Download Lab 4 Voltage Divider and Bridge Circuits

Villanova University
ECE 2053 Fundamentals of Electrical Engineering I Lab
4 – Voltage Divider and Bridge Circuits
Learning Objectives
The objectives are:
1. To develop an understanding of the voltage divider circuit,
2. To experience designing a circuit,
3. To develop an understanding of the bridge circuit.
Parts List
Low voltage center-tapped transformer: 16 V rms, end-to-end [the printed value of
12.6 V is incorrect]
Resistors: 10 Ω, 2.2 kΩ, 4.7 kΩ, 6.8 kΩ, 68 kΩ , 680 kΩ, other values (5 %, ¼ W
preferred)
Potentiometer: 2 kΩ potentiometer
Pot tweaker
References
Sections 2.3 and 2.4 in Franco, Electric Circuits Fundamentals
Exercise 1: The Voltage Divider
1. Copy the circuit of Figure 1 into your notebook. Compute the voltage at Node A –
this is the output voltage of the voltage divider circuit.
2. Collect two 4.7 kΩ and two 6.8 kΩ resistors; measure and record their values. Wire
your 25-volt power supply down to two buses on the PB as you have done in the past.
Hint: use black wire for the ground bus, and red wire for the positive bus. Set the
power supply to 5.0 volts with a current limit of 0.5 A. Complete the circuit on your
PB.
R2
A
+
4.7kohm
R1
5V
V1
Vs
6.8kohm
0
0
Figure 1. Basic voltage divider.
3. Measure the voltages Vs (source voltage) and V1 (the output voltage) with your DVM.
4. Determine the voltage across resistor R2 without measuring it.
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5. Change the source voltage to 10.0 V. Measure the output voltage.
6. Repeat the above for source voltages of 15.0 V, 20.0 V and 25.0 volts.
7. Make a table of your results as follows:
Source Voltage
(V)
Output Voltage
(V)
10 Ω Resistor Voltage Vs
(mV)
5.0
10.0
15.0
8. Insert a small value 10 Ω resistor between ground and R1. This small value resistor is called a
current sensing resistor Rs. Reset the source voltage to 5.0 volts. Now measure the
voltage across the current sensing resistor – designate this voltage Vs. Place data into
your table.
9. Make a new column in your table labeled “Circuit Current” with the unit μA
(microampere). Fill in the computed data.
10. Does the output voltage vary linearly or nonlinearly with the source voltage?
11. Does the circuit current vary linearly or nonlinearly with the source voltage?
12. Short out the 10 Ω resistor and reset the source voltage to 5.0 volts. Now shunt each
resistor with a resistor of its own value. Measure the output voltage.
13. How do the results from step 12 compare to the results in step 3 and 4?
14. Remove the shunt resistors.
15. Select a resistor that is 100 times the value of R1 – designate this as a load resistor RL.
Measure and record its value.
16. Predict the theoretical value of the output voltage V1 if resistor RL is connected
between nodes A and 0 (ground). Draw the circuit in your notebook.
17. Now connect this load resistor between nodes A and 0 on your PB. Measure the
output voltage. What do you observe?
18. Next, let’s measure the current in the load resistor without using a dc ammeter. To do
this, move your current sensing resistor (10 Ω) and insert it in series with resistor RL.
Now measure the voltage across the current sensing resistor – designate this voltage
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Vs. The current through Rs equals IRs = Vs/Rs. This is the same current through RL.
Compute this value and give your result in microamperes (μA).
19. Remove Rs and place it in series with resistor R1. Determine IR1, the current in R1.
20. Compare these two currents and decide the significance of the value of the load
resistor.
Exercise 2: A Potentiometer Circuit
1. A schematic drawing of a pot is shown below. Obtain a 2 kΩ potentiometer (pot) and
examine it. Note the 3 pins on the bottom side; they are designated End Terminal and
Wiper Terminal. The pins plug into the PB. The wiper terminal can slide (inside the
pot) back and forth by turning the small screw located on the end of the pot. Ask your
instructor for a pot tweaker. Insert it into the screw of the pot and rotate the screw.
How much does it turn from end to end?
End Terminal
End Terminal
Wiper Terminal
2. Insert the pot on your PB. Connect short wires to the three terminals. Measure the
resistance from one End Terminal to the other End Terminal using a digital ohmeter
(DOM). Then measure the resistance from one End Terminal to the Wiper terminal.
Repeat this for the other end.
3. Now connect the DOM between the wiper terminal and an end terminal and rotate the
screw (this moves the wiper) while measuring the resistance between an end terminal
and the wiper terminal. Record your results.
4. Now that you are familiar with a pot, refer to the voltage divider circuit in Figure 2.
Copy this into your notebook.
5. From this circuit predict the theoretical range of voltages for the output voltage V1 as
you move the wiper from one end to the other end.
6. Construct the circuit on your PB. Measure and record the output voltage for different
positions of the wiper arm.
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R2
2.2kohm
R1
Vs
2kohm pot
5Vdc
A
+
V1
0
Figure 2. Voltage divider.
Exercise 3: Voltage Divider Design
1. Design a circuit that has these specifications:
- Source voltage of 12 volts; negative terminal grounded.
- An unloaded output voltage, measured with respect to ground, that varies from
a minimum of 3 volts to a maximum of 9 volts. Note: The minimum voltage
is obtained when the wiper is at one end, and the maximum voltage is
obtained when the wiper is at the other end.
2. After you complete the design, show it to your instructor for his evaluation.
3. Then build the circuit on your PB and perform measurements in order to verify the
design.
Exercise 4: The Bridge Circuit
1. The circuit is shown in Figure 3. Change the 3.9 kohm resistor to 680 ohms. As
shown, it is two voltage dividers in parallel with a voltage source. Copy the circuit
into your notebook.
2. Set the wiper on your pot to its approximate midpoint position. Note that only one
end is wired into the circuit. Measure and record the values of the remaining
resistors.
3. Construct the circuit on your PB.
4. Use your DVM to measure the voltage at nodes A and B, with respect to node 0
(ground). Then measure voltage VAB (+ on the left).
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5. Check to see of VAB equals VA minus VB.
R3
R1
Vs
2.2kohm
A
3.9kohm
B
5Vdc
R2
4.7kohm
R4
2kohm pot
0
Figure 3
6. With your DVM connected between A and B, adjust the wiper arm until VAB is as
close to zero as possible.
7. Turn off the power and temporarily remove the pot from the PB. Measure and record
the resistance between the wiper and the active end of the pot for this wiper setting.
8. Substitute these values into the balanced bridge equation listed in your text. Does it
check?
Exercise 5: An AC Bridge Circuit
1. Readjust the wiper of the pot towards one end and place the pot back in your circuit.
2. Keeping the resistors in place, remove the dc source and replace it with the lowvoltage center-tapped transformer that you used previously.
3. Connect CH 1 ground clip to node O. Connect CH 1 of the scope to display the
signal vAO(t) and CH 2 of the scope to display the signal, vBO(t). Note: Node O is the
reference node.
Note: It is convenient, when observing power line signals, to use line triggering of
the scope. Proceed as follows: In the Trigger section of the front panel of the scope,
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press the Edge key and on the menu bar, select the Edge softkey and select Line; the
scope is now triggering with 60 Hz (line voltage frequency).
4. Adjust the scope controls to display about 2 cycles of the waveforms and so that the
waveforms are on top of one another. Adjust the wiper on the pot so that the two
waveforms are almost identical. In this case the bridge is balanced.
5. Remove the scope probes and now connect CH 1 of the scope to display the signal
vAB(t) [ground clip of CH 1 is now at node B]. Note: Node B is the reference node.
While observing the waveform vAB(t), adjust the wiper on the pot in the forward and
backward rotation and watch to see when the signal is positive, goes to zero, and
when it inverts. When is the bridge in balance?
6. Turn off the power and temporarily remove the pot from the PB. Measure and record
the resistance between the wiper and the active end of the pot for this wiper setting.
Is this the same setting as in step 7 in Exercise 4?
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