Download 07LAB1 - University of Guelph Physics

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Laboratory
1
DC Circuits
Preparation
/ Reading:
Text: 2.5 (or reference [1]: 2.5)
Appendix 16 &14
Lecture notes
Questions marked (*) in this lab script.
OBJECTIVE
Ohm’s Law; The diode; voltage divider; Thevenin model;
Oscilloscope & Function generator.
EQUIPMENT REQUIRED
Oscilloscope (Tektronix TDS210, dual channel, 60 MHz)
Function Generator (BK Precision 4011A, sine, triangle, square, logic
level; to 5 MHz)
Breadboard (PB-503, powered, with a built-in function generator)
Digital multimeter (DMM) (FLUKE 187, as DMM1)
another DMM (FLUKE 73, as DMM2)
1N914 (a general-purpose Silicon diode)
1k resistor
10 k resistor (2)
Decade Resistance Box (Model IN-17, selections of fixed resistors)
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Note: ------------------------------------------------------------------------------------------------------------------This is a right time to learn some conventions that may help you a little in this lab, a lot later
when you lay out more complicated circuits. Try to build your circuit so that it looks like its circuit
diagram:
1) Let signal flow in from left, exit on right (e.g., Fig. 1.1a);
(a) Good layout
(b) Bad layout
Fig. 1.1
2) Use color coding to help you follow your own wiring: use black for ground, red for
the positive supply, and blue for the negative supply.
3) It would be convenient to place the breadboard in horizontal orientation (you may
place the portable breadboard part as shown in Fig. 1.2, or rotate the base unit).
Place ground on a horizontal breadboard bus strip below your circuit. Place the
positive supply on a similar bus above your circuit. If your circuit involves negative
supply, place that on a bus strip below the ground bus.
Fig. 1.2
4) Build your circuit on the breadboard, not in air, with minimum number of wires. Plug
the components into the plastic breadboard strips (see Appendix 16). If possible, plug
wires into the breadboard so as to join the components to the banana jacks or the
BNC J1 and J2 connectors on PB-503 for measurements, and then bring external
meters or signal source to the breadboard through the banana jacks /BNC’s.
5) Always keep the power off during your construction of a circuit. Do not turn on
the power before all the connections are done.
----------------------------------------------------------------------------------------------------------------------------- -Lab 1-1: Ohm’s Law --- Verify that a resistor obeys Ohm’s law, by measuring V and I for a
few voltages.
1. Use a variable dc supply ( 0 – +15 V) provided by the breadboard PB-503, and construct a
circuit as shown in Fig. 1.3. Note that currents are measured through a part of a circuit with
DMM1, and voltages are measured between the points in the circuit with DMM2. (Note: Build the
circuit first, then add meters. Be careful on each meter’s polarity .)
DMM 1
DMM 2
Fig. 1.3
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* 2. Before you do the measurements on V and I for the 1 k resistor, answer the following
questions: *(1) DMM 2 is not measuring the voltage across the resistor. Why? Does that matter?
How can you fix the problem so the voltmeter measures what you want?
*(2) When you’ve made voltage measurement more accurate, what about the accuracy of the
current measurement? Can you measure the voltage and the current simultaneously? Explain.
(Hint: Consider errors in the readings due to the “internal resistance” of the meters. Ideally, meters should leave
the measured circuit unaffected --- say, an infinite impedance for voltmeters and zero impedance for ammeters,
but these are not the cases in reality.)
3.
Measure 5 values of V and I for the 1k resistor. Do not forget to record the units.
V(
)
|
I(
)
--------------------- |------------------|
|
|
|
|
|
|
4. Sketch the I-V curve based on your data. Though you knew what it would look like without
doing this experiment, we encourage you to draw the plot for checking if your data make sense,
also, for contrast with the device you will meet next --- its curve does not look like those of a
resistor, because it does not obey Ohm’s Law.
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Lab 1-2: The Diode (Non-linear device) --- We don’t expect you to understand how the
diode works yet; we just want you to meet it experimentally.
1. Wire up the circuit shown in Fig.1.4. Remember? Do not turn on the power before all the
connections are done. Note that you cannot just stick a voltage across a diode, as you did for
the resistor above. You’ll see why after you’ve measured the diode’s V-I curve.
Fig. 1.4
2. The 1k resistor limits the current to safe values. Vary R for at least 6 different values, using a
resistor substitution box (Decade Resistance IN-17), so that the total resistor R’ (= 1k + R) values
measured are close to the suggested values, and look at I vs V. Record your data.
R’ ( ) (suggested value) |
I( )
V( )
----------------------------- |------------------------------------------------(1k)
|
(1.5 k ) |
(2.0 k ) |
(4.0 k)
|
(11.0 k ) |
(1 M )
|
3.
Sketch the I – V curve based on your data.
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4.
See what happens if you reverse the direction of the diode. Write down your observations.
5. How would you summarize the V vs I behaviour of a diode, based on your experiments?
Explain why you cannot put 5 volts across the diode.
Lab 1-3: Voltage Divider --- Verify Thevenin Model
1. Construct the voltage divider shown in Fig.1.5. Apply Vin = + 15 V (use the dc voltages
provided on the breadboard PB-503).
Fig. 1.5
* 2. What is your expected output voltage value for the open circuit? And what is the value after
you attach a 10 k load? Give your calculation details below.
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3. Measure the (open circuit) output voltage. Then attach a 10 k load and see what happens,
write down your results. Compare your data with your calculated values.
* 4. What is the short circuit current? Show your calculation.
5. Now measure the short circuit current, i.e., short the output to ground, make the current flow
through your current meter (DMM1). Compare your result with your calculation.
(Note: Though the short circuit current in this case is very modest. you will rarely do again what you just did --short the output of a circuit to ground in order to discover its Rthevenin, or “output impedance”. Generally
speaking, this method is too brutal in the lab, as the current could be too large for the health of your circuit.)
6. Build the Thevenin equivalent circuit, using the variable dc supply on the breadboard PB-503,
and check that its Vopen circuit and Ishort circuit match those of the circuit that it models. Sketch your
equivalent circuit below.
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7. Then attach a 10 k load to your equivalent circuit, to see if it behaves as you expected from
your original voltage divider.
Lab 1-4: Oscilloscope & Function generator ---To get familiar with the oscilloscope and
function generator and to use them to obtain, and measure the amplitudes and period of various
voltage signals.
The oscilloscope (“scope” in short) is the most important instrument (it is primarily a
voltmeter) in labs, it permits the visual display of a voltage signal (amplitude vs time), and can
also be used to measure the average value, the root-mean-square (rms) value (which is usually
used to compute power), frequency, and period of a sinusoidal or non-sinusoidal signal.
The screen is divided into centimetre divisions in both vertical and horizontal directions.
The height of the vertical deflection of the displayed trace combined with the vertical sensitivity
setting (e.g., in Volts/div., where “div.”= division) yields the peak-to-peak (p-p) voltage of the input
signal. For example, if the vertical sensitivity of the scope is set to 0.1 V/div., then the peak-topeak voltage of a sinusoidal wave being displayed in Fig.1.6 is 0.4 V (= 4 div. x 0.1 V/div.).
V (t) = Vp Sin (2f t ) = (Vp-p/2) Sin (2t /T )
Fig. 1.6
Vrms = Vp /(2)1/2 = 0.707 Vp
Similarly, the time of one cycle, or the period T, can be determined with the horizontal scale (in
div.) combined with the horizontal sensitivity (or sweep rate, e.g., in sec/div.).
We’ll be using the scope in almost every lab from now on. You may need several labs to
get fully used to it, though some functions may still rarely be used at the semester end.
The function generator is essentially a voltage supply that typically provides a sinusoidal,
square-wave, and triangular voltage waveform with control of frequency, amplitude and dc offset
(a constant dc voltage added to the signal).
Note: --------------------------------------------------------------------------------------------------------------------------------------Both the scope and the function generator are built to withstand some abuse, so try various
combinations of settings to fully develop your abilities using them properly. Though you work in a group, do
not let one person perform all the experimental work, you should take turns to operate things to learn how to
use the equipment and to gain a firm understanding of the materials.
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1. Make all the necessary adjustments to clearly display a 5 kHz 6 Vp-p sinusoidal signal on the
scope. Establish the zero volt line at the center of the screen. Record the chosen sensitivities:
Vertical sensitivity
= __________
Horizontal sensitivity
= __________
Draw the waveform in the figure below (Fig. 1.7) carefully noting the required number of
horizontal and vertical divisions.
Fig. 1.7
Find the period (in sec. ) of the waveform on the screen using the number of horizontal divisions
for a full cycle you measured. Then, determine the frequency (in other words, the scope can be
used to check the frequency of the signal provide by the function generator).
T (measured)
= ______________
f (calculated)
= ______________
2. Switch the function generator to square waves, one may use the scope to measure the
“risetime” of the square wave (defined as time to pass from 10% to 90% of its full amplitude). You
may expect that the square wave rises instantaneously. The scope, properly applied, will show
you this is not so in reality. Check it yourself, and record the conditions (e.g, scope’s sweep rate,
signal frequency, etc.) for the risetime observation.
3. Put the “DC offset” function onto the signal from your function generator, then see what the
AC/DC coupling does on the scope. Write down your observation.
(Note: The AC coupling on the scope puts a capacitor in series with the scope input, this could produce
distortions of waveforms. Furthermore, the AC coupling washes away DC information, i.e. to make DC
information invisible on the scope screen. Sure, you don’t want to miss information unless when you choose to
do so purposefully.)
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4. (Optional) What comes out of the function generator’s TTL connector on the breadboard PB503? Look at this on one scope input channel while you watch a triangle/square wave from PB503 function generator on the other channel.
Lab 1-5: AC Voltage Divider
*1. First spend a minute thinking about the following question: How would the analysis of the
voltage divider be affected by an input voltage that changes with time?
2. Now connect the voltage divider from lab exercise 1-3 above (Fig.1.8), and see what is does
to a 1 kHz sine wave (use function generator and oscilloscope), comparing input and output
signals.
Fig. 1.8
Write down your observations. Explain to your own satisfaction.
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