University of Technology, Jamaica School of Engineering. Electrical Workshop Notes On Electrical components

Transcription

1 University of Technology, Jamaica School of Engineering Electrical Workshop Notes On Electrical components

2 Resistors Resistors are fundamental components in electronic circuits. A resistor is constructed to have a specific amount of resistance to current flow. The range of resistors may be from less than one ohm to well over 20 million ohms. Fixed resistors have a single value of resistance, but variable resistors can provide different values of resistance.

3 Resistors Resistors are rated by their value of resistance and the power they can safely dissipate. Every resistor has a power rating. The power rating is to a large degree, determined by its size. The greater the surface area, the more power it can dissipate. Power ratings of resistors vary from less than one tenth of a watt to many hundreds of watts.

4 Resistors Resistor tolerance is a measure of the resistor s variation from the specified value. Resistor tolerance is expressed as a percentage of its nominal value. Typical resistor tolerances are 1% and 5%, with tighter-tolerance resistors being somewhat more expensive. Resistors with tolerances lower than 2% are called precision resistors.

5 Resistance All materials exhibit electrical resistance The resistance of a material of uniform cross section is determined by: Material Length Cross-sectional area Temperature

6 Material Each material has a unique molecular structure Each material will react differently to pressures to establish a current through it Each material has a characteristic property called resistivity The symbol for resistivity is ρ (rho)

7 Length and Area The resistance R of a material of uniform cross-section is: Directly proportional to its length (longer, more resistance) Inversely proportional to its crosssectional area (bigger area, smaller resistance)

8 Resistivity Relationship R = ρl/a The resistance of a material of uniform cross-section is given by: R = ρl/a Where R = resistance (Ω) ρ = resistivity (Ω.m) l = length (cm) A = area (m 2 )

9 Resistor Technology There are four major classes of fixed resistor technology (see following slides): Carbon-composition Film resistors Wirewound resistors Surface-mount technology

10 Resistor Technology Carbon-composition Film resistors

11 Resistor Technology Wirewound resistors Surface-mount technology

12 Resistor Colour Code The resistor may have three, four or five coloured bands The first three bands represent the resistance value The fourth band represents the tolerance (accuracy) The fifth band (if present) represents a reliability factor

13 Resistor Color Code (3-4 Band) Since resistors are physically small, it is impractical to print the value of the resistor on it. Manufacturers mark resistors with three to five colored bands to indicate its value. Three- or four-band resistors can be interpreted by the following procedure: The first two bands represent the first two digits of the resistance value Multiply the digits obtained in step one by the multiplier value If a fourth band is present, it indicates the tolerance. If there is no fourth band, the tolerance is ±20%

14 Resistor Color Codes Bad beer rots our young guts but vodka goes well! Band Type Color Value Multiplier Tolerance Digit/Multiplier Black Digit/Multiplier Brown Digit/Multiplier Red Digit/Multiplier Orange Digit/Multiplier Yellow Digit/Multiplier Green Digit/Multiplier Blue Digit/Multiplier Violet Digit/Multiplier Gray Digit/Multiplier White Multiplier/Tolerance Gold ±5% Multiplier/Tolerance Silver ±10% Multiplier/Tolerance None - - ±20%

15 Variable Resistors Sometimes it is desirable to change the value of a resistor installed in a circuit. The resistance of a variable resistance is controlled by turning a knob, rotating a screw, or moving a slider. Two major classes of variable resistors are: Rheostats Potentiometers

16 Resistors Surface-mount resistor markings: Many surface-mounted components have no visible markings and must be measured with test equipment to determine their value. Some manufacturers indicate the value of a surface mount resistor with a numerical code. The first digit or digits indicate the value of the resistor and the last digit indicates the multiplier. A code of 103 would indicate a value of 10,000 ohms.

17 Resistor Symbols

18 Resistor Colour Code The resistor may have three, four or five coloured bands The first three bands represent the resistance value The fourth band represents the tolerance (accuracy) The fifth band (if present) represents a reliability factor

19 Resistor Colour Code Each of the first three bands represents a single digit number The resistance value is found by forming a two digit number from the first two bands The two digit number is then multiplied by a power of ten given by the digit representing the third band The resistor tolerance is given by the fourth band

20 Learning the Code Step 1: Memorise the colour values: 0 Black Multipliers 1 Brown 0.1 Gold 2 Red 0.01 Silver 3 Orange 4 Yellow Tolerance 5 Green 5% Gold 6 Blue 10% Silver 7 Violet 20% No band 8 9 Grey White

21 Learning the Code Step 2: The first three bands represent the resistance value Each of the first three bands represents a single digit number The third band digit is the power of ten that multiplies the number formed by the first two digits The third band represents the number of zeros following the first two digits

22 Sample Calculation Say the first three coloured bands are brown, red, orange The digits are: 1 = brown 2 = red 3 = orange (3 zeros) The value is 12x10 3 = 12,000Ω

23 Learning the Code Step 3: The fourth band gives the tolerance In this example the fourth band is silver 10% = silver

24 Learning the Code Thus the resistor in this example has a value of 12,000Ω (12KΩ) and a tolerance of 10%

25 Resistance

26 Capacitors A capacitor is a basic electronic component that stores electrical energy in the form of an electrostatic charge. The operation of nearly every electronic The operation of nearly every electronic system depends upon capacitors.

27 Inductors An inductor is also a fundamental electronic component, sometimes referred to as a coil. It consists of a spiraled or coiled wire. The inductor stores electrical energy in the form of an electromagnetic field.

28 Transformers A transformer is basically two or more coils whose electromagnetic fields interact. Transformers are used to increase or decrease alternating voltages.

29 Switches Switches are yet another group of basic electrical devices. Switches break (open) or make (close) circuit connections. See below and following slide for examples.

30 Switches

31 Basic Switch Operation The pole in a switch is the moveable part. The term throw refers to how many circuits are opened or closed during the switching operation. A double-throw switch opens or closes two circuits on each pole. Basic switch types include SPST, SPDT, DPST, DPDT, etc. Some switches do not maintain contacts when operated and are called momentary-contact switches.

32 Relays Relays are electromagnetically operated switches. They are used extensively in electrical systems, especially industrial systems. Relays are used for motor control circuits, circuits to protect workers, switching circuits, and power switching.

33 Transistors and Diodes Transistors and Diodes are solid-state devices or semiconductors. They are used in many electronic devices, including amplifiers, computers, and industrial controls. Diodes are used to alter information signals, convert AC current into DC current, and as protective devices and switches.

34 Single Way Switching

35 Two Way Switching

36 THANK YOU FOR YOUR ATTENTION!!