Resistor & Resistance

Resistance:

    Resistance is the property of material which opposes the flow of electric current.

Resistors :

    A resistor is an electrical component that limits the flow of electric current. or The resistance is the property of opposition to the flow of the current offered by the circuit elements.There are two type of resistor (i) fixed resistor (ii) variable resistor

Fixed resistors : 

    The fixed resistors is one in which the is nominal value of resistance is fixed. These resistors are provided with pair of leads.

    There are different types of fixed resistors which are given below.

Wire-wound resistor: Wire-wound resistors are manufactured by using resistance wire (nickel-chrome alloy called Nichrome) wrapped around an insulating core, such as ceramic porcelain, bakelite pressed paper.They are available in wattage ratings from one watt to 100 watts or more. The resistance can be less than 1 ohm and go up to few thousand ohms.Wire wound resistors are used for high current application.

Carbon composition resistors: These are made of fine carbon or graphite mixed with powdered.Carbon resistor are available in values of 1 ohm to 22 megohms and of different power ratings, generally 0.1, 0.125, 0.25, 0.5, 1.0 and 2 watts.

Metal film resistors: Metal film resistors are axial resistors with a thin metal film as resistive element. The thin film is deposited on usually a ceramic body.Metal film resistors are available from 1 ohm to 10 MΩ, upto 1W. Metal film resistors can work from 120°C to 175°C

Carbon film resistors:In this type, a thin layer of carbon film is deposited on the ceramic base/tube. Carbon film resistors are available from 1 ohm to 10 meg ohm and up to 1 W and can work from 85°C to 155°C.

Variable resistors:

    Variable resistors are those whose resistance values can be changed.

    There are different types of variable resistors which are given below.

Potentio meter: It is provided with 3 terminals and They are available with carbon tracks and wire wound  types. Trimmer potentio meters (or) resistor which can be adjusted with the help of a small screw drivers.

PTC resistors (Sensistors): In PTC resistor (positive Temp. coefficient resistor), as the temp increases, the resistance increases non linearly.

Ex-The resistance of PTC at room temperature may be of nominal value 100 Ω when the temperature rises say 10°C, it may increase to 150 Ω .

NTC Resistors (Thermistors) : In case of NTC resistors (Negative temperature co-efficient resistors) as the temperature increases, the value of resistance decreases nonlinearly.

Ex- NTC resistor, which has nominal value of resistance is 500 Ω at room temperature may decrease to 400 Ω with the rise of 10°C temperature.

VDR (Varistors) : The VDR (Voltage dependent resistor) resistance falls non-linearly with increasing voltage. For example, a VDR, may have 100 Ω resistance at 10 V, and it may decrease to 90 Ω at rise in 5V.The VDRS are used in voltage stabilisation, arc quenching and over voltage protection.

Light dependent resistor (LDR) :The LDRs are also known as photo- conductors. In LDRs the resistance falls with increase in intensity of illumination. These are also used for measuring the intensity of light.

Marking codes for resistors: Resistor Colour Coding uses coloured bands to easily identify a resistors resistive value and its percentage tolerance.

    The first two colour bands indicate the first two digits in the numeric value of resistance. The third colour band indicates the multiplier. The first two digits are multiplied by the multiplier to obtain the actual resistance value. The forth colour band indicates the tolerance in percentage.