Resistors

Resistors are so named because they resist current flowing through them.

Putting a resistor in a circuit causes a drop in voltage across the resistor, so resistors are commonly used to control the voltage that reaches various components. Most resistors that can carry large currents contain a coil of high-resistance wire wound around a ceramic former to dissipate heat.

Resistance is measured in ohms, represented by a Greek letter, omega, and so resistors are rated in ohms as well, to indicate how strongly they will oppose any current flowing through them. Resistors also have a wattage rating. This is because resistors work by converting some of the electrical energy passing through them into heat.

It is also important to remember that each electrical component also has a resistance of its own.

A resistor is a two-terminal electrical or electronic component that passes a current that is proportional to the potential difference between its terminals in accordance with Ohm's law. The ratio of potential difference (also called voltage) to current is known as its electrical resistance (or simply resistance). Resistors are used as part of electrical networks and electronic circuits.

Applications

In general, a resistor is used to create a known voltage-to-current ratio in an electric circuit. If the current in a circuit is known, then a resistor can be used to create a known potential difference proportional to that current. Conversely, if the potential difference between two points in a circuit is known, a resistor can be used to create a known current proportional to that difference.

An attenuator is a network of two or more resistors (a voltage divider) used to reduce the voltage of a signal.

A line terminator is a resistor at the end of a transmission line or daisy chain bus (such as in SCSI), designed to match impedance and hence minimize reflections of the signal.

The ideal resistor

The SI unit of electrical resistance is the ohm. A component has a resistance of 1 ohm if a voltage of 1 volt across the component results in a current of 1 ampere, or amp, which is equivalent to a flow of one coulomb of electrical charge (approximately 6.241506 × 1018 electrons) per second.

In an ideal resistor, the resistance remains constant regardless of the applied voltage or current flowing through the device or the rate of change of the current. While real resistors cannot attain this goal, they are designed to present little variation in electrical resistance when subjected to these changes, or to changing temperature and other environmental factors.

Non-ideal characteristics

A resistor has a maximum working voltage and current above which the resistance may change (drastically, in some cases) or the resistor may be physically damaged (burn up, for instance). Although some resistors have specified voltage and current ratings, most are rated with a maximum power which is determined by the physical size. Common power ratings for carbon composition and metal-film resistors are 1/8 watt, 1/4 watt, and 1/2 watt. Metal-film resistors are more stable than carbon resistors against temperature changes and age. Larger resistors are able to dissipate more heat because of their larger surface area. Wire-wound and sand-filled resistors are used when a high power rating is required.

Furthermore, all real resistors also introduce some inductance and a small amount of capacitance, which change the dynamic behavior of the resistor from the ideal.

Types of resistor

A few types of resistors may be fixed or variable.

• Fixed resistors
  Some resistors are cylindrical, with the actual resistive material in the center (composition resistors) or on the surface of the cylinder film resistors, and a conducting metal lead projecting along the axis of the cylinder at each end. This is called an axial package resistor. The photo above right shows a row of common resistors. Power resistors come in larger packages designed to dissipate heat efficiently. At high power levels, resistors tend to be wire wound types. Resistors used in computers and other devices are typically much smaller, often in surface-mount packages without wire leads. Resistors are built into integrated circuits as part of the fabrication process. A single IC may contain millions of resistors.
  
• Variable resistors
  The variable resistor is a resistor whose value can be adjusted by by turning a shaft or sliding a control. These are also called potentiometers or rheostats and allow the resistance of the device to be altered by hand. Variable resistors can be inexpensive single-turn types or multi-turn types with a helical element. Some variable resistors can be fitted with a mechanical display to count the turns.

Some examples include:

• Rheostat
  A variable resistor with two terminals, one fixed and one sliding. It is often used with high currents.
  
• Potentiometer
  A common type of variable resistor. One common use is as volume controls on audio amplifiers and other forms of amplifier.

Other types of resistor (Note that some of the following resistors do not obey Ohm's law)

• A metal oxide varistor (MOV) is a special type of resistor that has two very different resistance values: a very high resistance at low voltage (below the trigger voltage) and very low resistance at high voltage (above the trigger voltage). It is usually used for short circuit protection in power strips or lightning bolt "arrestors" on street power poles, or as a "snubber" in inductive circuits.
  
• A thermistor is a temperature-dependent resistor. There are two kinds, classified according to the sign of their temperature coefficients:
  
  • A Positive Temperature Coefficient (PTC) resistor is a resistor with a positive temperature coefficient. When the temperature rises the resistance of the PTC increases. PTC's are often found in televisions in series with the demagnetizing coil where they are used to provide a short-duration current burst through the coil when the TV is turned on. One specialized version of a PTC is the polyswitch which acts as a self-repairing fuse.
    
  • A Negative Temperature Coefficient (NTC) resistor is also a temperature-dependent resistor, but with a negative temperature coefficient. When the temperature rises the resistance of the NTC drops. NTC's are often used in simple temperature detectors and measuring instruments.
    
• A sensistor is a semicondutor-based resistor with a negative temperature coefficient, useful in compensating for temperature-induced effects in electronic circuits.
  
• Light-sensitive resistors are discussed in the photoresistor article.

Identifying resistors

Most axial resistors use a pattern of colored stripes to indicate resistance. SMT ones follow a numerical pattern. Cases are usually brown, blue, or green, though other colors are occasionally found like dark red or dark gray.

Source: CDX Global & Wikipedia - en.wikipedia.org