The R is the resistance value required to provide the desired current (I) or voltage. W ratings range from as low as 1/10 W to several hundred watts. The wattage rating specifies the maximum power the resistor can dissipate without excessive heat. Resistors with higher R values usually have lower wattage ratings.The lower the power rating (W), the smaller the actual physical size of the resistor; and, conversely, the higher the power rating (W), the larger the actual physical size of the resistor.

In a wire-wound resistor, resistance wire is wrapped around an insulating core. The insulating core is usually porcelain, cement, or plain pressed paper. The wire and core are then encased in an insulating material.
!(wire wound resistor)

Carbon-composition resistors from finely divided carbon or graphite mixed with a powdered insulating material and enclosed in a plastic case for insulation and strength. The axial leads are made of tinned copper for connecting to a circuit.
!(resistor cross section)

There are two kinds of film-type resistors: carbon film-type and metal film-type. Chip resistors have a carbon coating fired onto a solid ceramic substrate. These resistors have more precise R values and greater stability with temperature changes.

The fusible resistor is a wire-wound resistor made to burn open easily when the power rating is exceeded  It serves the dual functions of a fuse and a resistor to limit current.

Resistor Color Codings

Color codes have a numeric value for each resistor.
!(resistor bands and color codes example)

The third band of a resistor indicates the decimal multiplier for the first two digits in the R value. For example, if band 1 is blue, band 2 is grey, band 3 is orange, and band 4 is silver the resistance value is 68000 Ω. For resistors under 10 Ω the third stripe is either gold or silver, indicating a fractional decimal multiplier. When the third stripe is gold, multiply the first two digits by 0.1. If the third stripe is silver, then the multiplier is 0.01. Band 4 indicates a percentage of tolerance. If the band is silver the tolerance is ± 10%. If the band is gold the tolerance is ± 5%. If there is no band for tolerance, it is ± 20%. The characteristics of a resistor can be written in the form of 45000 Ω ± 5%.

In calculating the highest ohmic value, the decimal multiplier is converted to a decimal and is multiplied by the value of the first and second bands. This product is added to the nominal ohmic value to yield the total ohmic value. For instance, if the tolerance is 5% and the nominal ohmic value is 68000 Ω then the lowest ohmic value is 64600. Wire-wound resistors are big enough physically to have the R value printed on the case. Some small wire-wound resistors are color coded with stripes. Because of economics of scale, certain sizes are in large quantity and are easily available.

Variable Resistors

!(circular carbon and slide control resistors labeled)

!(shaft, rotating arm, and soldering lugs(1, 2, 3) of circular carbon resistor labeled)

When the shaft of a circular carbon resistor is turned, the rotating arm moves closer to the soldering lugs 1 or 3. The R value changes between this terminal and the rotating arm. An example of a variable resistor at work is a light switch that allows you to control the brightness of the light.

Tapered control is the way R varies with shaft rotation on a taper. With a linear taper, one half rotation changes R by one-half the maximum value. All values of R change in direct proportion to rotation.

For a non-linear taper, R can change more gradually at one end with larger changes at the opposite end. This is accomplished by different densities of carbon on the resistance element.

Rheostats and Potentiometers

Rheostats and potentiometers are variable resistors that are either carbon or wire-wound. These resistors are used to vary the amount of current or voltage for a circuit. Rheostat resistors can be either carbon or wire-wound resistors. A rheostat is a variable R with two terminals connected in series with a load. As the rheostat's terminal fluctuates, the total resistance will reflect those changes.
!(rehostat circiut schematic)

A potentiometer is a variable voltage divider with three terminals. The purpose of a potentiometer is to provide a variable voltage between two points in a circuit. The wattage rating varies between terminals 1 and 2 and also between terminals 2 and 3. It is possible to use a potentiometer as a rheostat by connecting one of the terminals and the variable terminal.
!(potentiometer circuit schematic)

Power Rating of Resistors

A resistor should have a wattage rating high enough to dissipate the power without becoming too hot. Carbon resistors in normal operation are often quite warm with a maximum temperature of about 85 degrees Celsius. A higher wattage rating allows higher power rating. The power rating depends on the resistor construction and especially physical size. A larger physical size indicates higher temperatures. Wire-wound resistors can operate at higher temperatures. Wire-wound resistors are larger with higher wattage ratings than carbon resistors. Shelf life describes resistors accurately because they do not change with age.

Series and Parallel Combinations of Resistors

Two or more resistors may be combined to gain a desired R value. The total resistance RT depends on the series or parallel connections. However, the combination has a power rating equal to the sum of the individual resistor's value. Two equal resistors in series double the resistance for RT. For two equal resistors in parallel, the resistance is 1/2 the RT.

Resistor Troubles

The most common trouble of a resistor is an open. When a volume or tone control makes a scratchy noise as the shaft is rotated, the resistance element is worn-out. Resistance measurements are made with an ohmmeter which is pictured bellow.

An open resistor reads infinitely high ohms. The ohmmeter must have an ohms scale large enough to read the resistance value, otherwise the meter will read infinitely high ohms. When checking resistance in a circuit, be sure there are no parallel paths across resistors. Due to heat and time, resistors can change their in value beyond tolerance. When troubleshooting, substitute a resistor of the correct value and monitor the effects to the circuit.

No comments:

Post a Comment