February 3
pre-lab due this day, 1:10pm
EGR 220: Engineering Circuit Theory
Lab 2: Equivalent Resistance; Voltmeter Internal Resistance
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Note that there are basically three experiments in this lab, and you should plan on spending
approximately 20 minutes in lab on each experiment. Think about how you will go about
performing each experiment before coming into lab so that you work as efficiently as possible.
PART I: MEASURING EQUIVALENT RESISTANCE
Objective
The objective for this experiment is to design simple resistor circuits and verify the calculated Req
with experimental measurements.
Overview
Design three circuits, each using at least three resistors (using three is recommended, but you can
you more if you want to), for which the equivalent resistance is (close to) 2kΩ.
 Circuit 1: Use three resistors in series
 Circuit 2: Use three resistors in parallel
 Circuit 3: Use three resistors in a combination of parallel and series (but not all three in
series or all three in parallel)
Part I pre-lab:
Prepare initial designs for the three circuits defined above, including
 The circuit diagram
 The values of the individual resistors you plan to use
 The calculation of the equivalent resistance, Req
o You may want to write these expressions out symbolically, using ‘R1,’ ‘R2’ and
‘R3’ so that you have them ready to plug in the actual R values in lab, because…
o You may need to adjust the resistor values you actually use, once you are in lab and
are limited to using the resistors that are available.
Tasks in lab
1. Build each circuit you or your lab-partner designed, or a different one.
2. Measure Req Method 1: Use the multi-meter (configured as an ohm-meter) to measure the
resistance of each circuit as well as the individual resistor values.
3. Measure Req Method 2 (calculate the ratio of V:I) Using the DC power supply as a voltage
source,
a. Select a source voltage value and apply this voltage to each of your circuits in turn.
b. Using the Agilent multimeter as an ammeter, measure the current supplied by the
DC voltage source (use the Fluke handheld for V and R measurements, but only the
Agilent meter for measuring current).
c. Calculate the equivalent resistance using the Vsrc and Isrc values.
4. Your comment for this week’s memo could relate to the comparison of the calculated and
measured values.
February 3
due February 10
a. If you use different resistor values in the actual circuits than you assumed for your
initial designs, be sure to re-calculate the expected Req with the actual resistor values
and compare that calculation to the measured values.
PART II: EQUIPMENT INTERNAL RESISTANCE
Objective
The goal of this experiment is to begin understanding that the laboratory equipment becomes part
of our circuits. The experiments are to measure the effect of our equipment on the measurements
we make. For this experiment, only the Agilent voltmeter is investigated. Note that this is an
experiment that allows you to investigate the concepts in the final homework problems from
chapter 2 in homework set #2.
Tasks IIa
1.
2.
3.
4.
5.
6.
7.
8.
Obtain a 1MΩ.
Measure the actual resistance value with the multi-meter configured as an ohm-meter.
Build the circuit below in Fig. 1 with the 1MΩ resistor as ‘R.’
Measure the power supply voltage, V, with the handheld multi-meter, to confirm for
yourself that it is 5V across these terminals.
Connect the Agilent multimeter into the circuit as shown, and configure it to measure the
voltage across its probes, Vm.
Use the measured values of R, V and Vm to calculate the internal resistance of the Agilent
voltmeter, Rm.
Repeat steps 1 – 6 using a 10kΩ resistor.
Pre-lab: Calculate the expected values for Vm for both circuits defined above (i.e. for R =
1MΩ and R = 10kΩ) if Rm were to equal infinity, and if Rm were to equal 10MΩ.
Fig 1: Voltmeter Rm in series with R
page 2
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February 3
due February 10
Tasks IIb
9. Obtain two 10kΩ resistors and measure their actual values (keep track of each resistor and
its corresponding value).
10. Connect the circuit shown in Fig. 2.
11. What is the measured value of Vm? Compare it to your prediction for the ideal voltmeter,
and explain any differences, in terms of electrical energy, the behavior of matter, and/or the
circuit laws and analysis methods we have used in class.
12. Repeat steps 9 – 11 for R = 10MΩ.
13. Pre-lab: What do you predict Vm, in Fig. 2, will be if:
a. The voltmeter is an ideal voltmeter? (An ideal voltmeter has infinite internal
resistance.) Briefly state why an ideal voltmeter would have infinite resistance.
b. The internal resistance, Rm, of the voltmeter were to be to equal 10MΩ.
Fig 2: Voltmeter Rm in parallel with load resistor, R
Questions to consider
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page 3
When might you need to consider the internal resistance of the voltmeter and when might it
be okay to assume that the voltmeter’s internal resistance is infinite?
If an ideal voltmeter has infinite internal resistance, what would be the ideal internal
resistance for an ammeter? Briefly explain your answer in terms of electrical energy,
circuits and/or circuit laws and analysis tools.
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