PHYS 2300 – DC Circuits Lab

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7 Οκτ 2013 (πριν από 3 χρόνια και 8 μήνες)

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PHYS 2300

DC Circuits Lab


Ohm’s Law


The first thing we will
do is verify that a resistor obeys Ohm’s law by measuring V
and I for a few voltages.


It is not expected that Ohm’s law will be a surprising new result to you (I hope not
anyway!). This is more of a way for you to become familiar with the breadboard,
powe
r supplies, and other equipment that you will be using extensively
throughout this course. The breadboard, in particular, gives students trouble the
first time they are exposed to it. Which of the holes are connected and which
aren’t? What’s with the littl
e spaces between sections? Are the holes powered
when I turn it on? You are meant to become intimately familiar with your
breadboard and by the end of the course you will be able to use it blindfolded.










 
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In the haze of uncertainty that many stude
nts have with regard to the
breadboard, they will avoid using it to build circuits. Resistors are commonly
“floated” in mid air by clip connections to power supplies. DON’T DO THIS! Learn
to build a clean, organized circuit on the breadboard.








In
building your circuits on the breadboard, it is helpful to maintain some design
conventions
, e.g., establishing power busses to distribute power to your circuit,
having the circuit flow from left to right, always placing transistors and chip
components
straddling a breadboard gap, etc.


















 
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Use

the
variable DC supply
built into your breadboard
and hook up the circuit
shown above. Note that voltages are measured
between
points on a circuit while
currents are measured
through
a part of a c
ircuit. This means you will usually
have to break the circuit to measure current.


NOTE: When choosing resistors, you need to consider tolerance and power
rating as well as the value. The tolerance (amount actual value may vary from
marked value) is
usually given by the 4
th
color band. For precision resistors,
value and tolerance may be written on the resistor body. The power rating is
related to physical size. The resistors in the small red cabinets vary from 1/8 to 2
watts. Estimate current and powe
r before using small resistors (small in physical
size and resistance value). See the supplemental document
Reading Resistors
.


NOTE:
Althoug
h the power supply on the bread
board says 15V MAX, the
maximum voltage is closer to 20 V with no load.


Measure a f
ew values of V and I for a 20k resistor (you may not find a 20k
resistor. Don’t panic! Consider how you might use some 10k’s).


Next try a 10k resistor. Sketch the two curves that these resistors define on a plot
of I vs. V. Now, you may be disinclined to
draw these figures because you knew
what they would look like already. Again, Ohm’s law isn’t expected to surprise
you. These figures, however, will be a good point of comparison to the devices
that come next…they do not obey Ohm’s law.


Now that you have
done the pedestrian part off this experiment, consider a
couple practical questions that arise in even this simplest of experiments.


A Qualitative View


The voltmeter in the circuit shown above is not measuring the voltage across the
resistor. Does it ma
tter? How could you fix the circuit so the voltmeter measures
across what you want? When you have done that, what about the accuracy of
 
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the current measurement? Can you summarize by saying what an ideal voltmeter
(or ammeter) should do to the circuit under
test? What does that say about its
“internal resistance”?


A Quantitative View


How large is each error, given a 20k resistor? Which of the two alternative
hookups is preferable? Would you have reached the same conclusion if the
resistor had been 20M? (do
n’t worry
if your uncertain, we will
cover this soon)



Nonlinear Devices (Ohm’s Law Defied!)


Incandescent lamp


You will now perform the same I vs. V measurement for an incandescent lamp.
DO NOT EXCEED 6.5 V! The lamp filament will burn out. Take a few
different
reading
s
and plot I vs. V. It is advised that you plot this on the same figure you
used to plot your resistor curves for comparison. Obtain enough points to show
that the lamp diverges from the resistor’s behavior.


Now the big question, what is
the
resistance
of the lamp? Is this even a
reasonable question? If the lamp’s filament is made from a material that is
fundamentally similar to that used to make the resistors, what accounts for the
funny shape of the I vs. V curve?



Diode


You aren’t expected to understand
why
the diode
does n’ t behav e Ohm’ s l aw.
Thi s i s j us t t o f ami l i ar i z e y ou wi t h s uc h a dev i c e, as t hey ar e us ef ul f or many
appl i c at i ons t hat we wi l l v i s i t l at er i n t he c our s e.


You need t o modi f y y our t es t s et up her e, bec aus e
y ou c an’ t j us t s t i c k a v ol t age
ac r os s a di ode, as y ou di d t he r es i s t or s and l amp. ( Wel l, y ou c an, but y ou c an’ t
do i t t wi c e wi t h t he s ame di ode!) You’ l l s ee why af t er y ou’ v e meas ur ed t he
di ode’ s I v s. V. Wi r e t he t es t c i r c ui t as s hown bel ow.






 
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NOT
E: Polarity matters. The band at the end of the diode corresponds to
the
bar
end of the circuit symbol. This side normally connects to the negative side of the
power supply
. Diodes also have voltage and current limits. The one you are
using has a 1 A limit
.


In this circuit, you are applying a
current
, and noting the diode voltage that
results; earlier, you applied a voltage and read the resulting current. The 1k
resistor limits the current to safe values. Vary
R
(use variable resistor, also call
potentiometer
, or pot) and look at the I vs. V. Sketch the plot in two forms: linear
and semi
-
log.

First, get an impression of the shape of the linear plot; four or five points should
define the curve. Then draw the same points on a semi
-
log plot. Which
co
mpresses one axis (it will be evident that it is the fast growing current axis that
needs compressing).




 
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See what happens if you reverse the direction of the diode.


How would you summarize the I vs. V behavior of a diode?


Now, what would happen if you
were to put 5 volts across the diode? (DON’T DO
IT!)



Voltage Divider





Construct the voltage divider circuit shown above. Apply V
in
=15 volts (use the DC
voltages on the breadboard). Measure the (open circuit) output voltage.


Attach a 10k “load” resi
stor across the output and see what happens.


We now want to m
easure the
short circuit current so that we can
determine the
Thevenin equivalent circuit. (Don’t be overly alarmed by the term “short circuit”.
Here it just means short the output to ground whi
le allowing current to flow
through your current meter. The current is modest in this example, however, you
will rarely do it again as it is just too brutal a method in most lab cases and we
can obtain R
Thevenin
in easier ways.)



Fro
m the I
short circuit
a
nd V
open circuit
you can calculate the Thevenin equivalent
circuit.


Now build the Thevenin equivalent circuit using the variable DC supply as a
voltage source and check that this circuit’
s open circuit voltage and s
hort circuit
current match those of the
circuit it models. Then attach a 10k load, just as you
did with the original voltage divider to see if it behaves identically.







 
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Oscilloscope


We’ll be using the oscilloscope in virtually every lab from now on. It is, therefore,
a good idea for you to
begin to familiarize yourself with how the scope works right
away.



Get familiar with the scope and function generator (outputs time
-
varying voltages:
waveforms
. Can be found on the top left of you breadboard.
)
Start
by generating
a 1000 Hertz
(1kHz) sine wave with the function generator and displaying it on
the scope.



Try to measure the output frequency with the scope (you should distrust the
m
arkings on the function generator
and trust what the scope tells you). You are
obligated to actuall
y measure a period, not frequency using the scope. You
should become proficient at this task.



AC Voltage Divider




Before you build the circuit above and begin to test it, first consider the following
question: How would the analysis of the voltage div
ider be affected by an input
voltage that changes with time?


Now construct the divider above and see what it does to a 1kHz sine wave (use
function generator and scope to set this up). Compare the input with the output.