Download Experiment 1-3

PHYSC 3322
Experiment 1.3
4 May, 2017
Diodes
Purpose
In this experiment, you will investigate the current-voltage characteristic of
semiconductor diodes, and consider their applications in DC power supplies.
Background
Select a 1 k resistor, and measure its exact resistance using the Fluke multimeter.
Select one of the diodes and construct the circuit shown in Figure 1, using the resistor
you just measured, the Tektronix power supply, and two Fluke multimeters. One meter
measures the voltage drop across the diode, while the other monitors the current
through the resistor and diode.
By adjusting the output voltage of the power supply, measure the voltage across and
current through the diode. As you vary the power supply voltage, monitor the power
dissipated in the resistor ( P  V 2 / R ). Do not allow the power to exceed 250 mW .
Next, reverse the polarity of the power supply connections and measure the diode
current and voltage in the reverse direction. Again, do not allow the resistor power
dissipation to exceed 250 mW . Plot the diode's complete (both negative and positive
polarity) I  V curve using Sigma Plot.
Repeat the above measurements with each of the four diodes (1N914, 1N34, 1N4733
and the red light-emitting diode). Plot your results.
Construct a half-wave rectifier using a 1N4007 diode as shown in Figure 2. Doublecheck your circuit before applying power—if the circuit is incorrect, you will likely
blow a fuse! View the output wave form with the oscilloscope, and sketch the wave
form shape. Do this with (1) no capacitor in the circuit, (2) a 22F capacitor, and (3) a
220 F capacitor.
Now convert the circuit into a full-wave rectifier by adding a second 1N4007 diode, as
shown in Figure 3. Again, sketch the output waveform for the three different output
capacitors.
Figure 1. Diode current vs. voltage
measurement.
1
PHYSC 3322
Experiment 1.3
4 May, 2017
Figure 2. Half-wave rectifier.
Figure 3. Full-wave rectifier.
Questions
Discuss and compare the I  V curves for the different diodes. A simplified model for
forward conduction in a semiconductor diode relates the current and voltage by


I  I 0 expV /V0   1 ,
(1)
where I 0 and V0 are constants that depend of the type of diode. Do your diodes obey
this relation? Estimate I 0 and V0 for each diode. To estimate I 0 , consider that when
V is negative and significantly larger than V0 in magnitude, Equation 1 reduces to
I  I 0 .
(2)
Once you have a value for I 0 , you can rearrange the equation and take the natural
logarithm of both sides to get a linear relationship with which you can determine V0
using a least-squares fitting procedure.
Explain the shapes of the output waveforms in the half-wave and full-wave rectifier
circuits.
2