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Resistor		Variable Resistor

A resistor is a two-terminal electrical component that passes a current through it that is proportional to the potential difference across its terminals . 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 incorporated into many electronic devices.

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 × 10 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.

Types of resistor

Resistors may be fixed or variable. .

Fixed Resistors Some resistors are cylindrical, with the actual resistive material in the centre or on the surface of the cylinder, 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. Resistors used in computers and other devices are typically much smaller, often in surface-mount packages without leads. Larger power resistors come in larger packages designed to dissipate heat efficiently. At high power levels, resistors tend to be wire wound types.

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 have a mechanical display to count the turns.

Variable resistors can be unreliable, because the wire or metal can corrode or wear. Some modern variable resistors use plastic materials that do not corrode and have better wear characteristics.

Some examples include:

• A rheostat is a variable resistor with two terminals, one fixed and one sliding. It is often used with high currents.
• A potentiometer is a common type of variable resistor. One common use is as volume controls on audio amplifiers.

Other Types of Resistor

• A Metal Oxide Varistor (MOV) is a special type of resistor which has 2 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 back electromotive force circuits.
• A thermistor is a temperature dependent resistor. There are two kinds, classified according to the sign of their temperature coefficients:
  • 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 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.
  • 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.
• Light-sensitive resistors are discussed in the photoresistor article.

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 capacitance, which change the dynamic behavior of the resistor from the ideal.

Identifying resistors

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

4-band axial resistors

4 band identification is the most commonly used colour coding scheme on all resistors. It consists of four coloured bands that are painted around the body of the resistor. The scheme is simple: The first two numbers are the first two significant digits of the resistance value, the third is a multiplier, and the fourth is the tolerance of the value. Each colour corresponds to a certain number, shown in the chart below. The tolerance for a 4-band resistor will be 2%, 5%, or 10%.

The Standard EIA Color Code Table per EIA-RS-279 is as follows:

Note: red to violet are the colours of the rainbow where red is low energy and violet is higher energy.

Resistors use specific values, which are determined by their tolerance. These values repeat for every exponent; 6.8, 68, 680, etc. This is useful because the digits, and hence the first two or three stripes, will always be similar patterns of colours, which make them easier to recognize.

Preferred values

Standard resistors are manufactured in values from a few milliohms to about a gigohm; only a limited range of values called preferred values are available. In practice, the discrete component sold as a "resistor" is not a perfect resistance, as defined above. Resistors are often marked with their tolerance (maximum expected variation from the marked resistance). On color coded resistors the color of the rightmost band denotes the tolerance:

silver 10%

gold 5%

red 2%

brown 1%.

Closer tolerance resistors, called precision resistors, are also available.

5-band axial resistors

5-band identification is used for higher tolerance resistors (1%, 0.5%, 0.25%, 0.1%), to notate the extra digit. The first three bands represent the significant digits, the fourth is the multiplier, and the fifth is the tolerance. 5-band standard tolerance resistors are sometimes encountered, generally on older or specialized resistors. They can be identified by noting a standard tolerance color in the 4th band. The 5th band in this case is the temperature coefficient.

SMT resistors

Surface-mount resistors are printed with numerical values in a code related to that used on axial resistors. Standard-tolerance SMT resistors are marked with a three-digit code, in which the first two digits are the first two significant digits of the value and the third digit is the power of ten. For example, "472" represents "47" (the first two digits) multiplied by ten to the power "2" (the third digit), i.e. ${\displaystyle 47\times 10^{2}=47\times 100=4700{\mbox{ ohms}}}$. Precision SMT resistors are marked with a four-digit code in which the first three digits are the first three significant digits of the value and the fourth digit is the power of ten.

Calculations

Ohm's law

The relationship between voltage, current, and resistance through an object is given by a simple equation which is called Ohm's Law:

${\displaystyle V=I\cdot R}$

where V is the voltage across the object in volts (in Europe, U), I is the current through the object in amperes, and R is the resistance in ohms. (In fact this is only a simplification of the original Ohm's law - see the article on that law for further details.) If V and I have a linear relationship -- that is, R is constant -- along a range of values, the material of the object is said to be ohmic over that range. An ideal resistor has a fixed resistance across all frequencies and amplitudes of voltage or current.

Superconducting materials at very low temperatures have zero resistance. Insulators (such as air, diamond, or other non-conducting materials) may have extremely high (but not infinite) resistance, but break down and admit a larger flow of current under sufficiently high voltage.

Power dissipation

The power dissipated by a resistor is the voltage across the resistor times the current through the resistor:

${\displaystyle P=I\cdot V=I^{2}R={\frac {V^{2}}{R}}}$

All three equations are equivalent, the last two being derived from the first by Ohm's Law.

The total amount of heat energy released per unit time is the integral of the power:

${\displaystyle W=\int _{t_{1}}^{t_{2}}v(t)i(t)\,dt}$

If the average power dissipated exceeds the power rating of the resistor, then the resistor will first depart from its nominal resistance, and will then be destroyed by overheating.

Series and parallel circuits

Resistors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent resistance (R_eq):

${\displaystyle {\frac {1}{R_{eq}}}={\frac {1}{R_{1}}}+{\frac {1}{R_{2}}}+\cdots +{\frac {1}{R_{n}}}}$

The parallel property can be represented in equations by two vertical lines "||" (as in geometry) to simplify equations. For two resistors,

${\displaystyle R_{eq}=R_{1}\|R_{2}={R_{1}R_{2} \over R_{1}+R_{2}}}$

The current through resistors in series stays the same, but the voltage across each resistor can be different. The sum of the potential differences (voltage) is equal to the total voltage. To find their total resistance:

${\displaystyle R_{eq}=R_{1}+R_{2}+\cdots +R_{n}}$

A resistor network that is a combination of parallel and series can sometimes be broken up into smaller parts that are either one or the other. For instance,

${\displaystyle R_{eq}=\left(R_{1}\|R_{2}\right)+R_{3}={R_{1}R_{2} \over R_{1}+R_{2}}+R_{3}}$

However, many resistor networks cannot be split up in this way. Consider a cube, each edge of which has been replaced by a resistor. Determining the resistance between (say) two opposite vertices requires matrix methods for the general case. However, if all twelve resistors are equal, the corner-to-corner resistance is 5/6 of any one of them.

See also

• Resistors in series and parallel circuits
• Wikibooks:Electronics:Component Identification

External links

• A good guide to resistor labeling, common values, and color codes
• Beginner's guide to potentiometers, including description of different tapers
• An article describing many aspects of resistors, including materials, tapers, and color code
• "The Original Color Coded Resistance Calculator"
• Resistor Colorcode Decoder

• Electrical components