The answer to two questions will help you identify a series or parallel connection: (1) Will the
identical current go through both components? If the answer is yes, the components are in series. (2) Are
both ends of one component connected directly to both ends of another component? If yes, the
components are in parallel. The components that are in series or parallel may be replaced with an
equivalent component. This process continues until the circuit is reduced to a simple series or parallel
circuit. After solving the equivalent circuit, the process is reversed in order to apply the solution to the
original circuit. This idea will be studied in this experiment.
PROCEDURE:
1.
Measure and record the actual values of the four resistors listed in Table 10-1.
2.
Connect the circuit shown in Figure 10-2. Notice that the identical current is through R, and R4,
so we know that they are in series. R2 has both ends connected directly to R3, SO these resistors are
in parallel.
Figure 10-2
3.
You can begin solving for the currents and voltages in the circuit by replacing resistors that are
either in series or in parallel with an equivalent resistor. In this case, begin by replacing R2 and R3
with one equivalent resistor. Label the equivalent resistor R2,3.Draw the equivalent series circuit
in the space provided in the report. Show the value of all components, including R2.3.
4.
The equivalent circuit you drew in step 3 is a series circuit. Compute the total resistance of this
equivalent circuit and enter it in the first two columns of Table 10-2. Then disconnect the power
supply and measure the total resistance to confirm your calculation. Enter the measured total
resistance, RT, in column 3.
5.
The voltage divider rule can be applied directly to the equivalent series circuit to find the voltages
across R,, R2,3,and R4. Find V,, V2.3, and V4 using the voltage divider rule. Tabulate the results in
Table 10-2 in the Voltage Divider column.
6.
Find the total current, IT, in the circuit by substituting the total voltage and the total resistance into
Ohm's law. Enter the computed total current in Table 10-2 in the Ohm's law column.
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P
7.
In the equivalent series circuit, the total current is through R1,R2,3,and R4. The voltage drop
across each of these resistors can be found by applying Ohm's law to each resistor. Compute V,,
V2,3, and V4 using this method. Enter the voltages in Table 10-2 in the Ohm's law column.
8.
Use V2.3 and Ohm's law to compute the current in R2 and Rj of the original circuit. Enter the
computed current in Table 10-2. As a check, verify that the computed sum of I2and I3is equal to
the computed total current.
9.
Measure the voltages V,, V2,3, V4, and Vs. Enter the measured values in Table 10-2.
10.
Change the circuit to the circuit shown in Figure 10-3. In the space provided in your report, draw
an equivalent circuit by combining the resistors that are in series. Enter the values of the
equivalent resistors on your schematic drawing and in Table 10-3.
Figure 10-3
11.
Compute the total resistance, RT,of the equivalent circuit. Then apply Ohm's law to find the total
current IT.Enter the computed resistance and current in Table 10-3.
12.
Complete the calculations of the circuit by solving for the remaining currents and voltages listed
in Table 10-3. Then measure the voltages across each resistor to confirm your computation.
FOR FURTHER INVESTIGATION:
Figure 10-4 illustrates another series-parallel circuit using the same resistors. Develop a procedure for
solving the currents and voltages throughout the circuit. Summarize your procedure in a laboratory report.
Confirm your method by computing and measuring the voltages in the circuit.
Q
Figure 1 0 4
APPLICATION PROBLEM:
For many years, resistive networks have been designed to control the signal level of audio or radio
frequency circuits and to match the resistance of the source and load. Circuits that perform these functions
are called attenuators, or pads. There are a number of variations in pad design, but in this problem we
will design an L-pad used in matching a higher resistance to a lower resistance. The circuit is shown in
Figure 10-5. The first dotted box represents a signal source with a source resistance of 600 a.(The
source resistance is internal on ac signal generators.) The L-pad consists of the two resistors shown in the
second dotted box, and the load is in the third dotted box and represents the circuit being driven by the
source.
, "y~yq-n=
L - pad
Source
I
I
1
,
0
I
I
I
1
Load
,
I
I
I
I
R2
I
I
I
I
I
I
I
I
I
0
I
I
1
.--------------------I
i
I
1
L------------------------J
load
100
R
L-----------------I
Figure 10-5
The resistors in the L-pad depend on the source resistance, the load resistance, and the desired
attenuation of the pad. In this design, the attenuation must be greater than the ratio of the source to load
resistance. The equations for determining these resistors are
where: Rs = source resistance
RL= load resistance
A = attenuation; the ratio of inputloutput voltage
The design requires an L-pad that matches a 600 0 source resistance to a 100 load resistance
with a 10: 1 attenuation from the input of the L-pad. Compute the values of the resistors in the L-pad and
construct the circuit based on your design using resistors as close as possible to the calculated values. The
source can be a signal generator with an internal 600 i-2 resistance set for a 1 kHz sine wave or a dc power
supply with a series 600 i2 resistor. Set the source voltage to 5.0 V when it is connected to the rest of the
circuit. If you have correctly calculated the values of the resistors in the L-pad, the output voltage
measured across the load resistor should be 0.5 V and the resistance of the circuit measured looking into
the L-pad should be 600 a.Summarize your results in your report.
Component
Listed
Value
RI
2.2 kn
R2
4.7 kn
R3
5.6 k f l
R4
10.0 kn
Measured
Value
C
-'
EVALUATION AND REVIEW QUESTIONS:
1.
The voltage divider rule was developed for a series circuit, yet it was applied to the circuit in
Figure 10-2.
(a)
Explain.
(b)
Could the voltage divider rule be applied to the circuit in Figure 10-3? Explain your
answer.
2.
As a check on your solution of the circuit in Figure 10-3, apply Kirchhoff's voltage law to each
of two separate paths around the circuit. Show the application of the law.
3.
Show the application of Kirchhoff's current law to the junction of R2and R4 of the circuit in
Figure 10-3.
4.
In the circuit of Figure 10-3, assume you found that IT was the same as the current in Rjand R4.
What are the possible problems?
(a)
(b)
"\
5.
How would you isolate the specific problem?
The circuit in Figure 10-6 has three equal resistors.
If the voltmeter reads +8.0 V, find the voltage drop across R1. V1=
(a)
What is the source voltage? Vs =
(b)
Figure 10-6
6.
What basic rules determine if two resistors in a series-parallel combination circuit are connected
in series or in parallel?