UNIVERSITY OF MASSACHUSETTS DARTMOUTH
COLLEGE OF ENGINEERING
EGR 101 INTRODUCTION TO ENGINEERING THROUGH APPLIED SCIENCE I
POWER CONTROL USING A POTENTIOMETER
INTRODUCTION
In this experiment, you will use a rotary potentiometer as a variable resistor to control the
brilliance of a light bulb.
The objective of this lab is to have you practice taking voltage and current
measurements, adjusting a potentiometer, using Ohm’s law and power equations and
using known information to infer something about a device.
The circuit you will build is called a dimmer circuit, as shown in Fig. 1.
Figure 1 A lamp dimmer using a potentiometer.
PROCEDURE: Breadboard

As a team, construct the circuit on your breadboard.

Set the power supply voltage to 10 V before you connect the rest of the circuit.

Insert the multirange analog ammeter to measure the circuit current. Remember
to “break” the circuit to insert the ammeter.

Adjust the setting of the rotary potentiometer using a screwdriver, as shown in
the 1st column in Table 1.

Use the DMM to measure the voltages across the potentiometer and the lamp.

Before you go to the next potentiometer setting, disconnect your circuit from the
10-V source and measure the resistance of the potentiometer at the existing turn
setting. Otherwise, you may be measuring the resistance of extra components
inside the breadboard, which are not visible to you.

Record the results in Excel as in Table 1.
Table 1 Summary Table for the Dimmer-Lamp Circuit
Potentiometer
Pot Effective
Resistance
( )
Pot setting
Breadboard Measurements
Current
(A)
Dimmer
Voltage
(V)
Bulb
Voltage
(V)
Calculated
Dimmer
Power
(W)
Bulb
Power
(W)
Bulb
Resistance
( )
0 turn
¼ turn
½ turn
¾ turn
Full turn

Plot on the same graph the bulb power as a function of the potentiometer
resistance for 1 team member’s MultiSim prelab data and the breadboard
data, and comment on your results.

From your measured voltages, is Kirchhoff’s Voltage Loop Law satisfied?

From your breadboard power data, is the power delivered to the circuit by
the breadboard’s power supply equal to the power dissipated in the
potentiometer and the bulb?

Is the calculated resistance for the actual bulb constant? If not, why do
you think it varies?
Turn in
1. MultiSim prelab tables from each team member.
2. One completed table for the breadboard section and the graph with both 1
team member’s multisim results and the breadboard results.
3. Comments on plots and answers to the all the questions.