the output voltage for each capacitor using
an input sine wave of 5 Vpp. Sketch the output waveform in each case.
Question 4: Comment on the effect of adding the capacitors.
3.4.
Full Wave Rectifier
Vin
C filter
R load
Figure 3.4: Full Wave Rectifier Circuit.
Construct the full wave rectifier circuit shown in Figure 3.4 with Rload = 10 kΩ, and
electrolytic capacitor Cfilter = 220 μF. For safety, check the polarity of the voltage across
the resistor before connecting the electrolytic capacitor. The capacitor must be inserted
with the correct polarity or it may explode!
Measurement 5: Using a 1 kHz sine wave from the function generator, observe the
output voltage across the resistor for input voltages of 0.5V, 1V, 2V and 5V peak to peak.
Measure the average output voltage and peak to peak ripple in each case, and sketch the
input and output waveforms.