LAB 3: Diodes

In this lab, we will determine the characterisitics of the diode characteristics and its behavior in a particular circuit. We will explain the use of the I-V curves and graphs with correspondence to a resistor, a diode, a Zenor diode, and a LED. We will use voltmeters and ampmeters to find the voltage and current, respectively, through the circuit in which the device under test (i.e. resistor, diode, etc.) will be examined. The device under test will be studied by it’s I-V graph. We will compare the device under test’s I-V graph to the basic characteristics to the behavior of that object.

- use a 1K ohm resistor to be a device under test

- the resistor will be our device we will be testing for this experiment

- the object is to find a linear or curve relationship between the current and voltage going through the resistor

- adjust the voltage going through the circuit by tuning the power supply

- use +20 V scale on the power supply

- connect the voltmeter’s V hole to the positive side of the resistor with a banana jack

- connect the voltmeter’s COM hole to the negative side of the resistor with another banana jack

- break the circuit to measure the current flowing through it

- turn power supply adjustment knob so the ampmeter will read the current of -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5 mA

- record the voltage from the voltmeter for each current reading

- plot the I-V curve of the resistor

R2 = 1 K ohm

gold = 5% tolerance

R2 range = 950 to 1050 ohms

power supply is at +20 scale

the resistor should behave in a linear relationship between the current and voltage through Ohm’s Law

table of expected values of I-V plot: resistor - experiment analysis

R1 = 988 ohms

R2 = 1005 ohms

table of expected values of I-V plot: resistor - actual measurement

Note: The graph plot can be viewed on the next page.

We found that the outcome plot has a linear relationship between the resistor’s current flow and voltage though the circuit. This is exactly what we expected. The linear relationship coincides with Ohm’s Law, the resistance being the slope of the line. As with the resistor, it does not matter if the voltage is negative. Because when the voltage is negative, the current is negative as well, causing a positive resistance.

- repeat as in experiment 1

- however, now use a 1N4148 diode as your device under test

- black band represents the negative side

- put the diode in the place of the resistor in the former problem

- diode will not connect at a negative direction

- do not apply more than 12 V on the circuit

- adjust power supply using +20 V source

- read the voltage across the diode for the current 0 to 5 mA

- read the diode current for the circuit by adjusting the voltage to -2, -4, -6, -8, -10 V

- record and plot the data

The graph of the diode plot should have a curve increasing from the zero current and have a limit infinty of about 0.7 V. The diode does care about the sign and should only work in positive voltage from 0 to 0.7 V. Negative voltage in a diode produces no current. We can estimate our values with diode equation, iD = Id - I0 = I0(e^(qvd/kT)-1).

table respect to diode current

table respect to diode voltage

According the our measurements and the I-V plot, the diode does not operate at all when there is the negative voltage. Diodes only operate when there is a positive voltage because it closes its gate when negative or zero voltage passes through. We see that the graph goes by the diode equation iD = Id - I0 = I0(e^(qvd/kT)-1). The curve on the graph represents the exponential of the diode. The curve somethimes depend on the temperature of the room, but such measurement differences are small. We see that the diode does have a limit to infinity at 0.7 V, no matter what the current might be (at least for positive values). We couldn’t measure the value of the diode breakdown, primarily because if we do, the diode will be french toast. The breakdown is probably farther negative in voltage.

- replace the last experiment with a 1N5232B Zener diode

- the black band is the negative side

- maximum voltage: +12 V on +20 V scale of power supply

- measure diode voltages for diode currents 0 to 5 mA

- measure diode currents for diode voltages -0.5 to -5 mA

- record and plot data for the Zenor diode

The Zenor diode is intentionally operated at negative current and voltage. Zenor diode should primarily work the opposite of a regular conventional diode.

table with respect to current of Zenor diode

table with respect to current of Zenor diode

table with respect to voltage of Zenor diode

The Zenor diode acts like a regular diode in the forward region. However, its limit is 0.8 V instead of the 0.7 V in a regular diode. In the reverse bias region, the Zenor diode appears to have a limit to infinity as well to -5.4 V. This may mean that the Zenor allows negative current and voltage through its gates, unlike the regular conventional diodes. Between the 0 to -5 V, absolutely no current flows through the circuit. So a Zenor diode regulates voltage only at 0 to 0.8 V and -5 to -5.40 V. The intention of the Zenor diode is to operate at the negative voltage and current, but we also see that the Zenor can also operate at positive voltage and current as well.

- repeat the last experiment, but now use a LED as our device under test

- measure the diode voltages for diode currents 0 to 5 mA

- measure the diode current for diode voltages -1 to -8 V

- record and plot the data

The LED should light between 1 V and 2 V. Other voltages should not work, including negative voltages.

In the LED, the device has a curve plot with a limit near 2.0 V. The LED does light from 1 to 2 V. It appears the light is on no matter how much positive current you put on the circuit. However, the LED will not light below 1 V. In the negative voltages, the current becomes zero.

In this lab, we had learned that the diode, Zenor diode, and the LED has a curve relationship in the positive current and voltage and has a limit in positive voltage. In the diode, the voltage limit is 0.7 V. We learned that the diode operates at this voltage, despite how much current goes through the diode. The Zenor behaves like a diode, but operates in the negative voltage and current. Hence, it is the opposite of a diode, having a limit and constant voltage of -5.4 V. The LED operates at a voltage of between 1 to 2 V. We learned that the LED will not operate at a negative voltage. The diode, Zenor, and

LED, when not operating, has it’s gates closed. Meaning the device will not regulate any current or voltage, unlike the resistor, which regulates at almost all voltage and current. This means that the diode regulates current at only positive voltage (0.7 V). The Zenor diode regulates current at lower than -5 V to -5.4V. The normal breakdown of a diode is about the region where the Zenor regularly operates.