EXPERIMENT
#4 PRELAB
So far in
the lab we've only dealt with linear (or piecewise linear)
devices in our circuits. In lab 2 we discovered that
resistors, motors, and incandescent bulbs all have I-V
graphs which can be characterized by sloped lines which
characterize the resistive behavior, outside of some
transition regions and "warming up" areas. However, in
order to control the ECE cars, we need our circuits to be
able to encode logic. The circuits need to be able to
represent the concepts of "true" and "false", "on" and
"off" and be able to perform basic Boolean operations using
these concepts. These devices will be used to implement the
two-state Boolean arithmetic that all of our digital
devices use to operate. The beauty of a two-state system is
that the simple concept of "on" and "off" can be mapped
onto the two states sometimes called "true" and "false" (if
we are speaking in terms of logic) or "1" and "0" (if we
are speaking in terms of Boolean logic).
- Propose a method of encoding the
concepts of "true" and "false" in an electrical circuit.
You can explain in general terms such how currents flow
or how voltage changes without necessarily assuming that
it can realistically be built by existing eletronics.
Play "what if..." Be sure to explain how to the concept
of 'true' and 'false' are differentiated in your
method.
- Would it be difficult to
implement your method using linear devices (such as
resistors)? Why or why not?
- Sketch an I-V graph for a device
that you could use to easily implement your encoding
method.
EXPERIMENT #4
Diodes
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 and
are still used in other applications where a device is
needed to provide a constant voltage drop that can be
turned on and off. There are several different types of
diodes - we will experiment with three types of diodes, a
basic diode, a zener diode, and a light emitting diode
(invented here by some folks in the microelectronics
group).
a. Basic Dioide
Figure 1: Circuit setup for testing a simple
diode.
Build the circuit shown above using a 1N5406 diode, 1kΩ resistor, test box, function generator, and an oscilloscope. Be sure to properly orient the diode in the circuit. See diagram below for explanation of diode orientation. 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.
Figure 2: Demonstration of diode component
labeling
Turn on the oscilloscope and put it in XY mode. Invert channel 2. Position the displayed dot at the center of the screen.
Turn on the function generator and set it to a 100 Hz sine wave with zero DC offset. 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.
- Sketch this curve in your lab notebook.
- Remember that the oscilloscope is a voltage
measuring instrument. How did we manage to measure
current on the Y-axis?
- To understand why we had to connect channel 2
with reverse polarity and then invert it, try reversing
the polarity of the oscilloscope wire measuring voltage
across the 1kΩ resistor. What happens to the
display of the I-V graph? Why does this occur? Hint: the
ground or negative terminal of ALL channels of the
oscilloscope are connected to the same ground and in fact
are connected to the same ground as the function
generator via the plug and therefore through the wiring
in the building.
- Note that the voltage drop across
the diode is fairly constant for a wide range of values
of the forward current. Measure this voltage drop on the
oscilloscope. How could a device with the characteristics
of a diode be useful to you in implementing the two-state
logic functions?
- How else might this device be useful?
b. Zener Diode
Use the same experimental setup as in part 1, but replace the diode with a 1N4734 Zener diode. Again vary the amplitude of the waveform from the function generator (to at least 15V) and the horizontal and vertical scales on the oscilloscope until you feel you have obtained a good image.
- Sketch the curve in your lab notebook.
- 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 to you or
others?
c. A unique Light Emitting Diode (LED)
Light Emitting Diodes (LEDs) function in exactly the same way as normal diodes except that they contain a substance that emits light when the diode is in the "on" state. For this lab we have a particularly interesting LED that behaves in a unique way.
Use the same experimental setup as in part 1, but replace the diode with the white capped LED. Be sure to orient the LED so that the longer of its two wires is connected to the positive terminal and the shorter of its two wires is connected to the negative terminal. Set the function generator to a 100 Hz sine wave with amplitude 9 V P-P and zero DC offset.
- What color is the LED emitting?
Be precise in describing the color.
- Now change the DC offset of the
function generator to its maximum positive setting (~5V).
What color is the LED emitting now?
- Now change the DC offset of the
function generator to its maximum negative setting (~
-5V). What color is the LED emitting now?
- 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? Confirm your thoughts by reducing the
frequency of the sine wave to 1 Hz.
- 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. Based
on the graph, do you think this LED is some kind of
"Zener LED", or merely two different normal LEDs
connected back-to-back? Explain your reasoning.
d. Using LEDs as Flags in Circuit Design
One common use of LEDs is to tell us whether a certain signal has a "high" voltage or a "low" voltage when we don't care what the exact voltage values are. An LED will be lit on a "high" voltage and unlit on a "low" voltage providing a visual cue to the value of a signal.
For this part of the lab, you will be using your protoboard with the array of ten LEDs which looks roughly like the component in the figure below:
Figure 3: Abstracted diagram of LED
array
Plug this component into your protoboard so that the device straddles the gap - each row of pins is parallel to the gap and each pin is inserted into its own hole. Ask your TA is you are uncertain about inserting the LED array into the protoboard.
Figure 4: Circuit for testing the LED
array
While in theory an LED should be lit up for any high voltage, in reality if the voltage becomes too high it will actually overload the LED and the LED will fail - and turn off.
- Create the circuit shown in the
figure above on your protoboard and fill in the table
below by using resistors with values of 0Ω (a
wire), 10Ω, 100Ω, and 1kΩ for
RL. Vary the voltage from the power supply
from 0-6 V and fill in each row of the table with the
voltage values from the power supply at which each state
is first reached. If the LED does not turn off by the time the voltage reaches 6V, just state this in your table.
- Typically, we want to consider
the range of 0 - 0.8 V as a "low" voltage and the range
of 2.0 - 6.0 V as a "high" voltage, and we don't care
about the range of 0.8 - 2.0 V. For which values of
RL is the LED on and off in the proper
ranges?
The LED array can be a very useful tool for checking signals when you are designing the circuit which will control your car, but you must make sure that it turns on and off in the proper voltage ranges to check signals. So if you want to use it later in the semester, you must include a resistor with resistance equal to what you found on each LED you use for testing.
Diode
Application - voltage regulation and
clipping
As you determined in lab
3 some voltage sources are non-deal in the sense that the
voltage actually changes as you modify the load attached to
it. For many applications this is unacceptable 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 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. This circuit
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
the same across the device and a large amount of current
can be drawn depending on the load. The diode is of course
not a source since it cannot provide power to anything. In
class you demonstrated its source-like behavior by modelled
the zener diode's I-Vcurve 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 though it
is quite small.
If the value of Vs 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 only the voltage
across the diode. When the diode is conducting current in
its breakdown region, the resistance of the device is so
low that no matter what load is applied the equivalent
resistance of the subcircuit including the diode and load
remains nearly constant. You can see this by deriving the
amount of current drawn from the non-ideal source/battery.
Looking at only the zener diode and load resistor - the
amount of current flowing into this subcircuit can be
derived (approximately) as:
As you can see this is just the current demanded by two resistive elements in parallel. If Rz << RL then the current will stable over a wide range of loads. The plot below shows this graphically - the x-axis is the load resistance and the y-axis is the equivalent resistance of the zener/load subcircuit. There are 4 lines on the plot each for a different value of the internal resistance of the zener diode Rz. You can see for the lower values of Rz once the load resistance reaches a value of about 5X the internal resistance of the zener diode the subcircuit's equivlaent resistance remains fairly constant. But the actual internal resistance of the diodes that you will be using is ~330Ω. This means that any load less than or equal to the internal resistance of the zener will not regulate well.
Build the circuit illustrated in the diagram below using the power supply as the 15V source (since this value is greater than 6V you must use the power supply in the +25V mode and use the other set of output jacks, a 120Ω 1W resistor (not the 1/4W variety that we have been using so far), and the zener diode that you used in the previous section. Be sure to build this circuit in the test boxes and not on the protoboard. The currents that we will be drawing might be too much for the traces inside the protoboards, and if some traces to get scorched, since you cannot see them from the outside they are rather hard to diagnose.
- Using a the variable resistor to
provide a wide range of load resistances, measure the
voltage across the load and plot the voltage
VL (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 turn remove the load from the
circuit to do so.
- How does the zener diode
'regulate' the voltage. As the resistance of the load
changes we know from 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?
- 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?