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The fact that a wire conducting current can become hot is evidence that the work done by the applied voltage in producing current must be accomplished against some form of opposition. This opposition, which limits the amount of current that can be produced by the applied voltage, is called resistance . Conductors have very little resistance; insulators have a large amount of resistance.
The atoms of a copper wire have a large number of free electrons, which can be moved easily by a potential difference. Therefore, the copper wire has little opposition to the fl ow of free electrons when voltage is applied, corresponding to low resistance.
Carbon, however, has fewer free electrons than copper. When the same amount of voltage is applied to carbon as to copper, fewer electrons will fl ow. Just as much current can be produced in carbon by applying more voltage. For the same current, though, the higher applied voltage means that more work is necessary, causing more heat. Carbon opposes the current more than copper, therefore, and has higher resistance.
What you are expected to learn
What you are expected to learn
At the end of this module, you should be able to:
Define resistance and explain its affect in a circuit.
Determine the tolerance range of a resistor.
Identify carbon composition, wirewound, and film resistors.
Identify potentiometers and rheostats.
Describe how a variable resistor operates.
Decode a resistor’s value using the color code or alphanumerical code.
Identify the three types of resistor circuits.
Calculate total resistance in series, parallel, and series-parallel circuits.
How to learn from this module
How to learn from this module
Going through this module can be both a fun and a meaningful learning experience. All you need to do is make use of your time and resources efficiently. To do this, here are some tips for you:
1. Take time in reading and understanding each lesson. It is better to be slow but sure than to hurry finishing the module only to find out that you missed the concepts you are supposed to learn.
2. Do not jump from one chapter to another. Usually, the lessons are arranged such that one is built upon another, hence an understanding of the first is essential in comprehending the succeeding lessons.
3. Be honest. When answering the test items, do not turn to the key to correction page unless you are done. Likewise, when performing experiments, record only what you have really observed.
4. Safety first. Perform the experiments with extra precaution. Wear safety gears whenever necessary.
5. Don’t hesitate to ask. If you need to clarify something, approach your teacher or any knowledgeable person.
What to do before (Pretest)
What to do before (Pretest)
Lesson 5: Resistance
Lesson 5: Resistance
The fact that a wire conducting current can become hot is evidence that the work done by the applied voltage in producing current must be accomplished against some form of opposition. This opposition, which limits the amount of current that can be produced by the applied voltage, is called resistance . Resistance is a measure of how much a material opposes current flow when it is connected to a voltage. Thus a material with a high resistance will have a lower current flow through it whereas a material with a low resistance will have a higher current flow through it. As a rule all materials offer resistance to current flow. Resistance is expressed in units called the Ohm . The Ohm is given the symbol Ω. Conductors have very little resistance; insulators have a large amount of resistance.
The atoms of a copper wire have a large number of free electrons, which can be moved easily by a potential difference. Therefore, the copper wire has little opposition to the fl ow of free electrons when voltage is applied, corresponding to low resistance.
Carbon, however, has fewer free electrons than copper. When the same amount of voltage is applied to carbon as to copper, fewer electrons will fl ow. Just as much current can be produced in carbon by applying more voltage. For the same current, though, the higher applied voltage means that more work is necessary, causing more heat. Carbon opposes the current more than copper, therefore, and has higher resistance.
Lesson 5.1: Resistor
Lesson 5.1: Resistor
Resistors are electrical-electronic components that are designed to have certain specific values of resistance and are used in a wide variety of applications in all types of electronic circuits.
Resistors are used in circuits to accomplish two functions, namely:
to limit the magnitude of current through it to certain values.
to place certain magnitudes of voltage across points in a circuit.
• Resistors are either fixed or variable.
• The tolerance of a resistor is the amount that its resistance can vary and still be acceptable.
•The most common types of resistors include carbon-composition, carbon film, metal-film, wire-wound, and surface-mount or chip resistors. Carbon-film and metal-film resistors are better than carbon-composition resistors because they have tighter tolerances, are less affected by temperature and aging, and generate less noise internally.
• Resistors are either carbon composition, wirewound, or film.
• Carbon composition resistors are the most commonly used resistors.
• Wirewound resistors are used in high current circuits that must dissipate large amounts of heat.
• Film resistors offer small size with high accuracy.
• A potentiometer is a variable resistor with three terminals. It is used to vary the voltage in a circuit.
• A rheostat is a variable resistor with two terminals. It is used to vary the current in a circuit.
The physical size of a resistor determines its wattage rating: the larger the physical size, the larger the wattage rating. There is no correlation between a resistor’s physical size and its resistance value.
All resistors have a maximum power rating and if exceeded will result in the resistor overheating and becoming damaged.
Standard resistor power rating sizes are 1/8 W, 1/4 W, 1/2 W, 1 W, and 2 W.
Low ohmic value power resistors are generally used for current sensing or power supply applications.
The most common trouble in resistors is an open. An ohmmeter across the leads of an open resistor will read infinite, assuming there is no other parallel path across the resistor.
Resistors are usually color-coded to indicate their resistance value in ohms. Either a four-band or a five band code is used. The five-band code is used for more precise R values. Chip resistors use a three- or four-digit code to indicate their resistance value.
Resistor values may be identified by colored bands:
—The first band represents the first digit.
—The second band represents the second digit.
—The third band represents the number of zeros to be added to the first two digits.
—The fourth band represents the tolerance.
—A fifth band may be added to represent reliability.
Resistor values of less than 100 ohms are shown by a black third band.
Resistor values of less than 10 ohms are shown by a gold third band.
Resistor values of less than 1 ohm are shown by a silver third band.
Resistor values for 1% tolerance resistors are shown with the fourth band as the multiplier.
Resistor values may also be identified by an alphanumerical system.
The BS1852 Standard uses letters and is used to identify large size resistors.
Tolerance is the percentage measure of the accuracy of a resistor from its preferred value with the E6 (20%), E12 (10%), E24 (5%) and E96 (1%) series of tolerance values available.
Resistors may be placed in three configurations—series, parallel, and compound.
Resistors that are daisy chained together in a single line are said to be connected in SERIES.
Series connected resistors have a common Current flowing through them.
Itotal = I1 = I2 = I3 …. etc
The total resistance in a series circuit can be found by the formula: RT = R1 + R2 + R3 . . . Rn•
Total circuit voltage is equal to the sum of all the individual voltage drops.
Vtotal = V1 + V2 + V3 …. etc
The total resistance of a series connected circuit will always be greater than the highest value resistor.
Resistors that have both of their respective terminals connected to each terminal of another resistor or resistors are said to be connected in PARALLEL.
Parallel resistors have a common Voltage across them.
VS = V1 = V2 = V3 …. etc
The total resistance in a parallel circuit can be found by the formula: RT = 1/ R1 + 1/ R2 + 1/ R3 . . . 1/ Rn
Total circuit current flow is equal to the sum of all the individual branch currents added together.
Itotal = I1 + I2 + I3 …. etc
The total resistance of a parallel circuit will always be less than the value of the smallest resistor.
• The total resistance in a series-parallel circuit is determined by both series and parallel formulas.
Important Terms
Important Terms
Carbon-composition resistor a type of resistor made of finely divided carbon mixed with a powdered insulating material in the correct proportion to obtain the desired resistance value.
Carbon-film resistor a type of resistor whose construction consists of a thin spiral layer of carbon on an insulated substrate.
Color coding a scheme using colored bands or stripes around the body of a resistor to indicate the ohmic value and tolerance of a resistor.
Metal-film resistor a type of resistor whose construction consists of a thin spiral film of metal on a ceramic substrate.
Potentiometer a three-terminal variable resistor used to vary the voltage between the center terminal and one of the outside terminals.
Rheostat a two-terminal variable resistor used to vary the amount of current in a circuit.
Surface-mount resistor a type of resistor constructed by depositing a thick carbon film on a ceramic base. (A surface-mount resistor is many times smaller than a conventional resistor and has no leads that extend out from the body itself.)
Taper a word describing the way the resistance of a potentiometer or rheostat varies with the rotation of its shaft.
Thermistor a resistor whose resistance value changes with changes in its operating temperature.
Tolerance the maximum allowable percent difference between the measured and coded values of resistance.
Wire-wound resistor a type of resistor whose construction consists of resistance wire wrapped on an insulating core.
Self Check
Self Check
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