WHAT IS A RESISTOR?

         

RESISTOR

                                               

            Resistor is a passive component which opposes or limit the electric current flowing in a circuit.Resistor is a component which exhibits the property of resistance. It is just a kind of circuit element which implements a electrical resistance in a circuit. It basically works on principle of ohm's law.
     
           Before learning about a resistor, first we should apprehend about the term resistance.
Resistance is a property(or measure of) which oppose the electrical current. It is quantity inverse to electrical conductance. This property has been discovered by a scientist Georg Ohm in 1824. The SI units of resistance is "Ohms". In other words, resistance is defined as the ratio of voltage to the current.

                                                       

                                                            where, V= Voltage
                                                                         I=Current
                                                                         R=Resistance.
            resistance is more helpful in many circuits where we can apply any voltage to any element by just placing a resistor between those. The resistance value can be taken by the above formula. As illustrated. Consider a LED having a rated maximum current of 20mA and a supply as battery of 9V & 500mAh. If we directly connect the supply to that it burn out due to heavy current. hence we need to place a resistor to overcome. by using formula, R=450 ohms. now the light glown without burn out.
            generally, in circuits, the resistance can be represented by a symbol as shown,

resistance symbol by american standards.

resistance symbol by IEC(International Electrotechnical commission) standards

  

            Resistance typically can be understood by an example. A water is flowing in a pipe having a same diameter throughout it. If we reduce the diameter at some place the flow of water tends to drop.

Here we can say water as  a current in the circuit which can be a voltage drop due to the resistance in a circuit.

   TYPES OF RESISTORS 

        Depending upon usage and construction of the resistors they are classified into varies types.

• Fixed resistors.
• variable resistors
• depending on physical quantities.

1)Fixed resistors :-

          these are of fixed resistance value. where these have constant fixed values like 500ohms, 2.2kohms etc. This are made up of different types such as carbon film, metal film, wire wound, metal oxide film.

     a) carbon film:  

                 

                A carbon film is deposited on an insulating substrate, and a helix is cut in it to create a long, narrow resistive path. Varying shapes, coupled with the resistivity of amorphorus carbon (ranging from 500 to 800 μΩ m), can provide a wide range of resistance values. Carbon film resistors feature a power rating range of 0.125 W to 5 W at 70 °C. This resistors has a operating  temperature ranging between -55 deg to 155 deg centigrade.

     b)Metal film:

        

                A common type of axial-leaded resistor today is the metal-film resistor. Metal Electrode Leadless Face (MELF) resistors often use the same technology, and are also cylindrical shaped but are designed for surface mounting. Note that other types of resistors (e.g., carbon composition) are also available in MELF packages.Metal film resistors are usually coated with nickel chromium (NiCr), but might be coated with any of the cermet materials listed above for thin film resistors. Unlike thin film resistors, the material may be applied using different techniques than sputtering (though this is one of the techniques).This resistors has a operating  temperature ranges about 150 deg centigrade.

     c)Wire wound type:

 

               Wire wound resistors are commonly made by winding a metal wire, usually nichrome, around a ceramic, plastic, or fibre glass core. The ends of the wire are soldered or welded to two caps or rings, attached to the ends of the core. The assembly is protected with a layer of paint, moulded plastic, or an enamel coating baked at high temperature. These resistors are designed to withstand unusually high temperatures of up to 450 °C.

2)variable resistors:

       these resistors can be changed to any resistances to whatever we required.

    a)potentiometer:

       

            A potentiometer or pot is a three-terminal resistor with a continuously adjustable tapping point controlled by rotation of a shaft or knob or by a linear slider. It is called a potentiometer because it can be connected as an adjustable voltage divider to provide a variable potential at the terminal connected to the tapping point. A volume control for an audio device is a common use of a potentiometer. The method was described by Johann Christian Poggendorff  around 1841. these are used to measure the voltages. For low-power applications (less than about 1 watt) a three-terminal potentiometer is often used, with one terminal unconnected or connected to the wiper.

     b)Rheostat:

             The most common way to vary the resistance in a circuit is to use a rheostat. The word rheostat was coined about 1845 by Sir Charles Wheatstone. The term "rheostat" is becoming obsolete,  with the general term "potentiometer" replacing it. For low-power applications (less than about 1 watt) a three-terminal potentiometer is often used, with one terminal unconnected or connected to the wiper. These have the resistance values from 1ohm to several kiloohms.

   3)Dependent type resistors:     

         a)thermistors:

            A thermistor is a type of resistor whose resistance is dependent on temperature. Thermistors are widely used as inrush current limiter, temperature sensors,((PTC)positive temperature coefficient) self-resetting overcurrent protectors, and self-regulating heating elements(NTC(negative temperature co-efficient)). 
     the operating temperature is , typically −90 °C to 130 °C.   

• With NTC, resistance Decreases as temperature rises to protect against inrush over voltage conditions. Commonly installed series in a circuit.
• With PTC, resistance Increases as temperature rises to protect against over current conditions. Commonly installed series in a circuit.

     b)Sensistors:   
             Sensistors are used in electronic circuits for compensation of temperature influence or as sensors of temperature for other circuits.
             Sensistor is a resistor whose resistance changes with temperature.The resistance increases exponentially with temperature,  Sensistors are used in electronic circuits for compensation of temperature influence or as sensors of temperature for other circuits.
     Sensistors are made by using very heavily doped semiconductors so that their operation is similar to PTC-type

     COLOUR CODING.
         You might have wondering what are the lines across the resistor figures. Those are the colour bands which helps us to find out the resistance values.  
         this figure gives an idea about how to find out the value of the resistance. 

                                      This is the schmatic diagram of a resistor.

  in this, 1st and 2nd band says certain value, multiplier says the multiples of 10 and tolerance say the range of that value of resistance.

                            This figure gives an exact idea of the colour coding.

     note: some resistors contains of five bands wherein first 3 bands wil be the value followed by multiplier and tolerance.
   

           Resistor is a component which has a huge role in electrical and electronics, without this no device can be manufactured. Resistor opposes current and also gives major contribution in circuit like voltage divider circuit, as an attenator, feedback network etc.




               This is the information gathered from different references books and internet. If you have any feedback or any data was wrong please don't hesitate to write a comment below..