Chapter 3
Summary Review of V, I, and R Voltage is the amount of energy per charge available to move electrons from one point to another in a circuit. Current is the rate of charge flow and is measured in amperes. Resistance is the opposition to current and is measured in ohms.
The most important fundamental law in electronics is Ohm s law, which relates voltage, current, and resistance. In a DC circuit, the current is directly proportional to the voltage and inversely proportional to the resistance. It takes one volt to push one amp through one ohm. I V R What is the current in from a 12 V source if the resistance is 10? 1.2 A
Ohm s law If you need to solve for voltage, Ohm s law is: V IR I V R What is the voltage across a 680 resistor if the current is 26.5 ma? 18 V
Ohm s law If you need to solve for resistance, Ohm s law is: R V I What is the (hot) resistance of the bulb? 132 OFF V I V R Hz 115 V V mv 10 A A Ra ng e Autorange Touch/Hold 1 s 1 s V 40 ma COM Fused
Summary Ohm s law
A student takes data for a resistor and fits the straight line Graph shown to of the Current data. What versus is the Voltage resistance and the conductance of the resistor? 16 The slope represents the conductance. 14 14.8 ma - 0 ma G 1.48 ms 10.0 V - 0 V The reciprocal of the conductance is the resistance: R 1 1 676 Ω G 1.48 ms I (ma) 12 8 4 0 0 2 4 V (V) 6 8 10
Graph of Current versus Voltage Notice that the plot of current versus voltage for a fixed resistor is a line with a positive slope. What is the resistance indicated by the graph? 27V 0V 10mA 0mA 2.7 k What is its conductance? 0.37 ms Current (ma) 10 8.0 6.0 4.0 2.0 0 0 10 20 30 Voltage (V)
Graph of Current versus Resistance If resistance is varied for a constant voltage, the current verses resistance curve plots a hyperbola. Current (ma) 10 8.0 6.0 4.0 Determine the voltage source for the curve? (2.4mA)(1.25kΩ) = 3V 2.0 0 0 1.0 2.0 3.0 (8.0mA)(375Ω) =3V Resistance (k )
Linear Resistance A linear resistance has a constant value of ohms. Its R does not change with the applied voltage, so V and I are directly proportional. + 0 to 9 Volts _ 2 Amperes 4 3 2 1 1 2 4 Carbon-film and metalfilm resistors are examples of linear resistors. 0 1 2 3 4 5 6 7 8 9 Volts The smaller the resistor, the steeper the slope.
Nonlinear Resistance In a nonlinear resistance, increasing the applied V produces more current, but I does not increase in the same proportion as the increase in V. Example of a Nonlinear Volt Ampere Relationship: As the tungsten filament in a light bulb gets hot, its resistance increases.
Nonlinear Resistance Another nonlinear resistance is a thermistor. A thermistor is a resistor whose resistance value changes with its operating temperature. As an NTC (negative temperature coefficient) thermistor gets hot, its resistance decreases. Thermistor Amperes Volts
Application of Ohm s law 15V/560Ω = 26.8 ma The resistor is green-blue brown-gold. What should the ammeter read? + DC Ammeter - Power Supply V A Gnd 5 V 2A +15 V - + - +
Application of Ohm s law? Applying Ohm s Law 20 V 4 I = 20 V 4 = 5 A V I R 1 A? 12 V = 1A 12 = 12 V 3 A 6 V? R = 6 V 3 A = 2
Open-Circuit An open circuit has zero current flow. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Short-Circuit A short circuit has excessive current flow. As R approaches 0, I approaches. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Prototyping Board Example of how components are Inserted in the protoboard
Summary Troubleshooting? APM Analysis, Planning, Measuring 1. Analyze clues or symptoms of the failure. - Is the power properly applied to circuit. - Has the circuit ever worked. Conditions of failure. - Check fuses or circuit-breakers, if applicable. - Any components burnt, solder splashes, etc.? 2. Planning Formulate a plan of attack and working knowledge of the circuit. - Schematics - Instruction manuals 3. Measuring Narrow the possible failures by isolating sections of the circuit and making carefully thought out measurements.
Linear Ohm s law Troubleshooting Selected Key Terms Characterized by a straight-line relationship. A law stating that current is directly proportional to voltage and inversely proportional to current. A systematic process of isolating, identifying, and correcting a fault in a circuit or system.
1. Holding the voltage constant, and plotting the current against the resistance as resistance is varied will form a a. straight line with a positive slope b. straight line with a negative slope c. parabola d. hyperbola
1. Holding the voltage constant, and plotting the current against the resistance as resistance is varied will form a a. straight line with a positive slope b. straight line with a negative slope c. parabola d. hyperbola
2. When the current is plotted against the voltage for a fixed resistor, the plot is a a. straight line with a positive slope b. straight line with a negative slope c. parabola d. hyperbola
2. When the current is plotted against the voltage for a fixed resistor, the plot is a a. straight line with a positive slope b. straight line with a negative slope c. parabola d. hyperbola
3. For constant voltage in a circuit, doubling the resistance means a. doubling the current b. halving the current c. there is no change in the current d. depends on the amount of voltage
3. For constant voltage in a circuit, doubling the resistance means V a. doubling the current I R b. halving the current c. there is no change in the current d. depends on the amount of voltage
4. A four-color resistor has the color-code red-violetorange-gold. If it is placed across a 12 V source, the expected current is a. 0.12 ma b. 0.44 ma c. 1.25 ma d. 4.44 ma
4. A four-color resistor has the color-code red-violetorange-gold. If it is placed across a 12 V source, the expected current is a. 0.12 ma b. 0.44 ma c. 1.25 ma d. 4.44 ma 12V 27k
5. If the current in a 330 resistor is 15 ma, the applied voltage is approximately a. 5.0 V b. 22 V c. 46 V d. 60 V
5. If the current in a 330 resistor is 15 ma, the applied voltage is approximately a. 5.0 V b. 22 V c. 46 V d. 60 V ( 15mA)(330 )
6. The current in a certain 4-band resistor is 22 ma when the voltage is 18 V. The color bands on the resistor are a. blue-gray-red-gold b. red-red-brown gold c. gray-red-brown-gold d. white-brown-red-gold
6. The current in a certain 4-band resistor is 22 ma when the voltage is 18 V. The color bands on the resistor are a. blue-gray-red-gold b. red-red-brown gold c. gray-red-brown-gold d. white-brown-red-gold 18V 22mA 818.2 820
7. The circuit with the largest current is a. (a) b. (b) c. (c) d. (d) +12 V R +15 V R +18 V R +24 V R 10 k 15 k 22 k 27 k (a) (b) (c) (d)
7. The circuit with the largest current is a. (a) b. (b) c. (c) d. (d) 24V 27k 0.89mA +12 V R +15 V R +18 V R +24 V R 10 k 15 k 22 k 27 k 12V 10k (a) (b) (c) (d) 1.2mA 15V 15k 1.0mA 18V 22k 0.82mA
8. The circuit with the smallest current is a. (a) b. (b) c. (c) d. (d) +12 V R +15 V R +18 V R +24 V R 10 k 15 k 22 k 27 k (a) (b) (c) (d)
8. The circuit with the smallest current is a. (a) b. (b) c. (c) d. (d) 24V 27k 0.89mA +12 V R +15 V R +18 V R +24 V R 10 k 15 k 22 k 27 k 12V 10k (a) (b) (c) (d) 1.2mA 15V 15k 1.0mA 18V 22k 0.82mA
9. Before troubleshooting a faulty circuit you should find out a. If the circuit ever worked b. The conditions that existed when it failed c. The symptoms of the failure d. All of the above
9. Before troubleshooting a faulty circuit you should find out a. If the circuit ever worked b. The conditions that existed when it failed c. The symptoms of the failure d. All of the above
10. A troubleshooting method that starts in the middle and works toward a fault is a. short-circuit testing b. comparison testing c. half-splitting d. resistance testing
10. A troubleshooting method that starts in the middle and works toward a fault is a. short-circuit testing b. comparison testing c. half-splitting d. resistance testing