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What Is A Resistor ?

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat, may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity. Resistors are common elements of electrical networks and electronic circuits and are ubiquitous in electronic equipment. Practical resistors as discrete components can be composed of various compounds and forms. Resistors are also implemented within integrated circuits. The electrical function of a resistor is specified by its resistance: common commercial resistors are manufactured over a range of more than nine orders of magnitude. The nominal value of the resistance falls within the manufacturing tolerance, indicated on the component.

Electronic symbols and notation

Two typical schematic diagram symbols are as follows:

(a) resistor, (b) rheostat (variable resistor), and (c) potentiometer

The International Electrotechnical Commission resistor symbol

The notation to state a resistor’s value in a circuit diagram varies.

One common scheme is the RKM code following IEC 60062. It avoids using a decimal separator and replaces the decimal separator with a letter loosely associated with SI prefixes corresponding with the part’s resistance. For example, 8K2 as part marking code, in a circuit diagram or in a bill of materials (BOM) indicates a resistor value of 8.2 kΩ. Additional zeros imply a tighter tolerance, for example 15M0 for three significant digits. When the value can be expressed without the need for a prefix (that is, multiplicator 1), an “R” is used instead of the decimal separator. For example, 1R2 indicates 1.2 Ω, and 18R indicates 18 Ω.

Theory of operation

Ohm’s law

The behavior of an ideal resistor is dictated by the relationship specified by Ohm’s law:


Ohm’s law states that the voltage (V) across a resistor is proportional to the current (I), where the constant of proportionality is the resistance (R). For example, if a 300 ohm resistor is attached across the terminals of a 12 volt battery, then a current of 12 / 300 = 0.04 amperes flows through that resistor.

Practical resistors also have some inductance and capacitance which affect the relation between voltage and current in alternating current circuits.

The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. An ohm is equivalent to a volt per ampere. Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm (1 mΩ = 10−3 Ω), kiloohm (1 kΩ = 103 Ω), and megaohm (1 MΩ = 106 Ω) are also in common usage

Series and parallel resistors

The total resistance of resistors connected in series is the sum of their individual resistance values.

What Is A Resistor ?

For example, a 10 ohm resistor connected in parallel with a 5 ohm resistor and a 15 ohm resistor produces 1/(1/10 + 1/5 + 1/15) ohms of resistance, or 30/11 = 2.727 ohms.
What Is A Resistor ?
What Is A Resistor ?

A resistor network that is a combination of parallel and series connections can be broken up into smaller parts that are either one or the other. Some complex networks of resistors cannot be resolved in this manner, requiring more sophisticated circuit analysis. Generally, the Y-Δ transform, or matrix methods can be used to solve such problems.

Resistor Characteristics

Dependent on the application, the electrical engineer specifies different properties of the resistor. The primary purpose is to limit the flow of electrical current; therefore the key parameter is the resistance value. The manufacturing accuracy of this value is indicated with the resistor tolerance and is expressed as a percentage of the resistance value (for example ±5%). Many other parameters that affect the resistance value can be specified, such as long term stability or the temperature coefficient. The temperature coefficient, usually specified in high precision applications, is determined by the resistive material as well as the mechanical design.

In high frequency circuits, such as in radio electronics, the parasitic capacitance and inductance can lead to undesired effects. Foil resistors generally have a low parasitic reactance, while wirewound resistors are among the worst. For accurate applications such as audio amplifiers, the electric noise of the resistor must be as low as possible. This is often specified as microvolts noise per volt of applied voltage, for a 1 MHz bandwidth. For high power applications the power rating is important. This specifies the maximum operating power the component can handle without altering the properties or damage. The power rating is usually specified in free air at room temperature. Higher power ratings require a larger size and may even require heat sinks. Many other characteristics can play a role in the design specification. Examples are the maximum voltage or the pulse stability. In situations where high voltage surges could occur, this is an important characteristic.

Sometimes not only the electrical properties are important, but the designer also has to consider the mechanical robustness in harsh environments. Military standards sometimes offer guidance to define the mechanical strength or the failure rate.

Resistor Standards

Many standards exist for resistors. The standards describe ways to measure and quantify important properties. Other norms exist for the physical size and resistance values. Probably, the most well known standard is the color code marking for axial leaded resistors.

Resistor color code

What Is A Resistor ?
What Is A Resistor ?

Resistor with a resistance of 5600 ohm with 2 % tolerance, according to the marking code IEC 60062.

The resistance value and tolerance are indicated with several colored bands around the component body. This marking technique of electronic components was already developed in the 1920s. Printing technology was still not far developed, what made printed numerical codes too difficult on small components. Nowadays, the color code is still used for most axial resistors up to one watt. In the figure above, an example is shown with four color bands. In this example the two first bands determine the significant digits of the resistance value, the third band is the multiplying factor and the fourth band gives the tolerance. Each color represents a different number and can be looked up in a resistor color code chart or using a resistor color code calculator.

Resistor color code calculator

The color code can easily be decoded using this calculator

Resistor Values (Preferred values)

In the 1950s, the increased production of resistors created the need for standardized resistance values. The range of resistance values is standardized with so called preferred values. The preferred values are defined in E-series. In an E-series, every value is a certain percentage higher than the previous. Various E-series exist for different tolerances.

SMD resistors

What Is A Resistor ?

For SMD (Surface Mount Device) resistors, a numerical code is used, because the components are too small for color coding. SMD resistors are, just as for leaded resistors, primarily available in the preferred values. The size of the component (length and width) is standardized as well, and is referred to as resistor package. An example of an SMD resistor on a PCB is given in the picture. 1206 means 0.125×0.060 inches.

Resistor Applications

There is a huge variation in fields of applications for resistors; from precision components in digital electronics to measurement devices for physical quantities. A few popular uses are described below.

Resistors in series and parallel

In electronic circuits, resistors are very often connected in series or in parallel. A circuit designer might, for example, combine several resistors with standard values (E-series) to reach a specific resistance value. For series connections, the current through each resistor is the same and the equivalent resistance is equal to the sum of the individual resistors. For parallel connections, the voltage across each resistor is the same. The inverse of the equivalent resistance is equal to the sum of the inverse values for all the parallel resistors. The articles resistors in parallel and resistors in series provide detailed introduction to these concepts and calculation examples. To solve even more complex networks, Kirchhoff’s circuit laws may be used.

Measure electrical current (shunt resistor)

Electrical current can be calculated by measuring the voltage drop over a precision resistor with a known resistance, which is connected in series with the circuit. The current is calculated by using Ohm’s law. This is a called an ammeter or shunt resistor. Usually this is a high precision manganin resistor with a low resistance value.

Resistors for LEDs

LED lights need a specific current to operate. A too low current will not light up the LED, while a too high current might burn out the device. Therefore, they are often connected in series with resistors to set the current. These are called ballast resistors and passively regulate the current in the circuit.

Blower motor resistor

In cars, the air ventilation system is actuated by a fan that is driven by the blower motor. A special resistor is used to control the fan speed. This is called the blower motor resistor. Different designs are used. One design is a series of different size wirewound resistors for each fan speed. Another design incorporates a fully integrated circuit on a printed circuit board.

Source : eepower

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