47Electric Quantities and Components47
POSITION OF SWITCH STATE OF LAMP
2.11 Safety Practices for Technicians
Electrical and electronic circuits sometimes have lethal voltages and currents. Therefore, it is essential for a technician to have a solid knowledge of appropriate safety practices. While it is certainly important for a technician to respect dangerous voltages and currents, it is also important not to fear them. Fear can obstruct your thinking, interfere with the perfor-mance of your job, and actually create a more dangerous situation. Fear often stems from lack of knowledge, so learn all you can about potentially dangerous circuits.
The LED is a polarized component and must be connected such that the cathode terminal is more negative than the anode terminal.
KEY POINTS
LEDs require current- limiting resistors to pro-tect them from damage by excess current.
KEY POINTS
Figure 2-50. Several example LEDs.
Exercise Problems 2.10 Exercise Problems 2.10
Effects of Voltage or Current
The human body offers a certain resistance to current flow. Ohm’s Law can easily confirm that the higher the voltage applied to the body, the higher the current flow. However, it is surprising how small the voltages and currents can be and still affect the body. Voltages of only a few volts and currents of only a few milliamperes can cause serious injury under the right conditions. As the value of current through the body increases, the effects get progres-sively more severe and include such things as mild tingling sensations, involuntary muscle contractions, difficulty in breathing, heartbeat irregularities, burned skin, and death. One of the most dangerous voltage sources that all technicians work around is the 120-V power line. It is very powerful and can inflict the full spectrum of effects listed previously.
Safety Guidelines
Following are some specific safety guidelines that represent good technician practices:
1. Never work alone on circuits with lethal voltages and currents.
2. Always know the location of circuit breakers, and be sure other people in the area also know where the breakers are located.
3. If practical, remove the power to circuits before working on them.
4. Never trust another person’s statement that power has been removed from a cir-cuit; verify it yourself.
5. Work with one hand as much as possible. Keep the other hand in your pocket or otherwise restrained so that it cannot inadvertently contact the circuit.
6. Wear shoes with insulative soles or stand on a rubber mat.
7. Only use tools with insulated handles.
8. Remove all neckties and jewelry before working on equipment.
9. NEVER allow horseplay or practical jokes in the vicinity of a live circuit.
This last rule deserves extra attention since its effects are not always obvious. Suppose a technician is working around a high-voltage circuit. He is not scared, but he is rightfully alert with very focused attention. A would-be prankster sneaks up behind the technician and slams a book closed. Of course, it scares the technician, but what about serious injury?
In his eagerness to get away from the circuit, the technician may inadvertently contact a high-voltage point. In the process of jumping back from the circuit, the technician may trip and fall into other equipment, causing injuries. And finally, the technician may be unable to find the humor in such mindless acts and will report the prankster to his super-visor. This behavior would be grounds for termination in some companies.
Chapter Summary
• When a material has an unequal number of electrons and protons, we say it has a charge.
An excess of electrons in a body produces a negative charge; a deficiency of electrons (excess protons) in a body produces a positive charge. Charge is measured in coulombs (C) where 1 C is the amount of charge provided by 6.25 × 1018 electrons. A charged body is surrounded by an electrostatic field, which is a region that can exert a force on another charged body. Unlike charges attract while like charges repel.
• When two unequal charges are separated by some distance, there is a potential for doing work. This potential difference, called voltage and measured in volts (V), is capable of causing charged particles to move. The movement of charged particles is called current flow. If the potential difference between two charges is sustained even though charged particles are moving from one point to the other, then we call the potential difference an electromotive force or emf.
• An ideal voltage source provides a constant voltage over an infinite time period regard-less of the value of current flowing in the circuit. Practical voltage sources have internal resistance. The internal resistance causes the available voltage to decrease as current flow increases.
• Current is generally the movement of charged particles. When 1 C of charge moves past a given point in a 1-s interval, we say there is 1 ampere (A) of current flow.
Many technicians think of current flow as the movement of electrons. Current viewed in this way is called electron current flow and moves through a circuit from negative to positive.
• Ideal current sources are capable of maintaining a constant current flow regardless of the amount of resistance in a circuit. Practical current sources provide reasonably constant currents over a range of resistances.
• Resistance is the opposition to current flow and is measured in ohms (Ω). One ohm of resistance will limit the current to 1 A if 1 V is applied to the circuit. Different mate-rials have different amounts of resistance. Insulators have high resistance, conductors have low resistance, semiconductors have moderate resistance, and superconductors have no resistance at all.
• Power is a measure of the rate at which we use energy and often takes the form of heat in electrical components. Power is measured in watts (W). Electrical power measured in watts can be compared to an equivalent mechanical energy expressed in horsepower by using the relationship 1 hp = 746 W. Electrical power is usually calculated with the power formulas which use current, voltage, and resistance as factors in the equation.
• Resistors are components that are designed to provide a certain amount of resistance.
Fixed resistors are manufactured with a specific amount of resistance. Variable resistors can be adjusted for different amounts of resistance. Rheostats (two-terminal devices) and potentiometers (three-terminal devices) are two forms of variable resistors. All resistors, fixed and variable, have power ratings, which indicate the amount of elec-trical power (heat) they can dissipate without damage. Fixed resistors often use a color code to indicate their value. Resistors may have three, four, or five bands. The bands provide information such as resistance value, tolerance, and reliability.
• Switches are used to open or close an electrical circuit. The pole of a switch is the movable element and is often drawn as an arrow on schematic diagrams. The throw of a switch is a term that identifies how many circuits are switched by a given pole. The various contact forms (SPST, DPDT, SPDT, and so on) can be made with different types of actuator mechanisms: toggle, rocker, push button, or rotary. Momentary-contact switches change states while they are being activated, but they automatically return to the normal state when they are released. Schematic symbols always show the normal or relaxed state.
• Fuses and circuit breakers are devices that protect circuits from excessive current flow.
Fuses are usually faster than circuit breakers, but they must be replaced with a new fuse once they have blown. Circuit breakers can be reset (manually or automatically) after they have tripped.
• Incandescent, neon, and LED indicators provide sources of light. Incandescent sources emit light from a heated tungsten filament. The light may be used as a point source or as a diffused source. Neon bulbs require higher voltages to operate. They emit light when the neon gas becomes ionized. LEDs are solid-state devices that require low voltages and currents and emit red, yellow, green, or blue light. Both neon bulbs and LEDs must have an external current-limiting resistor, and both are used as point sources of light.
• Ohm’s Law (I = V/R ) describes a very important relationship. It states that the current in a circuit is directly proportional to the voltage across the circuit and inversely proportional to the amount of resistance in the circuit.
• Ohm’s Law and the power formulas are so important, they should be committed to memory. However, some technicians use the following memory aid when they are first learning these relationships.
Review Questions Review Questions
Section 2.1: Charge
1. When a body has more electrons than protons it has a (positive, negative) charge.
2. What polarity of charge is on a proton?
3. An electron is a (positively, negatively) charged particle.
4. If two electrons were near each other would they attract, repel, or have no effect on each other?
V P
I IV R
IR
I2R V2 R V2
P P I2
V I V R P V P R P I PR
The Ohm’s Law and power equation memory wheel
5. How many electrons must be removed from a neutral body to create 1 C of charge?
6. Calculate the number of electrons required to produce a charge of 230 µC.
7. If a certain material has a deficiency of 2.5 × 1019 electrons, what is the polarity and magnitude of its charge?
8. Calculate the force exerted by two bodies separated by 0.25 m, if each body has a neg-ative charge of 3.75 C.
9. Will the bodies described in Question 8 attract or repel one another?
10. How much force is exerted by two charged bodies with charges of +3 C and –2.7 C, if they are separated by a distance of 1.75 m?
11. If a body with a +2-mC charge is separated from another charged body by a distance of 0.15 m, how much charge must be on the second body in order to produce an attraction of 375 mN?
12. The arrows representing an electrostatic field indicate the direction in which a (posi-tive, negative) charge would move.
13. What does the closeness of line spacing indicate on a sketch of an electrostatic field?
Section 2.2: Voltage
14. Difference of potential is measured in __________________.
15. Voltage is measured in __________________.
16. Electromotive force (emf ) is measured in __________________.
17. Work can be measured in foot-pounds, newton-meters, or __________________.
18. Explain the difference between difference of potential and electromotive force.
19. All differences of potential are electromotive forces. (True or False) 20. All electromotive forces have differences of potential. (True or False)
21. Five pounds of force is being applied to a certain object. Express the force in terms of newtons.
22. An ideal voltage source maintains a __________________ voltage across its terminals.
23. The output voltage of a practical voltage source varies with current flow because of its __________________ resistance.
Section 2.3: Current
24. Current can be defined as the movement of _________________.
25. If current is considered to be a flow of electrons, it is called __________________
current and flows from (positive, negative) to (positive, negative).
26. Conventional current is considered to be moving in the same direction as (positive, negative) charge.
27. If 7.5 C pass a certain point in a conductor every second, how much current is flow-ing in the wire?
28. How much current is flowing in a conductor if 400 µC pass a given point in 500 ms?
29. If a current of 400 mA flows past a certain point for 3 s, how much charge will be transferred?
30. How much charge is transferred if 2.7 A flows in a wire for 2 minutes?
31. An ideal current source maintains a __________________ current through its terminals.
32. A practical current source is relatively unaffected by changes of in-line resistance.
(True or False)
Section 2.4: Resistance
33. Write a brief definition of resistance as it applies to electrical circuits.
34. Insulators have (high, low, moderate, zero) resistance.
35. Superconductors have (high, low, moderate, zero) resistance.
36. Semiconductors have (high, low, moderate, zero) resistance.
37. Conductors have (high, low, moderate, zero) resistance.
38. Describe the meaning of the term breakdown voltage with reference to insulators.
39. Resistance is measured in __________________.
Section 2.5: Power
40. Mechanical power is the rate of doing __________________.
41. Mechanical power is measured in __________________.
42. Electrical power is the rate of using electrical __________________.
43. Electrical power is measured in __________________.
44. How much power is dissipated if 175 J of energy are used over a period of 2.7 s?
45. During what time interval must 5 J of energy be expended if 500 mW are to be produced?
46. Express 350 W as an equivalent number of horsepower.
47. Express 75 hp as an equivalent number of watts.
48. How much power is dissipated in an electrical circuit when 75 V is connected across a 5-Ω resistance?
49. How much current must flow through a 2.7-kΩ resistor in order to produce 375 mW of power?
50. How much voltage is required to produce 200 mW in a 270-Ω resistor?
Section 2.6: Ohm’s Law
51. Current is (directly, inversely) proportional to the resistance in a circuit.
52. Current is (directly, inversely) proportional to the voltage in a circuit.
53. Write the three forms of Ohm’s Law.
54. How much current flows through a 6.8-kΩ resistor that has 500 mV across it?
55. How much current flows through a 2-MΩ resistor that has 100 V across it?
56. If a 390-kΩ resistor has 200 mA flowing through it, how much voltage is across it?
57. If a 180-Ω resistor has 10.5 mA flowing through it, how much voltage is across it?
58. How much resistance is required to limit the current to 150 µA when 6.2 V are applied?
59. How much resistance does a circuit have if 150 V causes 200 mA of current to flow?
60. Determine the current (I ) that is flowing in Figure 2-51.
61. What is the value of the resistor shown in Figure 2-52?
Section 2.7: Resistors
62. The two general classes of resistors are __________________ and __________________.
63. Since resistors cannot be manufactured with exactly the right value, they are given a __________________ rating.
64. What characteristic or rating of a resistor is largely determined by its physical size?
65. If a 39-kΩ resistor has a ±5% tolerance, what is the maximum deviation between the actual and marked values of the resistor that is considered to be within tolerance?
66. What is the highest resistance a 27-kΩ, ±20% resistor can measure and still be within tolerance?
67. What is the lowest a 3.3-MΩ, ±2% resistor can measure and still be within tolerance?
68. What type of resistor has no leads and is soldered directly to pads on a printed circuit board?
69. Name three factors used by the manufacturer to control the value of a wirewound resistor.
70. What type of resistor is often associated with high power ratings?
71. In your own words, explain how to interpret the color code of a three-band resistor.
72. Explain how to interpret the fourth band of a four-band resistor.
73. If the fifth band of a certain resistor is silver, what is its tolerance?
74. If the fourth band of a four-band resistor is silver, what is its tolerance?
75. What is indicated by the tolerance band of a resistor if the band is red?
76. Interpret the information provided by the color codes on the resistors listed in Table 2-7.
77. Figure 2-53 shows a variable resistor connected as a __________________.
78. A potentiometer is a __________________-terminal device.
79. A rheostat is a __________________-terminal device.
80. Does the resistance between the end terminals of a rheostat vary when the control is adjusted?
81. Does the resistance between the end terminals of a potentiometer vary when the con-trol is adjusted?
Figure 2-51.
How much current is flowing in this circuit?
VT R
25 V I 5 kΩ
VT R
10 mA 120 V
Figure 2-52.
What is the value of the resistor?
Figure 2-53.
How is the variable resistor connected?
Section 2.8: Switches
82. How many connections would be on a SPST switch?
83. How many connections would be on a three-pole, four-position rotary switch?
84. Explain the meaning of the labels NO and NC, which are sometimes used with refer-ence to switches.
85. If two switch symbols on a schematic diagram are linked with a dotted line, what does this mean?
Section 2.9: Fuses and Circuit Breakers
86. Explain the purpose of the voltage rating on a fuse.
87. Explain the purpose of a current rating on a fuse.
88. When might a slo-blo fuse be a better choice than a fast-acting fuse?
89. Name one major advantage of a circuit breaker over a fuse.
Section 2.10: Optical Indicators
90. Name three factors that can cause an incandescent lamp to burn out before its rated lifetime.
91. What name is given to describe the amount of voltage required to illuminate a neon bulb?
92. LEDs require an in-line current-limiting resistance for proper operation.
(True or False)
Section 2.11: Safety Practices for Technicians
93. Carelessness around electrical/electronic circuits can result in death. (True or False) 94. It takes voltages in excess of 500 V to be considered dangerous. (True or False)
FIRST BAND
SECOND BAND
THIRD BAND
FOURTH BAND
FIFTH BAND
INDICATED VALUE
Yellow Violet Brown Gold —
Green Blue Orange — —
Orange Orange Silver Silver —
Brown Gray Orange Red Silver
Blue Red Green Silver Brown
Table 2-7. Determine the Value of These Resistors
The lights in your shop are presently controlled by two switches. The lights can be turned on or off by either switch. This operation is similar to the light switches at the top and bottom of stairs in houses. Your supervisor has located a schematic that he believes shows how the switches are connected. The schematic is shown in Figure 2-54.
Your supervisor has given you two assignments. First, he wants you to verify that the schematic shown in Figure 2-54 really does provide the intended operation. Second, a friend has told him that the lights can be controlled from a third location if a DPDT switch is added between the two SPDT switches already installed. It is your job to determine how to connect the third switch. Figure 2-55 provides a schematic view of the problem.
TECHNICIAN CHALLENGE TECHNICIAN CHALLENGE
Shop lights Shop
power
S1 S2
Figure 2-54. A schematic showing how two SPDT switches can control a light from either of two locations.
Shop lights Shop
power
S1 S2
DPDT switch
Figure 2-55. How can a DPDT switch be connected in order to provide lamp control from any of three locations?
Equation List Equation List
(2-1) 1 C = 6.25 × 1018 electrons (2-2)
(2-3) (2-4) (2-5)
(2-6)
(2-7) (2-8) (2-9) (2-10) (2-11) (2-12) (2-13) (2-14)
F kQ1Q2
d2
---=
I Q
----t
=
Power energy
---time
= 1 hp = 746 W I V
----, orR
= I E
---R
=
R V
----I
= V = IR P = VI P V2
---R
= P = I2R
maximum deviation = tolerance×marked value resistance range = marked value±maximum deviation RAC = RAB+RBC