A resistor is a device which has a known value of resistance to oppose the flow of electric current through it.
A resistor is required to limit the current through a circuit. When current flows through the resistor, temperature rises as I2R losses occurred in the resistor. The maximum power in form of heat, which produces temperature within permissible limit of resistor, is known as power rating of resistor and the maximum voltage across the resistor to limit the power dissipation is known as voltage rating of resistor.

Mainly the resistors are two types, 1) Linear resistor and 2) Non linear resistor.
1) LINEAR RESISTOR: Those resistors, whose value of resistance do not change with the applied voltage or potential difference is known as linear resistor.
The V-I characteristics of such resistor is straight line, that means current is directly Proportional to the applied voltage. The two types of linear resistor are a) Fixed resistor b) Variable resistor.
a) FIXED RESISTOR: It consist of fixed value of resistance and these are

(i) CARBON COMPOSITION RESISTOR: The various proportion mixture of carbon or graphite particles and binding resin for desirable resistance is required to form this type of resistor. Metal caps with leads are connected to the two ends. A plastic cover with different color code painted to prevent the moisture and external damage enclosed outside the resistor. These cheap type resistors are available in the range of 1Ω to 25MΩ and power rating of 1/8 to 2 watt and voltage rating 250 to 500 volt. But these types of resistors have high temperature co-efficient and a noise.


Every carbon composition resistor consists of a color bands outside its cover. It indicates the resistance value as well as tolerance range. Each color has a specific value and there are four color bands printed outside the body of the resistor.
Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Grey=8, White=9.

In college we have learned a sentence to remember this color value as B.B.R.O.Y of Great Britain has Very Good Wife. Every capital letter indicates a color and their digit in increasing order. B= BLACK=0, B=BROWN=1, R=RED=2, O=ORANGE=3, Y=YELLOW=4, G=GREEN=5, B=BLUE=6, V=VIOLET=7, G=GREY=8, W=WHITE=9.
The first and second color band represents the two digit number. If first band is Red and second is Yellow, then the value is 24.
The third color band value is power of ten. If the third color is orange, then value will be 24 X 103 Ω.
The fourth band indicates the tolerance. Golden color tolerance band represent ±5% tolerance, silver color = ±10% tolerance, if provided no band, then ±20% tolerance.

(ii)THIN FILM RESISTOR: Basically thin film resistors are made of, by an insulating rod made by ceramic material and a thin film layer of resistive material outside the insulating rod.When a very thin carbon film overlaid around the insulating material rod in a resistor, it is known as carbon film resistor. In electronic circuit these types of resistor are widely used cause of wide operating range and low noise.
In case of metal film resistor, vaporized metal is used instead of carbon. Mixture of metal-oxide is used as resistive film outside the insulating rod. This type of resistor has excellent temperature co-efficient and highly reliable. But these are much more costly.

(iii) THICK FILM RESISTORS: In case of thick film resistors, a thick layer of resistive material instead of thin film is used around the insulating rod.
When a metal-glass mixture also known as metal-glaze, is applied as a thick film to a ceramic material to make a resistor, it is known as Metal-oxide or metal-glaze resistors. This type of resistors is used at high voltages and values from 1Ω to several MΩ.
Cermet or ceramic-metal resistors construction are little different. In this type outside of ceramic insulating material, an alloy of carbon or metal fixed with the ceramic metal. The form of resistors is rectangular or square shape. These types of resistors are highly stable and accurate resistance value.
In expensive electronic circuit like T.V a fusible resistor is used. When power rating increase from a specific value, it opens the circuit. Thus this type of resistor performs to limit current as well as fusing element in the circuit.

(iv) WIRE WOUND RESISTOR: This is the costlier resistor among fixed type and here no film or resistive coating is used. It consists of an insulating rod and a resistive wire wrapping around the rod. The resistive wires are made by different alloy to make different resistance. Common resistive wires are Tungsten, Manganin, Nickel e.t.c. These resistors have highest stability and highest power rating, mainly used in high power rating devices, testing and measuring devices.

b) VARIABLE RESISTOR: In this type linear resistor, the resistance values can be changed manually.
The types of variable resistors are (i) Potentiometer, (ii) Rheostats, (iii) Trimmers.

(i) POTENTIOMETER: Potentiometer is used to control the voltage level in the circuit. It consists of three terminals, two of them are constant and the middle terminal connected with the moving contact, for which movement, resistance values also vary. The range of resistance up to 10 MΩ.

(ii) RHEOSTATS: Rheostats are used for current limiting purposes. A nichrome resistive wire wrapped around a ceramic core. An adjustable sliding contact is used to vary the resistance value. This is also known as variable wire wound resistors ranges from 1Ω to 150Ω with power rating 5 to 50 watt.
Rheostats and potentiometers work are same and there is no basic difference between them, besides rheostats uses for controlling current whereas potentiometers use for controlling the voltage level.
(iii) TRIMMERS: When we use an additional adjustable screw with the rheostats or potentiometers for better efficiency are known as trimmers.

2) NON LINEAR RESISTOR: Non linear resistor are those resistor, where the flowing current through it is not exactly directly proportional to the applied voltage, that means this type of resistors does not obey the V-I characteristics.
The types of non linear resistors are (i) Thermisters, (ii) Varisters, (iii) Photo resistor or
(i) THERMISTERS: Thermister is a thermally sensitive resistor available in positive and negative temperature co efficient types. Mainly its resistance value changes with change in temperature. In positive temperature co-efficient type resistance value increases with increasing temperature. In negative temperature co-efficient type resistance value decreases with increasing temperature. It made from cobalt, Nickel, Strontium, and the metal oxide of manganese.
(ii) VARISTOR: Varistors are voltage dependent resistors (VDR), whose resistance decreases with increasing voltage. It is used to eliminate the high voltage transient from the circuit.
Metal-oxide materials are used to make varistors.
(iii) PHOTO RESISTOR: Photo resistor or Light dependent resistor (LDR) is a resistor, whose resistance value changes with light intensity. Cadmium sulfide, lead sulfide are used to make photo conductors. As it has negative temperature co-efficient, when light falls on the photo conductive cells, the resistance value decreases. Uses of LDRs are smock detector, light detector, photographic device etc.

(i) For current limiting.
(ii) To control voltage.
(iii) For protection purpose.
(iv) Heater, immersion rod.
(v) To control temperature.
(vi) Blocking of electric current in circuit.
(vii) Transform electric energy to heat energy.


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