Laboratory 3
Voltage and current dividers
Objective:
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Series and parallel connections,
Ohm’s Law,
Voltage divider,
Current divider,
Experimental implementation of the voltage divider,
Experimental implementation of the current divider.
Equipment:
o Digilent Electronics Explorer Board,
o Digital multimeter,
o Resistors.
Theoretical support:
o Lecture 2 (Microsoft Power Point Support):
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The topology of electric circuits,
The correspondence between electrical scheme – equivalent circuit,
Series and parallel connections,
Ohm’s law,
Series connections of resistance,
Voltage divider,
Parallel connections of resistance,
Current divider.
Prelab:
1. The following circuit is given:
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VR
I
Fig. 1
Write the statement and the analytical expression of the law with which you can
calculate the value of the current that passes through the resistance R.
2. The following circuit is given:
VR1
VR2
Fig. 2
a. Write the analytical expression of the equivalent resistance across the terminals
AB.
b. Write the analytical expression of the voltage drop across resistance R1.
3. The following circuit is given:
I
IR2
Fig. 3
a. Write the analytical expression of the equivalent resistance across the terminals
AB.
b. Write the analytical expression of the current that flows through resistance R2.
Procedure:
1. The model of a real voltage source:
The following circuit is given:
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A
Vopen
Vsource
B
Fig. 4. The model of a real voltage source.
a. Set the voltage source to 5V.
b. Measure the voltage drop across the source with a voltmeter.
c. Connect at the terminals AB of the voltage source a 100 Ω resistance. Draw
the equivalent circuit and measure again the voltage drop at the terminals AB.
d. Explain the differences between the two measured values at b and c, and write
the analytical expressions by which it can be calculated the voltage drops for
the drawn circuit.
2. Voltage divider
For the following circuit, are given: R1=5.1kΩ, R2=1.5kΩ, and Vgol – the voltage drop
at open circuit measured at AB terminals:
VR1
A
VR2
Vopen
Vopen
B
Fig. 5. Voltage divider with two resistances.
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A) The equivalent circuit of the voltage divider from Fig.5 is shown below:
VR1
A
Vsource
The model of a real voltage source
Vopen
VR2
B
Fig. 6. The equivalent circuit of the voltage divider from Fig. 4.
If Vsource=5V and R0=50Ω:
a. Calculate the voltage across the terminals AB, Vopen. Note the calculated
values in Table 1 (Appendix 1).
b. Calculate the equivalent resistance at the terminals AB.
c. Calculate the values of the voltages VR1, and VR2. Note the calculated values
in Table 1 (Appendix 1).
d. Repeat steps b. and c. for R1=R2=2kΩ.
e. In series with R1 and R2 is connected a resistance R3=1kΩ. Calculate the
voltage drops VR1, VR2, and VR3. Note the calculated values in Table 1
(Appendix 1).
B)
a. Using an Ohmmeter, measure the resistances values of the two resistors
chosen in Fig.4: R1=5.1kΩ, R2=1.5kΩ.
b. Using the resistors chosen at step a., implement on Digilent Explorer board
the circuit presented in Fig. 5.
c. Using an Ohmmeter, measure the value of the equivalent resistance at
terminals AB.
d. Set the voltage supply value to 5V. Measure using a Voltmeter the voltage
drop at terminals AB, Vopen. Note the measured value in Table 1(Appendix 1).
e. Measure using a Voltmeter the voltage values VR1, and VR2. Note the
measured values in Table 1(Appendix 1).
f. Repeat steps b. and c. for R1=R2=2kΩ.
g. In series with R1 and R2 is connected a resistance R3=1kΩ. Measure using a
Voltmeter the voltage drops VR1, VR2, and VR3. Note the measured values in
Table 1 (Appendix 1).
3. Series and parallel connections
The circuit from Fig.6 is implemented on the Digilent Explorer Board.
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Fig. 7
Using an Ohmmeter:
a. Measure the equivalent resistance value at the terminals AB, if terminals CD
and EF are in open-circuit.
b. Measure the equivalent resistance value at the terminals AB, if terminals CD
and EF are in short-circuit.
c. Measure the equivalent resistance value at the terminals CD, if terminals AB
and EF are in open-circuit.
d. Measure the equivalent resistance value at the terminals CD, if terminals AB
and EF are in short-circuit.
e. Measure the equivalent resistance value at the terminals EF, if terminals AB
and CD are in open-circuit.
f. Measure the equivalent resistance value at the terminals EF, if terminals AB
and CD are in short-circuit.
Questions for Lab Report:
1. For the circuit from Fig.6:
a. Calculate the equivalent resistance value at the terminals AB, if terminals CD and
EF are in open-circuit.
b. Calculate the equivalent resistance value at the terminals AB, if terminals CD and
EF are in short-circuit.
c. Calculate the equivalent resistance value at the terminals CD, if terminals AB and
EF are in open-circuit.
d. Calculate the equivalent resistance value at the terminals CD, if terminals AB and
EF are in short-circuit.
e. Calculate the equivalent resistance value at the terminals EF, if terminals AB and
CD are in open-circuit.
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f. Calculate the equivalent resistance value at the terminals EF, if terminals AB and
CD are in short-circuit.
2. Explain the difference between the measured values and calculated values.
3. For the circuit from Fig.2, calculate the total current and the currents that passes through
each resistance.
4. The following circuit is given: R1  5k , R 2  2 k , R 3  R 4  R 5  R 6  8k and
V=5V.
Vsursă
a.
b.
c.
d.
Fig.8
Specify which circuit elements are connected in series and which circuit elements
are connected in parallel.
Calculate the equivalent resistance of the circuit.
Calculate the voltage drops across each resistance.
Calculate the currents that flow through each resistance.
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Appendix 1
Table 1
Voltage divider
R1 =
R2 =
Calculated
R1 =
R2 =
Measured
(Multimeter)
R1 =
R2 =
Calculated
R1 =
R2 =
R1 =
R2 =
R3 =
R1 =
R2 =
R3 =
R1 =
R2 =
R3 =
R1 =
R2 =
R3 =
Vopen, [V]
VR1, [V]
VR2, [V]
VR3, [V]
Measured
(Multimeter)
Calculated
Measured
(Multimeter)
Calculated
Measured
(Multimeter)
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Appendix 2
Detailing of the measurements of voltage drops for the circuit from Fig.5
Measuring the voltage VR1 using a voltmeter (voltage drop across resistance R1)
-
+
Measuring the voltage VR2 using a voltmeter (voltage drop across resistance R2)
+
-
Measuring the voltage V using a voltmeter
+
-
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