Project1-The Diode.

If you have access to electronic equipment, you may want to perform the simple project described in the next few problems. If this is the first
time you have tried such a project, get help from an instructor or someone who is familiar with electronic projects.
When building electronic circuits, eventually you'll make a mistake (as all of us do), and sometimes those mistakes cause circuits to fry. If you smell hot electronic components, disconnect the battery from the circuit, and then check the circuit to determine what connections you should change.
When fixing a circuit, follow some simple safety rules. Do not try to rearrange connections with the battery connected because you may short leads together.
Also, don't touch bare wires with live electricity. Even with batteries, you have a chance of being burned or seriously injured. If your skin is
wet, it forms an electrical connection with lower resistance, allowing more current to flow, potentially injuring you.
If you do not have access to equipment, do not skip this project. Read through the project, and try to picture or imagine the results. This is
sometimes called “dry-labbing” the experiment. You can learn a lot from reading about this project, even though it is always better to actually
perform the project. This advice also applies to the other projects in many of the following chapters.

Objective

The objective of this project is to plot the V-I curve (also called a characteristic curve) of a diode, which shows how current flow through the

diode varies with the applied voltage. As shown in Figure 2.10, the I-V curve for a diode demonstrates that if low voltage is applied to a diode, current does not flow. However, when the applied voltage exceeds a certain value, the current flow increases quickly.

Fig 2.10

General Instructions

While the circuit is set up, measure the current for each voltage value. As you perform the project, observe how much more rapidly the current increases for higher applied voltages.
Parts List

• One 9 V battery
• One snap battery connector
• One multimeter set to measure current (mA)
• One multimeter set to measure DC voltage
• One 330-ohm, 0.5-watt resistor
• One 1N4001 diode
• One breadboard
• One 1 MΩ potentiometer
• One terminal block

Step-by-Step Instructions

Set up the circuit as shown in Figure 2.11. The circled “A” designates a multimeter set to measure current, and the circled “V” designates a multimeter set to measure DC voltage. If you have some experience in building circuits, this schematic (along with the previous parts list) should provide all the information you need to build the circuit. If you need a bit more help in building the circuit, look at the photos of the completed circuit in the “Expected Results” section.

Fig 2.11

 Carefully check your circuit against Figure 2.11, especially the direction of the battery and the diode. The diode has a band at one end.
Connect the lead at the end of the diode with the band to the ground bus on the breadboard.
After you check your circuit, follow these steps, and record your measurements in the blank table following the steps:

1. Set the potentiometer to its highest value. This sets the voltage applied to the diode to its lowest possible value.
2. Measure and record the voltage applied to the diode.
3. Measure and record the current.
4. Adjust the potentiometer slightly to give a higher voltage.
5. Measure and record the new values of voltage and current.
6. Repeat steps 4 and 5 until the lowest resistance of the potentiometer is reached, taking as many readings as possible. This results in the highest voltage and current readings for this circuit. At this point, the potentiometer resistance is zero ohms, and the current is limited to approximately 27 mA by the 330-ohm resistor. This resistor is included in the circuit to avoid overheating the components when the potentiometer is set to zero ohms. If your circuit allows currents significantly above this level as you adjust the potentiometer, something is wrong. You should disconnect the battery and examine the circuit to see if it were connected incorrectly. If V gets large—above 3 or 4 volts —and I remains small, then the diode is backward. Reverse it and start again.
7. Graph the points recorded in the table using the blank graph shown in Figure 2.12. Your curve should look like the one shown in Figure 2.10.

Fig 2.12

Figure 2.13 shows the breadboarded circuit for this project.

Figure 2.14 shows the test setup for this project.

 Compare your measurements with the ones shown in the following table:

Further reductions in the resistance below the 330 ohm included in the circuit causes little increase in the voltage but produces large increases in the current.
Figure 2.15 shows the V-I curve generated using the measurements shown in the preceding table.

Fig 2.15

 The V-I curve (or diode characteristic curve) is repeated in Figure 2.16 with three important regions marked on it.

Fig 2.16

The most important region is the knee region. This is not a sharply defined changeover point, but it occupies a narrow range of the curve where the diode resistance changes from high to low.
The ideal curve is shown for comparison.
For the diode used in this project, the knee voltage is about 0.7 volt, which is typical for a silicon diode. This means (and your data should verify this) that at voltage levels below 0.7 volt, the diode has such a high resistance that it limits the current flow to a low value. This characteristic knee voltage is sometimes referred to as a threshold voltage. If you use a germanium diode, the knee voltage is approximately 0.3 volt.