Resistors
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Resistors
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Click here to read the lecture notes on this topic. Please take the practice quiz below. This is completely anonymous and no records of results are maintained.
If you do not see the quiz above your browser may not support Java Applets. You may wish to download an update for your browser. Resistance is a property of every electrical component. The purpose of a resistor is either to reduce or limit current to a specific or safe value or, by using a series of resistors to provide a desired voltage.
TYPES OF RESISTORS
Carbon-Composition Resistors. One of the most common types of resistors is the molded-composition resistor, usually referred to as the carbon resistor.
They have a resistance range from 2.7 ohms to 100M ohms.
The physical size of the resistor is related to its power rating, which is the ability of the resistor to dissipate heat caused by the flow of current through the resistance.
In the manufacture of carbon resistors, fillers or binders are added to the carbon to obtain various resistor values. The fillers cause the overall conduction characteristics to change.
Carbon resistors are easy to manufacture, inexpensive, and have a tolerance adequate for most applications.
Their disadvantages are that they have a tendency to change value as they age and they have limited power-handling capacity.
Carbon-film resistors have a then coating of resistive material on a ceramic insulator. They have a resistance range from 10 ohm to 25 M ohm. The main advantage is that they can be manufactured to greater accuracies, having a tolerance between 2 % and 5 %. They also generate less noise because of random electron motion. This is helpful for audio and communications circuits.
Allen-Bradley is replacing Carbon-composition resistors with carbon-film, metal-film and wirewound resistors.
Metal-Film ResistorsMetal-film resistors are used where a high degree of accuracy is required. Tolerances of less than 1% are available. They are manufactured by spraying spraying a relatively then layer of metal on a ceramic cylinder. They have a resistance range from 0.1 ohm to 22 M ohm.
Metal-Oxide Film ResistorsMetal-oxide film resistors are made by oxidizing tin chloride on a heated-glass substrate. These resistors have low noise and excellent temperature characteristics. They are popular where precision resistors of high ohmic value are required. They have a resistance range from 1 K ohm to 30 G ohm.
Resistor NetworksLow-power resistors are also available in packages that resemble integrated circuit chips; as a result, they are clled chip resistors. Chip packages are fabricated from then film and thick film metals to exacting tolerances. They have a resistance range from 1.0 ohm to 100 M ohm. Chip resistors are designed for printed circuit board installation and are very popular in surface-mount technology.
Wirewound ResistorsWirewound resistors have very accurate values and possess a higher current-handling capability than carbon resistors. Several materials are used to make wirewound resistors. Nickel-chromium is the most common. Copper-nickel, and gold-platinum are also used. They have a resistance range from 0.01 ohm to 178 K ohm. One disadvantage is that it takes a large amount of wire to manufacture a resistor of high ohmic value.
Wirewound resistors do not use a color code. The value is usually stamped on the resistor case.
VARIABLE RESISTORS There are two types of variable resistors, one called a potentiometer (pot) and the other a rheostat). An example of a pot is the volume control on your radio, and an example of the rheostat is the dimmer control for the dash lights in an automobile.
Rheostats usually have two connections, one fixed and the other moveable. The pot always has three connections, two fixed and one moveable. Pots are always connected as voltage dividers.
Pots can be made using wirewound or carbon elements with a wiper to contact the element at various points.
The relationship between the angle of rotation and resistance, called taper, is either linear or nonlinear (logarithmic). The resistance is evenly distributed the taper is linear. Volume controls are often nonlinear because the human ear differentiates sound waves in a logarithmic fashion.
Trimmer ResistorsTrimmer resistors are variable resistors that are used where small and infrequent adjustments of a resistance are necessary to maximize circuit performance. Adjustment of trimmers is usually performed with a small screwdriver. This miniature potentiometers are also known as trim pots. They have a resistance range from 10 ohm to 100 K ohm.
ADDITIONAL TYPES OF VARIABLE RESISTORS
BolometersA bolometer changes resistance when heat energy is applied. There are two types of bolometers, the barretter and the thermistor. The barretter increases in resistance as the dissipated power increases. The thermistor decreases in resistance as the power increases.
BarretterThe barretter resembles a fuse with a fine wire in the middle.
ThermistorA high degree of precision is made possible by the thermistor. It is one of the most common heat-sensitive devices found in power meters. A semiconductor is used as the active material. A bead of semiconductor is encased in a glass capsule.
Light-Sensitive Devices
Photoconductive CellsLight-sensitive materials such as cadmium sulfide (CdS) or cadmium selenide (CdSe) are used. Two metal electrodes are deposited on the material as thin layers.
They are used in devices such as intrusion detectors and automatic door openers.
RESISTOR POWER RATING When a current is passed through a resistor, heat is developed within the resistor. The resistor must be capable of dissipating this heat into the surrounding air; otherwise, the temperature of the resistor rises, causing a change in resistance or causing the resistor to burn out.
Hot-spot temperature is the maximum temperature measured on the resistor due to both internal heating and ambient operating temperature.
Heat dissipation depends on the amount of surface area, therefore larger resistors generally have higher power ratings. Carbon resistors are commonly rated 1/8 W ¼ W ½ W 1 W and 2 W.
RESISTOR VALUE CODING
Alphanumeric Markings
When the physical size is large enough such as potentiometer or a wirewound resistor, that value markings are stamped on.
Resistor Color Coding
General-Purpose Resistors
Ten colors are used for indicating the resistive value, and two colors are used for indicating the tolerance. The four-band system for general purpose resistors is very common. General-purpose resistors have a tolerance between 5% and 20%.
Black 0 Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Gray 8 White 9 Gold 5% Silver 10% None 20%
Precision Resistors
Precision resistors (usually a film-type) with tolerances of 2% or less use a five-band system. The fourth band is the multiplier, the fifth band is the tolerance.
STANDARD RESISTOR VALUES
Specific resistor values are manufactured in large quantities so that they are less expensive. 10,12,15,18,22,27,33,39,47,56,68,82
TROUBLESHOOTING RESISTOR FAULTS
The most common problems to occur with fixed resistors are a change in resistance or a complete failure. A complete failure occurs when the resistor overheats and burns out. When the resistor interior is burned out, it produces an open circuit.
Burned out resistors are often caused by the failure of some other component in a circuit causing an excess of current. A cold solder joint can also cause an open circuit.
The internal composition of resistors make an internal short-circuit nearly impossible. However short circuits can occur on printed circuit boards or at the connections of resistors.
Variable resistors are subject to the same kinds of failures are fixed resistors. A condition refered to as a dirty, or noisy, pot is common with potentiometers. After againg the contact assembly between the wiper and the resistaive track can become oxidixed. This can cause noise to be generated in audio circuits via the volume or tone control.
The ohmmeter may be used to check resistors. The ohmmeter should never be used on a live circuit. To accurately test a resistor it must be isolated from other components in the circuit. One lead may be removed. Click here to move back to the top of this page.
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