UNIVERSITY OF MASSACHUSETTS DARTMOUTH
COLLEGE OF ENGINEERING
EGR 101 INTRODUCTION TO ENGINEERING THROUGH APPLIED SCIENCE I
POWER CONTROL USING A POTENTIOMETER - PRELAB
INTRODUCTION
In this experiment, you will use a 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.
By inserting the potentiometer in series with the lamp (bulb), you can control the current
and hence the power and brilliance of the lamp.
In this experiment, you will construct the circuit on a breadboard and take real
measurements. However, prior to the experiment, you are charged on your own to
simulate this circuit using Multisim as explained next.
PROCEDURE: MULTISIM PRELAB
Use Multisim to simulate the circuit shown in Figure 1. To measure current and voltage
in Multisim, insert an ammeter and 2 voltmeters in the circuit as shown in Figure 2.
Beginning with the dimmer potentiometer set to 0%, record the current and the voltage
measurements in Excel as shown in Table 1. Repeat the measurements by adjusting
the potentiometer in 25% increments throughout its range. From the voltage and current
measurements determine the resistance of the dimmer potentiometer and the lamp
(bulb). Determine also the power absorbed by the potentiometer and the bulb.
U1
+
0.000
A
DC 1e-009Ohm
DimmerCurrent
V
U2
DC 10MOhm
PotVoltage
V
U3
DC 10MOhm
BulbVoltage
+
0.000
R1
500Ω
50 %
Key=A
Dimmer Potentiometer
V1
10 V
X1
12 V
+
-
0.000
Figure 2. The lamp dimmer with instrumentation.
Table 1 Summary Table for the Dimmer-Lamp Circuit
Dimmer (pot)
Pot effective
Resistance
( )
Pot setting
(%)
MultiSim Measurements
Current
(A)
Dimmer
Voltage
(V)
Bulb
Voltage
(V)
Calculated
Dimmer
Power
(W)
Bulb
Power
(W)
Bulb
Resistance
( )
0%
25%
50%
75%
100%
DELIVERABLES

Complete the Multisim simulation PRIOR to arriving in class.

Print a copy of the completed Excel spreadsheet. This is your “ticket” in order to
perform the experiment.