EET 251

Electricity and Electronics

Lesson Two

ELECTRICAL COMPONENTS AND MEASUREMENTS

Most electrical equipment is made of several parts or components that work together. It would be almost impossible to explain how electrical equipment operates without using symbols and diagrams. Electrical diagrams show how the component parts of equipment fit together. Common electrical components are easy to identify. It is also easy to learn the symbols used to represent electrical components. The components of electrical equipment work together to form an electrical system.

Another important activity in the study of electricity and electronics is measurement. Measurements are made in many types of electrical circuits. Learning the proper ways of measuring resistance, voltage, and current are important as these are the three most commonly measured quantitites.

2.1 Components, Symbols, and Diagrams.

Wire: symbol for wire, symbols for wire crossing and connecting

Lamp, battery

Switch: toggle switch, single-pole single-throw (SPST)

Resistor: typical color-coded resistor, potientiometer (pot)

2.2 Resistors

Wire-wound resistors, precision-wound resistors

Carbon resistors

Variable resistors or potentiometers or rheostats (figure 2-11)

Resistor color codes, color bands to indicate resistance value and tolerance.

Power rating of a resistor

2.3 Electrical Units

Four units of electrical measurement:

Voltage, Current, Resistance and Power.

Electrical Quantity	Unit of Measurement	Symbol	Description
Voltage	Volt (V)	V	Electrical pressure that causes current flow
Current	Ampere (A)	I	Amount of electron movement through a circuit
Resistance	Ohm (S)	R	Opposition to current flow.
Power	Watt (W)	P	Rate of use of energy as current flows through a circuit

Small and Large units: The electrical unit used to measure a certain value is often less than a whole unit (less than 1). Examples of this are 0.6V, 0.005 W. When this occurs, prefixes are used. Examples of this are 5mW. Sometimes, electrical units of measurement are quite large, such as 20,000,000 W. When this occurs, prefixes are needed to make these large numbers easier to use.

Prefix	Abbreviation	Fractional Part of a Whole Unit
giga	G	1,000,000,000
mega	M	1,000,000
kilo	k	1,000
milli	m	0.001
micro	µ	0.000001
nano	n	0.000000001
pico	p	0.000000000001

2.4 Scientific Notation

A number that has many zeros to the right or to the left of the decimal point is made simpler by putting it in the form of scientific notation (powers of 10).

2.5 Schematic Diagrams

Schematic diagrams are used to represent the parts of electrical equipment or circuits.

2.6 Block Diagram

Block diagrams show the functions of the subparts of any electrical system.

2.7 Wiring Diagram

Wiring diagram show the actual location of parts and wires on equipment.

2.8 Measuring Resistance

Meters: ohmmeter, multimeter (VOM), analog meter, digital meter

Resistance tests or continuity checks

2.9 Measuring Voltage

Voltmeter

Dc voltmeter

Voltage drop

2.10 Measuring Current

Anmeter

Procedure for measuring current:

1. Turn off the circuit’s voltage source.

2. Set the meter to the highest current range.

3. Connect the negative test lead of the meter to the terminal nearest to the negative side of the voltage source.

4. Connect the positive lead of the meter

5. Turn of the switch to apply voltage to the circuit

6. Look at the meter needle to see how far it has moved up the scale.

7. Adjust the meter range until the needle moves to between the center of the scale and the right side.

2.11 Digital Meters

Digital meters employ numerical readouts to simplify the measurement process and to make more accurate measurements.

Resolution of a digital multimeter

REVIEW

1. Why are symbols used to represent electrical components?

2. Draw the symbols for the components listed her

a. Resistor

b. Ground connection

c. SPST

d. Potentiometer

e. Lamp

f. Fuse

3. Draw the symbols for (a) conductors crossing (b) conductors connected

4. Draw the symbols for a battery placing the positive (+) sign and the negative (-) sign at the proper sides of the symbol.

5. Identify each of the meter symbols illustrated here.

(a) FA (b) mA (c) V (d) S (e) A

6. Draw a diagram of a circuit that has one battery, two resistors, a single-throw switch, and a current meter connected to form one path.

7. Why is scientific notation used?

8. Why is it necessary to convert large or small electrical units by using prefixes?

9. On which part of the ohms scale of a meter are the most accurate resistance measurements made?

10. What is the X1000 range of the ohmeter?

11. What is meant by zeroing the meter?

12. If the range of the ohmmeter is adjusted to X100,000 and the needle points to 10.6 on the ohms scale, what is the value of the resistance being measured?

13. What values are shown by the first and second color band of a color-coded resistor?

14. What value is shown by the third color band of a color-coded resistor?

15. What value is shown by the fourth color band of a color-coded resistor?

16. What unit of electrical measurement is used for resistance?

17. If a fourth color band is not indicated on a resistor, what is its tolerance value?

18. When measuring an unknown voltage, what meter range should be used?

19. What determines how the meter is connected to measure the voltage drop across a resistor?

20. When is it necessary to multiply or divide a scale reading on a VOM?

21. How is the proper voltage range of a VOM selected?

22. Could 4 V be measured on the 3-V range of a VOM? Why?

23. Who is the VOM properly connected into a circuit to measure current flow?

24. Why is proper polarity important when measuring direct current?

25. What is the proper procedure to be followed when attempting to measure an unknown current?