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 experiment is to practice measuring voltage and current, 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 Figure 1.
R1
500Ω
50 %
Key=A
Dimmer Potentiometer
V1
10 V
X1
12 V
Figure 1 A lamp dimmer using a potentiometer.
PROCEDURE: BREADBOARD

Construct the circuit on your breadboard.

Set the power supply voltage to 10 V before connecting to the circuit.

Insert the multirange analog ammeter to measure the circuit current. Remember,
you must break the circuit to install (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 setting, measure the resistance of the potentiometer at
the existing setting. In order to avoid including the resistance of other devices
hidden from view but possibly connected to the circuit, disconnect one of the
potentiometer leads.

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 axes a graph of the bulb power as a function of the
potentiometer resistance for both the Multisim prelab data and the
breadboard data. Comment on your results.

From your measured voltages, is Kirchhoff’s Voltage Law satisfied?

From your breadboard power data, is the power delivered to the circuit by
the breadboard’s internal power supply equal to the power dissipated in
both the potentiometer and the bulb?

Is the calculated resistance for the actual bulb constant? If not, why do
you think it varies?
WHAT NEEDS TO BE TURNED IN
1. Multisim prelab table.
2. One completed table for the breadboard section and the graph with both
the Multisim results and the breadboard results.
3. Comments on plots and answers to the all the questions.