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AA320 F07 AA320 F07

Lab 1 - Week of Oct. 1.

Introductory note: The labs will generally start with an explanation of goals by instructor. There are
five lab stations, and the lab exercises will be carried out by pairs of students. You must prepare a
report on each lab (one report per student pair), including schematics of the circuits you construct,
measurements taken, and answers to exercise questions and follow-up (post-lab) questions. You are
urged to keep lab notes (what you did, what you measured, questions, etc) in a notebook; this will be
handy for writing your reports. Reports should be concise and neat. For each experiment, describe the
objective, what you did (including schematics of circuits you build), results from your measurements,
and any comments. A good rule of thumb to follow is that someone who has not read the lab
instructions should be able to fully understand what you did from your report. Although word-
processed reports are desirable, handwritten reports are acceptable if neat. To pass the course you
must turn in a report on every lab.

Lab 1 objectives: Become familiar with use of lab power supplies and digital multimeter (DMM) for
current and voltage measurement. Measure voltage and current relationships for resistor networks, to
become familiar with use of Ohm's Law.

1. Verify Ohm's Law. Hook up your power supply to a 1 k
resistor with the digital multimeter
(DMM) in series with the resistor to measure current.




While varying the voltage of the supply in 1-Volt steps from 0 V to 10 V, measure the current
through the circuit at each voltage. [Note: Use your 2
nd
DMM to measure the power supply
voltage, since the DMM indicator has higher resolution than the meter on the power supply.]
Make a table listing voltage, current, computed resistance (R
meas
=V/I), and power P dissipated in
the resistor. Explain any deviations between R and R
meas
.

2. Resistive voltage divider. Hook up the voltage divider across the power supply, as shown below,
using R1= 10 k
and R2= 20 k. Set the supply voltage to 10 Volts.



a) Predict the voltages across R1 and R2.

b) Measure the voltages across R1 and R2.

c) What is the calculated current in R2? In R1? 3. Unknown resistance. Now replace R2 for exercise 2 with the unknown component which will be
supplied to you. Set the supply voltage to some value of your choosing and measure the voltage
V2 across the component.

a) Determine the resistance of the component from your measurements and the known value of R1.

b) Verify this determination of R2 by a direct measurement using the resistance function of your
DMM.

c) Now repeat your voltage measurement of V2 while grasping the component between your
fingers to warm it up. You should find that the component has a different resistance. [The
component is a thermistor , having a temperature-dependent resistance; it is used for measuring
temperature.]


4. Potentiometer. A potentiometer (or "pot") is a resistor which has a 3
rd
lead which can be connected
to an arbitrary position on the resistor. The position of this "tap" is varied by rotating a control. It
has many uses, one of which is as a variable voltage divider. Pots are commonly used for
continuous adjustments on instruments - e.g., volume control on radios and stereos.

Hook up a 10 k
, multiturn pot across the power supply as shown, and set the supply voltage to 10
Volts. Use your DMM to measure the voltage between the tap "b" and the "negative" side of the
circuit.


a) Adjust the control from full counter-clockwise to full clockwise and note the variation of
voltage. How does the current through the pot vary as you do this?

b) Adjust the pot so the tap voltage is 6 volts. Predict the resistance between the tap and the
negative side.

c) Leaving the pot in the position from (b), disconnect the power supply and measure the
resistance from the tap to the negative side. [Use the DMM's resistance function to measure
this resistance.] Compare with your prediction.

Lab cleanup: Please disassemble your breadboard circuits and put all components and wires back in
storage. Follow-up questions.

General question: Suppose you inadvertently connected a wire between the output terminals of the
power supply ("shorted" it). Wires (solid lines on schematics) are usually modeled as perfect
conductors (zero resistance), but of course they do have a finite resistance. Suppose the total resistance
of the wire and contact-points was R=0<b>.</b>05
and your supply voltage was set to 12 Volts. Compute
the current through the short and the power dissipated by the short. Do you think the power supply
could actually deliver this current and power? [Note: An electric range provides powers of a few
kilowatts to the heating elements - consider the size of the AC conductors for an electric range.]

Lab exercise 1: What currents would you expect to measure if the leads from the power supply were
reversed? What powers would you predict?

Lab exercise 2: Compute powers P1 and P2 dissipated in resistors R1 and R2?
Consider the series resistors to be equivalent to a single resistor R. Compute the power P dissipated in
R. You should find P=P1+P2.

Lab exercise 3. Suppose your measurement of V2 for the unknown component was only accurate to
±1% and that the resistance R1 was only known to ± 2%. What would be the uncertainty in your
determination of R2?

Lab exercise 4. Suppose that there was a load on the voltage divider output, having a resistance R
L
=50
k
. With the supply voltage set to 10 Volts, predict the loaded output voltage (tap to negative) when
the pot is: a) fully counterclockwise, b) fully clockwise, c) set at midpoint.



From this you can see that when using a pot as a variable voltage source, the pot resistance should be
much less than the load resistance if the voltage supplied by the pot is to be independent of the load.
Where this is a concern, we typically "buffer" the output voltage of the pot with an op-amp to lower
the effective resistance of the pot - we will look at this when we discuss operational amplifiers.

An observation: You might question whether your measurements of voltages across resistors might be
influenced by the measurement instrument itself (DMM). Certainly the DMM has a finite (not infinite)
resistance which will act like a load resistor.
The answer is that modern voltmeters are constructed with input circuitry having extremely high
resistance. In most applications the DMM draws a current which is negligible compared to currents in
the circuits being measured.