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.

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 / Breadboard Measurements / Calculated
Pot setting / Pot Effective
Resistance
() /

Current

(A) / Dimmer
Voltage
(V) / Bulb
Voltage
(V) / 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.