ECEN 1400 - Introduction to Digital and Analog Electronics

Peter Mathys, Spring 2014

Lab 1: i-v Characteristics

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Goals of this Lab

This lab is a group activity.
The group assignments are given here. One lab report per group needs to be turned in on D2L. The responsibilites for the successful completion of the lab consist of three parts: The prelab, the actual lab measurements, and the writing of the report. The report will be graded according to three criteria: Correctness, completion, and clarity. On the cover page you must clearly state which group member had the main responsibility for the prelab, for the lab measurements, and for the report writing. All group members need to be knowledgeable for all three parts, but each member has a specific role in the group. The responsibilities must be rotated for future labs so that each group member will have experienced all three roles.



P1. Download and Install Multisim. Go to the Software page for instructions to download and install the National Instruments Multisim software. Note that this software is designed for the Windows operating system. It will also run on Windows emulators for the Mac and Linux, but this will still require a Windows operating system. One alternative is to use the computers in the lab for Multisim assignments. Another possibility is to use browser based online tools such as Circuit Lab or Do Circuits.

P2. Measure Resistor i-v Characteristic in Multisim. A video tutorial for this is available (here not yet working) or here. Use the setup shown below in Multisim to measure the i-v characteristic of resistor R1. Note that every circuit in Multisim needs to have at least one ground Ground symbol as reference, otherwise you get the following message when you try to run a simulation.

No Ground Consistency Check Error

Use a resistance value of your choice in the range of 100 ohm to 1 kohm for R1. If the resistor can dissipate 0.25 W of power, what is the largest positive or negative value that the voltage source V1 can take on so that the power dissipation in the resistor R1 does not exceed 0.25W? Show your cacluations.

Multisim setup for measuring i-v characteristic of R

Make sure that the multimeter XMM1 is set as an amp (current) meter (right-click on it, select properties and then click on the "A" button) and measure several (i,v) pairs for positive and negative values of v by changing the value of V1 (within the range that does not exceed 0.25 W power dissipation for R1). Make a labeled plot of the i-v characteristic of R1.

P3. Measure Red LED i-v Characteristic in Multisim. Look at the datasheet of a typical red LED. In Multisim a red LED is selected under Diodes as follows:

Selecting a red LED in Multisim

You will notice that a safe operating point for a typical red LED is a continuous current of about 20 mA which results in a forward voltage vF of about 2 volts. In the Multisim setup shown below for measuring the i-v characteristic of a red LED, compute the value of R1 (using Ohm's law) such that a current of approximately 20 mA flows through the LED when the voltage source V1 is set to 5 V.

Multisim setup for measuring i-v characteristic of red LED

The purpose of R1 is only to limit the current that flows through the LED (and prevent its destruction when you measure a real device) when it is driven by a voltage source. The (i,v) pairs for the i-v characteristic are measured with respect to the LED only. Make sure again that the XMM1 multimeter is set to measure current and measure several (i,v) pairs (using the two multimeters) for V1 in the range of about -1 V to +5 V. What is the smallest value of V1 that yields a nonzero current through the LED? Use the measured (i,v) pairs to make a labeled plot of the i-v characteristic of the red LED. You may discover when you make the plot that there are areas where you did not make enough measurements to make a good plot. If that is the case, go back and make a few more measurements in those areas. Use the insight gained from this to develop a strategy of how you are going to measure a real LED in the lab.

Lab Experiments

Note: You will need at least one lab kit per group. The lab kits are available for $100 (plus tax) at the EE store in ECEE 1B10.

E1. Identify Lab Equipment. In the picture below identify the following pieces of lab equipment:

Picture of Lab Equipment

User manuals for most of these are available under References

Another piece of equipment that is used in the lab is the handheld TENMA 72-2050 Multimeter shown below.

TENMA 72-2050 Handheld Multimeter

Having both a benchtop and a handheld multimeter available is very useful for the i-v characteristic measurements in experiments E2 and E3.

E2. Measure i-v Characteristic of a Resistor. The goal of this experiment is to perform the same measurements as in prelab problem P2, but this time on a real resistor using the equipment in the lab. Pictures of resistors and the color code for resistors can be found here.

The schematic below shows the measurement setup. Use the same resistor value the you used in P2. Use the handheld multimeter for the measurement of the current (XMM1 in the schematic) through R1 and the benchtop multimeter for the measurement of the voltage (XMM2 in the schematic) across R1.

Schematic for measuring i-v characteristic of a resistor

A possible implementation of the circuit on the breadboard in your lab kit is shown next.

Breadboard setup for measuring i-v characteristic of a resistor

Use a banana plug cable to connect the "+6 V" output of the power supply to the "mA" input of the handheld multimeter. Set the multimeter selector swith to the "mA" position. Use another banana plug cable to connect the "COM" terminal of the multimeter to the red banana jack on the breadboard. To close the loop, use a banana plug cable from the black banana jack on the breadboard to the "-6 V" connector on the power supply. To measure the voltage across the resistor, use a banana plug cable to connect the black banana jack on the breadboard to on of the "LO" inputs of the benchtop multimeter. Use a second banana plug cable to connect the green banana jack on the breadboard to the "V (HI)" input (top right) of the benchtop multimeter. Set the benchtop multimeter to "DC V". Then turn on the power supply and measure several (i,v) points by adjusting the output voltage of the power supply. Take care not to exceed the 0.25 W power rating of the resistor (check the maximum voltage you computed in P2). To obtain (i,v) pairs for negative v, swap the cables that connect to the power supply. Note: You need to record the voltage values read (with the multimeter) directly across the resistor, not the voltage displayed by the power supply. The main reason for this is that there is a voltage drop across the amp meter. Once your measurements are completed, make a labeled plot of the i-v characteristic of the resistor that you chose for this experiment. Check that the slope of the i-v gaph is indeed 1/R.

E3. Measure i-v Characteristic of a Red LED. In this experiment you will perform the measurement practiced in prelab problem P3 on a real red LED from your lab kit.

The schematic of the measurement setup is shown below. Use the resistor value for R1 that you computed in P3.

Schematic for measuring i-v characteristic of a red LED

A possible arrangement of the components on the breadboard is shown in the next picture. Observe the polarity of the LED. The longer lead is the anode which is connected to the more positive side. The flat side of the LED case is on the cathode side which is connected to the more negative potential. Check the drawing in the datasheet.

Breadboard setup for measuring i-v characteristic of a red LED

Use the same wiring and the same general approach as in E2 to measure several (i,v) points of the red LED i-v characteristic. Use the insights you gained from P3 to select the values of v at which you measure i (or vice versa) to obtain a good i-v graph. Make sure that you stay within the current limit of the LED (don't go beyond approximately 30 mA).

E4. Use Waveform Generator to Drive Red LED. In this experiment the red LED will be driven by the sinusoidal output of the waveform generator (through resistor R1 to limit the current). The oscilloscope will then be used to observe the voltage across the LED and one of the multimeters will be used to measure the dc voltage across the LED. The schematic for this, where XFG1 is the waveform generator, XSC1 is the oscilloscope, and XMM1 is the multimeter, is shown in the following figure.

Schematic for driving red LED from waveform generator

Use the same breadboard setup as in E3, but this time connect the output of the waveform generator to the red and black banana jacks. Set the waveform generator to output a sinewave with a frequency of 1 kHz and an amplitude of 8 Vpp (pp stands for peak-to-peak). Leave the dc offset at 0 V. Connect one of the multimeters to the green and black banana jacks and set it to measure dc volts (not ac volts). What do you expect the measurement to be? A positive voltage, zero, or a negative voltage? Why? Next, use the oscilloscope probe (from your lab kit) shown below to display the voltage across the red LED on the oscilloscope. Measure the average voltage Vavg with the oscilloscope. Is it positive, zero, or negative?

Oscilloscope Probe

The oscilloscope probe has a switch where you can choose a 1:1 or a 10:1 attenuation factor. In general, you obtain more precise results with the 10:1 setting. To obtain correct amplitude readings from the oscilloscope you have to set the scope channel on which you use the probe to the 10:1 setting. Sketch and/or plot 2 or 3 periods of the signal displayed on the oscilloscope (including time and amplitude scaling). What is your interpretation of the result? When is the LED conducting current? Is it an ideal "one-way street" for current?

Set the waveform generator to 1 Hz and leave all other settings unchanged. What result do you expect to see on the LED? Is it the same as what you actually see when you turn on the waveform generator output? For what purpose do you think the circuit that you analyzed in this experiment could be used?