ENGR 1110: Introduction to Engineering
Lab 3
Items needed:
• Digital Multimeter (DMM)
• Power Supply
• 1 Breadboard
• 1 wire kit
• 4 1k Ohm resistors
• 1 xF capacitor
• 1 LED
Pre-Lab Preparation
Work the circuit analysis problems indicated below in italics before coming to lab. This will help you to know
what to expect when you do the actual measurements.
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 Dia. 1. Using Ohm’s law, calculate the current flowing in the resistor given
a supply voltage of 9V and a resistance of 1kOhm.
c. Wire up the circuit shown in Dia. 1 on your breadboard, referring to Fig. 1 if you need help with
breadboard wiring. 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?
Diagram 1
Figure 1: Breadboard layout of Diagram 1
d. Set up the DMM to measure current, and insert its leads into the circuit to measure the current
flowing through the resistor, as shown below. Remember, the DMM leads must be in the path of
current flow (inline with the circuit) in order to measure current. How does the measured current
compare with the current calculated using Ohm’s law?
2.
Resistors in series
a. Look at the circuit in Dia. 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 9V and resistor values of 1kOhm 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 Dia. 2 on your breadboard, referring to Fig. 2 if you need help with
breadboard wiring. Ensure the output voltage of the power supply is 9V, 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.
Diagram 2
Figure 2: Breadboard layout of Diagram 2
c. Set up the DMM to measure current, and insert its leads into the circuit to measure the current
flowing through the resistors, as shown below. How does the measured current compare with the
current calculated using Ohm’s law?
3.
Resistors in parallel
a. Look at the circuit in Dia. 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 9V and
resistor values of 1kOhm each. Remember, circuit elements in parallel with each other have the
same voltage drop across them. Calculate the current flowing through each resistor,
remembering the formula for a current divider.
b. Wire up the circuit shown in Dia. 3 on your breadboard, referring to Fig. 3 if you need help with
breadboard wiring. Ensure the output voltage of the power supply is 9V, 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.
Diagram 3
Figure 3: Breadboard layout of Diagram 3
c. Set up the DMM to measure current, and insert its leads into the circuit to measure the current
flowing through each one of the resistors, as shown below. 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.
4.
Series/parallel combination
a. Look at the circuit in Dia. 4. Calculate the voltage drop across each resistor and the series
resistor pair, given a supply voltage of 9V and resistor values of 1kOhm each. Remember, the
rules for voltage in parallel and series combinations. Calculate the current flowing through leg of
the circuit, remembering the formula for current division.
b. Wire up the circuit shown in Dia. 4 on your breadboard, referring to Fig. 4 if you need help with
breadboard wiring. Ensure the output voltage of the power supply is 9V, 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.
Diagram 4
Figure 4: Breadboard layout of Diagram 4
c. Set up the DMM to measure current, and insert its leads into the circuit to measure the current
flowing through each leg of the circuit, as shown below. 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.
5.
Diode circuit
a. Look at the circuit in Dia. 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
given voltage drop across them, usually around 0.6V. Given this, a power supply voltage of 9V
and a resistor value of 1kOhms, calculate the voltage drop across the resistor and the current
flowing in the circuit.
b. Wire up the circuit shown in Dia. 5 on your breadboard, referring to Fig. 5 if you need help with
breadboard wiring. 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?
Diagram 5
Figure 5: Breadboard layout of Diagram 5
6.
Diode in parallel circuit
a. Wire up the circuit shown in Dia. 6 on your breadboard, referring to Fig. 6 if you need help with
breadboard wiring. 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 is dimmer in this circuit. This is because the LED has less current flowing through it.
Measure the voltage drop across the LED, noticing that it is the same as in the previous setup.
Diagram 6
Figure 6: Breadboard layout of Diagram 6
7.
Diode in parallel with capacitor
a. Look at the circuit shown in Dia. 7 below. 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), turn
off the power supply or disconnect it from the circuit. Watch the light output of the LED as the
capacitor discharges and the voltage drops. Record your observations.
Diagram 7
Homework (to be done individually and turned in at the beginning of the next lab):
Write a memo to Dr. Reeves using MS Word to report expected values for each circuit, measured values or
observed behavior, and an explanation of any differences.