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Resistor

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What is a Resistor ?

In many circuit applications, resistance must be inserted into the circuit in the purpose of reducing the current or to produce a desired IR voltage drop.  The component used for this are resistor and labeled with the letter R in the circuit diagrams.  Resistor are the most commonly found component in all electronic equipment, from a small AM radio to a color television receiver. The common type of resistor is the carbon resistor as shown in the image.

TYPE OF RESISTORS:

The two main characteristics of a resistor are its R in ohms and the wattage rating.  Resistors available values ranges from a fraction of an ohms up to many mega ohms.  The power rating can be as high as several watts or as low as 1/10 W.

The power rating of a resistor is important because it specifies the maximum wattage the resistor can dissipate without producing excessive heat that can damage the component and or other components in the circuit.  Dissipation means that the power is wasted as I2R loss. Since the result heat is not used, the generated heat which is usually too much can make the resistor burn open.

When power dissipation is about 5 watts or more wirewound resistors are used, specially in devices that require high current handling capability, heat dissipation and resistance stability and accuracy.  For 2W or less, carbon resistor are preferable because they are smaller and cost less.

Carbon resistors are most common in electronic equipment because they are smaller and cost less as compared to wirewound resistor, Usually higher values have smaller wattage because they have less current.

Both carbon and wirewound resistors can be either fixed or variable.  A fixed resistor have a specific R that cannot be adjusted, while a variable resistor can be adjusted for any value between zero ohms up to its maximum R value.

Carbon composition variable resistors are commonly used for control, such as the volume control in a radio receiver or the contrast control in a television receiver.  An application for a variable resistor is to divide the voltage from a power supply.

1. WIREWOUND RESISTOR:  Resistance wire such as manganin wrapped around an insulating core, Commonly used insulating materials are cement, porcelain, or just plain compressed paper.

The wire is bare, but usually the entire unit is encased in an insulator. The length of wire used and its specific resistivity determine the resistance of the unit.

Wirewound ResistorSince wirewound resistors are generally for low-resistance high-power applications, wirewound resistors are available in power ratings from 5W to several hundred watts, with resistance range of less than 1Ω to several thousand ohms. In addition, wirewound precision resistors are used where accurate, stable resistance values are require.

2. CARBON COMPOSITION RESISTOR: This type of resistor are made of finely divided carbon or graphite mixed with a powdered insulating material in the proportions needed for the desired value. The resistor element is usually enclosed in a plastic case for insulation and mechanical strength.

Joined to the two end of the carbon resistance elements are metal caps with leads of tinned copper wire for soldering the resistor connections into a circuits. Carbon resistors are commonly available in resistance values of 1/10, 1/8, ¼, ½, 1 or 2 watts.

3. CARBON FILM RESISTOR: Carbon film resistors are manufactured by depositing a carbon film on a ceramic substrate. In many ways semilar to carbon composition resistors, they can be interchange with each other, the cost is less than for the hot-molded carbon-composition type.
4. METAL-FILM RESISTORS:
These are form by means of vacuum deposition, a process by which any of the number of different metal or metal oxide films are deposited on a suitable insulating mandrel or core.

Nickel and chromium are deposited on the alumina ceramic core and the unit is then subjected to laser trimming. Metal film resistors are laser trimmed or helixed to obtain the desired resistance value before the protective insulation coat is applied.

TYPES OF METAL FILMS RESISTORS

  1. METAL-OXIDE DEPOSITION — This process makes use of a chemical vapor deposite to a tin-oxide film onto a glass substrate. Although this tin oxide resistor is similar in performance to evaporated or sputtered metal film, the technique is obviously not capable of achieving the level of precision possible with other thin-film process.
  2. BULK METAL. — This process produces a flat instead of cylindrical elements. It is an expensive process but produces resistor with a tight tolerance and excellent temperature coefficient of resistance characteristics, used exclusively for ultra-precision applications. This process entails the laminations of metal foil to a substrate and later etched chemically to produce a serpentine rather than helical conduction path.
  3. THICK FILM — This process coats ceramic substrate with a glass-metal matrix material. Later, it is fired at an extremely high temperature to produce a metallic film. Unlike in other processes wherein the resistive elements are subjected to a very temperature in order to vaporize materials, the thick film process does it to cure.
  4. VACUUM DEPOSITION — Also known as evaporated metal film, it is the original metal film resistor manufacturing process. This techniques superheats a nickel chromium alloy wire by resistance or electron — beam heating in a vacuum. As the alloy evaporizes, it is deposited on ceramic substrates which are then loaded into a vacuum container. To achieve the resistance ranges, manufacturers make use of contaminants called dopants.
  5. SPUTTERING — This is a more recently developed techniques and closely related to vacuum deposition. It is likewise yields a product consisting of nichrome resistor on a ceramic substrate.5. CERMIT-FILM RESISTORS: These have a carbon coating fired onto a solid ceramic substrate.  The purpose is to have a exact or precise R values and greater stability with heat.  They are often made in a small square, with leads to fit a PC board.  A flat package with multiple leads can be used for several resistors in one unit.

  6.  VARIABLE RESISTORS:

    Variable resistors can be wirewound or a carbon type.  Inside the metal case, the control has a circular disk that is the carbon-composition resistance element.  Joined to the two end are the outside soldering-lug terminals 1 and 3.  The middle lug 2 is connected to the variable arm contacting the resistor element by a metal spring wiper.  As the shaft of the control is rotated, the variable arm moves the wiper to make contact at different points.

    When the contact moves closer to one end, the resistance decreases between this end and the variable arm.  The variable resistance is zero when the wiper contacts this end but is maximum with the variable arm  at the opposite end.  Between the two outside ends, the resistance is not variable but always has the maximum resistance of the control.

    RESISTORS COLOR CODING :

    In addition to having the required resistance value in ohms, a resistor should  have a wattage rating high enough to dissipate the I2R power produced by the current flowing through the resistance, without becoming too hot.  If a resistor becomes too hot because of excessive power dissipation, it can change appreciably in resistance value or burn.

    The power rating is a physical  property depending on the resistor construction, specifically physical size.
    Note:

            1. The larger the physical size of the resistor indicates a higher power rating.

            2. Higher-wattage resistors can operate at higher temperature.

            3. Wirewound resistors are physically larger with higher wattage rating than carbon resistor.

    For both types, a higher power rating allows a higher voltage rating.  The rating gives the highest voltage that may be applied across the resistor without internal arching.  In wirewound resistors, excessive voltage can produce an arc between turns; in carbon resistors, the arc is between carbon granules.

    SERIES AND PARALLEL COMBINATION:

    In some cases two or more resistors are combines in series or parallel to obtain a desired resistance value with a higher wattage rating as shown.

    series parallel resistors

    The total resistance depend on the series and parallel connections. However, the combination has a power rating equal to the sum of the individual wattage ratings, whether the resistors are in parallel or series. The reason is that the total physical size increases with each added resistor, Such combinations can be used to obtain a higher power rating.

    Figure a, the two equal resistors in series double the resistance. Also the power rating of the combination is twice the value for one resistor.Figure b, the two equal resistor in parallel have one-half the resistance. However the combined power rating is still twice the value for one resistor.

    Figure c, the series parallel combination of four resistors makes RT the same on each resistor. However, the total power rating is four times the value for one resistor.

    CHECKING RESISTOR WITH OHMMETER :

    An open resistor reads infinitely high ohms. For some reason, infinite ohms is often confused with zero ohms. Remember though that infinite ohms means an open circuit. The current is zero but the resistance is infinitely high. Furthermore, it is practically impossible for a resistor to become short-circuited in itself. The resistor may be short circuited by some other part of the circuit, however, the construction of resistors is such that the trouble they develop is an open circuit, with infinitely high resistance in ohms.