III. RESISTORS AND VOLTAGE DROPS
Equipment List:
DC power supply
HP-DMM
Simpson VOM
Bread Board
& wires
Assorted
resistors
Light bulb and
holder
Assorted
bananas
Purpose: To explore and confirm
the predictions of DC circuit theory with combinations of resistors.
Procedure:
Part I - Bread-board, Simpson
VOM, no power supply. Choose two different resistors somewhere around
1,000 to 10,000 ohms. Call one resistor R1 and the other R2.
A. With the HP-DMM or whatever is available, measure the resistance of
R1 and R2; compare to the color code. Record these values as you will
need them later. See the last page to understand how breadboard
connections are made.
B. Connect your two resistors in series on the bread board as shown in
the diagram. With your VOM, measure the total resistance across the two
resistors and confirm that the total resistance is the sum of the two
individual resistors.
Rtotal = R1 + R2
C. Connect the two resistors in parallel on the bread board as shown.
Measure the total resistance across the two resistors and confirm that
they are in parallel.
1/Rt = 1/R1 + 1/R2
Part II - Bread-board, Simpson
VOM (voltmeter), HP-DMM (ammeter), power supply.
In this part of the lab, TAKE NO RESISTANCE MEASUREMENTS DIRECTLY!
A. Construct the circuit as shown below.
B. From circuit theory, find the four algebraic values IR2, IR1, VR1,
and VR2 as a function of R1, R2, and V (no numbers).
C. Now confirm the predicted values in part B by taking measurements
with the VOM and HP-DMM. To make a numerical prediction from these
formulas you will need to know the numerical values of R1, R2, and V.
From part I-A above, you know the values of R1 and R2; to find the
value of the power source V, you will first construct your circuit and
with the power supply connected, turn up the supply to some arbitrary
value (above 10 volts) and then measure the supply voltage V. Don't
preset the supply voltage to some "nice" value like "exactly" 10 volts;
this is bad experimental technique as it prejudices you to a
measurement before you've made it.
You should now calculate all four values from the formulas derived in
part B. These calculations represent your precited values. Note that
after you have made these numerical predictions you can't change the
power supply voltage.
Now measure all four values (IR2, IR1, VR1, and VR2) directly. With
your circuit still operational and at the same total voltage setting of
the power supply as you have already measured in part C, use the VOM as
a voltmeter to measure the voltage "drop" across R1 and R2. Does your
VOM measured values for VR1 and VR2 agree with your prediction from the
part B theory?
D. Now confirm the values of IR2 and IR1. This is more difficult
because to measure current with an ammeter you must "open" the part of
the circuit that you wish to measure the current. Your instructor will
demonstrate this for you. Use your HP-DMM as your ammeter. See the
diagram below for help as well.
Again, from the known values of V, R1, and R2 you should have a
prediction of what I1 and I2 should be. Confirm these predictions with
your HP-DMM ammeter.
Repeat all of part II for the new circuit found below.
Conclusion: Comment on the
accuracy of your predictions when compared to your measurements. To
what extent and in what manner does the usage of a measuring instrument
affect the circuit you are trying to measure? Should a voltmeter have a
large or small internal resistance? Why? Should an ammeter have a large
or small internal resistance? Why?
------------------------------
The following diagram should help you see how the holes in your
breadboard are connected.