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Laboratory #4 ­ introduction to diodes and diode circuits

Objectives

* Learn how to use oscilloscope to make IV graphs * Generate IV curves for various types of diodes * Learn how to properly perform multiple channel voltage measurements with oscilloscope * Investigate how effective Zener diodes are as a voltage regulator"

Lecture References Orange Book

* 0.22- Reference vs. Ground * 4.1-4.10- IV characteristics * 6.1-6.3- Diode * 6.4-6.8- Linear models of diodes * 6.12-6.19- Zener diode * 6.13- Light Emitting Diode (LED)

Transparencies

* Brunet Handouts 7- IV Characteristic * Brunet Handouts 8- Diodes * Haken Set 1- pg. 29-47

Mallard

* Homework 6- IV Characteristics * Homework 7- Diodes

EXPERIMENT #4

prelab

In lab 2 we measured the I-V characteristics of resistors, and, optionally, motors, and incandescent bulbs. Within the range of voltages that we used to measure the current, the resistors had a linear I-V curve, a linear relationship characterized by Ohm's Law ­ V=RI, or I=V/R. You found the voltage to be directly related to the current by a simple constant. The other devices, the bulb and the motors had more interesting, non-linear I-V relationships. During this lab you will be investigating the behavior of a class of devices ­ diodes ­ that have very useful, nonlinear I-V characteristics. In lecture, you are on a threshold where you will be turning from the analog world of sources, resistors, capacitors, and inductors to a digital world where all information is passed as discrete states rather than a continuum of states. Devices that are used in most digital circuits are based on devices that can hold two states ­ a switch. A switch can be ON or OFF ­ it can connect a signal to a HIGH or LOW voltage, ideas that can be mapped onto the abstract framework of Boolean Logic. During the last half of the semester you will be working with devices ­ logic gates ­ that can be modeled using this framework. Transistors are the device most commonly used to build these logic functions in hardware. But the fundamental building block that allows the transistor to behave the way it does is the p-n junction which is a diode. Digital logic functions will play a key role in your final design. To control the decision making circuitry that interprets the information from the InfraRed sensors looking down at the table, you will be using circuits on 'chips' that perform logical operations, or in this years' final design you will use a microcontroller buillt up from many logic elements. Using these chips/board you will design a circuit to navigate the vehicles to follow a white piece of tape on a black table. To appreciate the complexity of this simple device ­ a switch, try and draw an I-V curve for a manual switch. Conceptually, this would be the perfect device to implement a system based on two-state logic, but a manual switch has an obvious drawbacks for all applications where speed, size, complexity, and autonomy are desired. The electronic counterpart of the switch is the diode. The characteristics of the diode that make it a good switch is that the diode will conduct current only when the voltage applied across the terminals is greater than a predetermined voltage. Low voltage -> OFF or FALSE or 0 High voltage -> ON or TRUE or 1 The two voltage ranges can be mapped onto one of these two states. Transistors are the components that are actually used to implement logic on chips since one of the many behaviors of a transistor is that in some configurations it behaves like a switch. But ­ the simpler device first...

The beauty of a two-state system is that the simple concept of "on" and "off" can be mapped onto a range of input voltages so that in electrically noisy conditions the circuit performs as desired. 1. Explain how a diode can be used as a two-state system mapping the concepts of "true" and "false" in an electrical circuit.

2. Sketch an I-V graph for a diode and indicate the voltage ranges representing the two state.

3. Can you think of a drawback of using just a diode? .

EXPERIMENT #4

procedures

In this lab you will play with a new class of device - diodes. These devices are often used as switches and have traditionally been used in building digital logic gates. There are also many other applications that use the analog behavior of the diode: i) a rectifier, ii) voltage regulators, iii) fuses, iv) bridges, v) light emitting diode array as a display... One such application will be investigated at the end of the lab ­ the voltage regulator where a zener diode is used to provide a constant voltage drop across a wide range of loads ­ a design you saw in your homework. There are several different types of diodes - we will experiment with three, a basic diode, a zener diode, and a light emitting diode (invented here by some folks in the microelectronics group).

Diodes -- Basic Dioide Build the circuit shown using a diode, 1k resistor, test box, function generator, and an oscilloscope. Be sure to properly orient the diode in the circuit. You should be old hands at building circuits, but if you are still confused ask your TA for help. Always build your circuit first without the test equipment (e.g. the oscilloscope probes). Pay particular attention to the polarity of the oscilloscope probes ­ remember the negative side of both channels of the oscilloscope are tied to a common point inside the oscilloscope.

Set up the oscilloscope to display the I-V curve of a common variety diode

Connect the diode and resistor in series. Be sure you get the diode oriented properly ­ ask your TA if you have questions. The circuit will not work properly if you connect the diode backwards. The line on the casing of the diode marks the negative terminal. Connect the function generator across the series connection of the diode and resistor. Turn on the function generator and set it to a 100 Hz sine wave with zero DC offset. Probe the diode in the direction indicated in the figure with channel 1 of the oscilloscope. Probe the voltage across the resistor in the direction indicated in the figure with channel 2 of the oscilloscope. Invert channel 2. Put the oscilloscope into x-y mode. Position the displayed dot at the center of the screen. Vary the amplitude of the waveform from the function generator and the horizontal and vertical scales on the oscilloscope until you feel you have obtained a good image.

Don't dismantle the setup when you are finished with this part.

1. Sketch this curve using the graph paper provided or using the computer. 2. Remember that the oscilloscope is a voltage measuring instrument. manage to measure current on the Y-axis? How did we

3. For this particular diode what would be the voltage threshold (or tun-on voltage) where the diode changes from not conducting to conducting.

-- Zener Diode Use the same experimental setup as in part 1 , but replace the diode with a Zener diode. Again vary the amplitude of the waveform from the function generator and the horizontal and vertical scales on the oscilloscope until you feel you have obtained a good image.

Set up the oscilloscope to display the I-V curve of a zener diode

Replace the regular diode with a zener diode without disconnecting anything else. Again, remember to orient the diode properly. Vary the amplitude of the waveform from the function generator and the horizontal and vertical scales on the oscilloscope until you feel you have obtained a good image.

Don't dismantle the setup when you are finished with this part.

4. Sketch the curve in your lab notebook. 5. The "Zener breakdown voltage" is the negative voltage value at which the graph drops sharply. Measure this voltage on the oscilloscope. How could a device with the characteristics of a Zener diode be useful?

-- A unique Light Emitting Diode (LED) Light Emitting Diodes (LEDs) function in exactly the same way as normal diodes except that they are made of a semiconducting material that emits light when the diode is in the "on" state. For this lab we have a particularly interesting LED ­ can you discover its special ability?

Set up the oscilloscope to display the I-V curve of a special light-emitting diode

Replace the regular diode with the white capped LED without disconnecting anything else. And again, be sure to orient the LED properly. This device is not marked with a line like the regular and zener diodes. In fact, it does not look like the other two devices that you have used so far. The polarity is marked by making one lead shorter than the other. The longer lead marks the positive terminal, the shorter lead the negative. Set the function generator to a 100 Hz sine wave with amplitude 9 V Peak-toPeak and zero DC offset.

6. What color is the LED emitting? Be precise in describing the color.

7. Now change the DC offset of the function generator to its maximum positive setting (~5V). What color is the LED emitting now?

8. Now change the DC offset of the function generator to its maximum negative setting (~ -5V). What color is the LED emitting now?

9. Does the LED actually emit the in-between color in the middle section of the graph, or is this merely an optical illusion caused by the rapid switching between the two other colors? You might find additional insight into this question by reducing the frequency of the sine wave to 1 Hz.

10. Draw the full I-V characteristic of this LED, and label the color it emits (or if it is not emitting at all) for each region of the graph.

11. Based on the graph, do you think this LED is some kind of "Zener LED", that is a single device whose characteristics are different under forward and reverse biasing, or merely two different normal LEDs connected in reverse parallel? Explain your reasoning.

Diode Application -- voltage regulation and clipping In lab 3 you experimented with voltage sources that are non-ideal. They are non-ideal in the sense that the voltage changes as you modify the load attached to it. For many applications this is unacceptable (our power supplies as an example) and a circuit is added to the source to regulate the voltage so that it remains the same for a wider range of load resistances. Similar designs can also be used to clip and rectify signals (circuits that you encountered during lecture and in Mallard problems) suggesting an application for AC-DC conversion. The voltage regulating circuit you will be experimenting with in this portion of the lab is one that you encountered for homework and is also a simplified version of the circuit inside your vehicle that regulates the voltage from the battery to provide the 5V for the logic circuits. The regulator comprises a simple non-ideal voltage source - such as a battery with some internal resistance and a Zener diode. From the I-V curve of the zener diode (which you just drew in your notebooks) you can see two regions where the diode behaves like a source in the sense that the voltage remains nearly the same across the device for a wide range of currents flowing through the device. The diode is, of course, not a source since it cannot provide power. In class, you modeled its "source-like" behavior by replacing the zener diode's I-V curve with a piecewise linear model consisting of 2 vertical lines. A slightly more exact model would take into account that the zener does not emulate an ideal source but has some resistance of its own. Ideally, if the value of V s is sufficiently large to drive the Zener diode into the breakdown region then any load that is connected across the terminals of the Zener diode will see the voltage across the load held at the zener breakdown voltage ­ no matter how the load might change. But circuits never behave ideally ­ for this circuit there is a range of load resistances for which the circuit behaves as an ideal regulator. Outside of this range the circuit no longer regulates, or keeps the voltage across the load at the desired level.

The voltage is regulated or held constant across the zener diode because the the diode is capable of handling a wide range of currents with a very small change in voltage. Let's investigate how the circuit works given a constant, positive V s and Rs so that the voltage across the zener diode is sufficient to put the device into the breakdown region for at least some values of the load R L. From what we know of circuit theory we can think about the two extremes of R L. If RL is infinite then the voltage across the diode will be V Z. As the load decreases from infinity eventually it will become small enough that the diode will be almost shorted causing the diode to turn off. So, there are two ranges in all possible R L, one in which the diode is off, and one in which the diode operates in the breakdown region where it can act as a simple voltage regulator. The graph illustrates how the voltage and current depend on the value of the load resistance given that the source voltage is fixed. On the left are plots of the current flowing through different parts of the circuit for any choice of load and on the right, the voltage. The voltage plot shows that once the load resistance is large enough to put the zener diode into the breakdown region, the voltage becomes constant. In contrast, the voltage across the load without the regulating diode (the orange line) increases asymtotically to the value of the voltage of the source.

The current plot shows how the current is distributed among the different components. The orange curve shows the current that would flow through the load if the zener diode where not there. With diode in the circuit, the voltage across the load is equal to the breakdown voltage, so the current flowing through the load is completely determined by V z (the curve just below the orange on labeled I L). Once the diode starts conducting in the breakdown region, the current drawn from the battery is constant depending only upon the source voltage, the internal resistance of the source, and the breakdown voltage of the diode ­ none of which changes even when the load changes. As the resistance of the load increases, thereby decreasing the current needed to maintain a constant voltage across the load, the diode steps up and handles the extra.

Build a simple voltage regulating circuit and test the range of operation. Set up the circuit illustrated below using the power supply voltage V in to bias the circuit properly. Since this value is greater than 6V you must use the power supply in the +25V mode, tapping the voltage from the other set of terminals. Use a 100 1W resistor

rather than the 1/4W variety that we have been using, and the zener diode provided by the TAs.

100

+ Vin R

12. Using a the variable resistor to provide a wide range of load resistances, measure the voltage

across the load and plot the voltage V L (the voltage across the load) as a function of the load resistance. It might help to make a table first - entering values of the voltage across the load resistor for different settings of the variable resistor (REMEMBER - you can measure the resistance using the multimeter but you must remove the load from the circuit to do so).

13. How does the zener diode 'regulate' the voltage? As the resistance of the load changes we know from the last lab that the voltage across a non-ideal source (which we are modeling with an ideal voltage source and a resistor) changes. How does the zener diode prevent this change?

14. Is there a range of load resistances where the regulation is not very good? That is, does the voltage stray far from the zener breakdown voltage?

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