ENGR 1110: Introduction to Engineering
Lab 3 – Circuits and Tools
Items used (these will be provided to each team in lab):
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Digital Multimeter (DMM)
Power Supply
1 Breadboard
Wire
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Three 1 kΩ resistors
1 capacitor, 1000 µF, 16 V or higher
1 LED, red or green
Pre-Lab Preparation
Each student should work the circuit analysis problems indicated below in italics (In-Lab Activity paragraphs 1b,
2a, 3a, 4a, 5a ) before coming to lab. This will help you to know what to expect when you do the actual
measurements. Your individual in-lab score will be reduced if you do not have this completed when you
arrive for lab.
In-Lab Activity
In this lab you will examine the relationships between basic circuit elements, voltage, current, and resistance.
1.
Single resistor
a. Turn on the power supply, making sure the leads are not shorted (connected). Set the output
voltage of the power supply to 9 volts. Set your DMM to read voltage and connect the positive
lead (red) of the power supply to the positive lead (red) of the DMM. Connect the negative lead
or common (black) of the power supply to the common lead (black) of the DMM. Read the
measured voltage on the DMM. Does it read exactly 9 volts? Why or why not? Switch the leads
so that the positive lead of the power supply is connected to the common of the DMM, and vice
versa. What does the DMM say the voltage across it is now?
b. Look at the circuit in Fig. 1. Using Ohm’s law, calculate the current flowing in the resistor given
a supply voltage of 9 V and a resistance of 1 kΩ.
Note: the symbol Ω is the Greek capital letter “omega.” It is used in electrical engineering to
represent the unit of resistance called the “ohm” (rhymes with “home”). The prefix k stands for
1000. Thus the notation 1 kΩ stands for 1000 ohms, which can also be written as 1 kilohm or
1 kohm, and is pronounced like “one kill ohm”.
c. Wire up the circuit shown in Fig. 1 on your breadboard. Ensure the output voltage of the power
supply is 9V, and measure the voltage drop across the resistor with the DMM. Remember the
DMM leads are placed on either side of a component to measure a voltage drop across it. How
does the measured voltage compare to the supply voltage?
d. Turn off your power supply. Set up the DMM to measure current. The DMM leads must be in the
path of current flow (inline with the circuit) in order to measure current. DO NOT PROCEED
until your GTA has verified your setup is correct. Turn on your power supply. How does the
measured current compare with the current calculated using Ohm’s law?
Figure 1: Single-resistor circuit diagram and breadboard layout.
Figure 2: Series-combination circuit diagram and breadboard layout.
2.
Resistors in series
a. Look at the circuit in Fig. 2. Using Ohm’s law and the formula for a voltage divider, calculate the
voltage drop across each resistor as well as the drop across both resistors, given a supply voltage
of 9 V and resistor values of 1 kΩ each. Remember, the sum of the voltage drops in a series
circuit is equal to the supply voltage. Calculate the current flowing through the circuit
(remember, the same current flows through both resistors).
b. Wire up the circuit shown in Fig. 2 on your breadboard. Ensure the output voltage of the power
supply is 9 V, and measure the voltage drop across each resistor and across both resistors with the
DMM. How do the measured voltages compare to the supply voltage? Do the measurements
agree with your calculations? If not, explain why.
c. Turn off your power supply. Set up the DMM to measure current. DO NOT PROCEED until
your GTA has verified your setup is correct. Turn on your power supply. How does the
measured current compare with the current calculated using Ohm’s law?
Figure 3: Parallel-combination circuit diagram and breadboard layout.
3.
Resistors in parallel
a. Look at the circuit in Fig. 3. Using Ohm’s law and the formula for a parallel resistor
combination, calculate the voltage drop across the resistors, given a supply voltage of 9 V and
resistor values of 1 kΩ each. Remember, circuit elements in parallel with each other have the
same voltage drop across them. Calculate the current flowing through each resistor.
b. Wire up the circuit shown in Fig. 3 on your breadboard. Ensure the output voltage of the power
supply is 9 V, and measure the voltage drop across the resistors with the DMM. How does the
measured voltage compare to the supply voltage? Does the measurement agree with your
calculations? If not, explain why.
c. Turn off your power supply. Set up the DMM to measure current. DO NOT PROCEED until
your GTA has verified your setup is correct. Measure the current flowing through each one of
the resistors. How does the measured current compare with the current calculated using Ohm’s
law and current division? Do the measurements agree with your calculations? If not, explain why.
Figure 4: Series-parallel-combination circuit diagram and breadboard layout.
4.
Series/parallel combination
a. Look at the circuit in Fig. 4. Calculate the voltage drop across each resistor and the series
resistor pair, given a supply voltage of 9V and resistor values of 1kΩ each. Remember, the rules
for voltage in parallel and series combinations. Calculate the current flowing througheach leg of
the circuit.
b. Wire up the circuit shown in Fig. 4 on your breadboard. Ensure the output voltage of the power
supply is 9 V, and then measure the voltage drop across each resistor and both series resistors
with the DMM. How do the measured voltages compare to the supply voltage? Does the
measurement agree with your calculations? If not, explain why.
c. Turn off the power supply. Set up the DMM to measure current. DO NOT PROCEED until
your GTA has verified your setup is correct. Measure the current flowing through each branch
of the circuit. How do the measured currents compare with the currents calculated using Ohm’s
law and current division? Do the measurements agree with your calculations? If not, explain why.
Figure 5: LED circuit diagram and breadboard layout.
5.
Diode circuit
a. Look at the circuit in Fig. 5. Remember, diodes (such as LEDs) only conduct current in one
direction (when the triangle of their symbol faces in the direction of current flow). Diodes have a
voltage drop across them. Assuming a diode voltage drop of 1.8V, a power supply voltage of 9V
and a resistor value of 1kΩ, calculate the voltage drop across the resistor and the current flowing
in the circuit.
b. Wire up the circuit shown in Fig. 5 on your breadboard. The longer leg on an LED is the positive
lead, corresponding to the line on the end of the triangle in the LED circuit drawing. Ensure the
output voltage of the power supply is 9V, and measure the voltage drop across the resistor and the
current flowing in the circuit with the DMM. How do the measured voltages compare to the
supply voltage? Does the measurement agree with your calculations? If not, explain why. Turn
the LED around in the circuit so its positive and negative leads are swapped. What happens to the
LED light? What about the voltage drop? What about the current flow?
Figure 6: LED circuit diagram and breadboard layout.
6.
Diode in parallel circuit
a. Wire up the circuit shown in Fig. 6 on your breadboard. Notice that the circuit forms a current
divider network, so the current flowing through each leg of the circuit is less than the total
amount flowing in the circuit. Notice that the LED shines as brightly in this circuit as the one in
Fig. 5, because there is still 9 V across the LED-resistor branch. Measure the voltage drop across
the LED, noticing that it is the same as in the previous setup.
1 kΩ
1000 µF
Figure 7: LED with parallel capacitor circuit diagram.
The capacitor value shown is 1 mF = 1000 uF. The actual value used in lab
may be different.
7.
Diode in parallel with capacitor
a. Look at the circuit shown in Fig. 7. Placing the capacitor in parallel with the power source
charges the capacitor, the capacitor acting much like a rechargeable battery. The process of
charging the capacitor causes the voltage across the capacitor leads to rise slowly from zero to the
supply voltage. With a large enough capacitor, the voltage rises slowly enough that it is possible
to watch the LED increase in brightness as the voltage increases. Similarly, if the power source is
removed or turned off, the capacitor will discharge like a small battery, supplying current to the
circuit. As the capacitor discharges, the voltage produced by the capacitor drops until it reaches
zero and current stops flowing. With a large enough capacitor, one can watch the LED dim and
finally turn off as the voltage supplied by the capacitor falls.
b. Wire up the circuit shown in the diagram, making sure the power supply is disconnected. Make
sure the power supply is at the correct voltage, and connect it to the circuit. Quickly watch what
happens to the LED as the capacitor fills up and allows more voltage across the LED. After the
capacitor is fully charged (the LED is at a constant voltage and is producing a stable light),
disconnect it from the circuit. Watch the light output of the LED as the capacitor discharges and
the voltage drops. Record your observations.
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Homework (to be done individually and turned in at the beginning of the next lab):
Write a report on this lab exercise. Follow the Lab Homework Format given in the syllabus. In the body
of the report, present a table giving the expected values for each circuit, the measured values or observed
behavior, and an explanation of any differences.