What is a Resistor? What are the Types of Resistors? What are the Applications of Resistor?

What is a Resistor?

A resistor is a passive two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohm's law. The electrical resistance is equal to the voltage drop across the resistor divided by the current through the resistor. 

Figure 1: A typical axial-lead resistor.

Figure 2: Two common schematic symbols of resistor.

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.

Theory of Operation:

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

             V = I/R

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), kilohm (1 kΩ = 10³ Ω), and megohm (1 MΩ = 10⁶ Ω) are also in common usage. 

Figure 3: A few types of resistors.

Types of Resistors:

1.     Linear resistors.

                                 i.         Fixed resistors

a)    Led arrangement

b)    Carbon composition

c)     Carbon Pile

d)    Carbon film

e)    Printed carbon resistor

f)      Thick and thin film

g)    Metal film

h)    Metal oxide film

i)      Wire wound

j)      Foil resistor

k)    Ammeter shunt

l)      Grid resistor

m) Special verities

                                    ii.         Variable resistor

a)       Adjustable resistor

b)       Potentiometers

c)        Resistance and decade boxes

d)       Special devices.

2.     Non-linear resistors.

Applications of Resistors:

• 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.  
• Current-limiting. By placing a resistor in series with another component, such as a light-emitting diode, the current through that component is reduced to a known safe value.  
• A series resistor can be used for speed regulation of DC motors, such as used on locomotives and train sets.  
• 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.  
• All resistors dissipate heat. This is the principle behind electric heaters.  

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