The resistance R of a given material is proportional to its length L and inversely proportional to its area of cross section A. Thus R varies as L/A and = P(L/A), where p is the constant of the material known as its specific resistance.

The specific resistance are expressed in OHMS.

**Types of resistors**

Fixed Resistors

Carbon composition (In the range upto 100M OHMS, and power ratings 1/8 to 2W)

Carbon film (R value range from 10 ohms to 10 M ohms with 2 Watts)

Metal film (Range from 10 ohms to 1 M ohms with power rating upto 5 Watts)

Wire wound. (Two types 1. Power style and 2. precision style)

**Variable Resistors**

Potentiometer (pot)

This is a variable resistor either of carbon or wire wound type. It smaller in size refereed as “pot”

Wire wound pot: There are two types: Single turn, and (b) Multi Turn Range is 50 ohms to 5 ohms and in power ratings of 2 to 5W.

**Multi turn pot:-**

Multi turn or helical pots are used in applications the requires precise setting of a resistance value. The range is 50 ohms to 250k ohms and power ratings upto 5W.

**Rheostat:-**

A wire wound pot that can dissipate more than 5 watts is referred to as a Rheostat.

The resistance wire wound on and open tube of ceramic which is covered with enamel, except for the track of the movable contact. Operating temp: 300c

**TYPE OF RESISTOR.**

Variable resistor | Variable resistor has an adjustable resistance (2 terminals) |

Potentiometer | Potentiometer has an adjustable resistance (3 terminals) |

Photo-resistor | Reduces resistance when exposed to light |

Power resistor | Power resistor is used for high power circuits and has large dimensions. |

Surface mount(SMT/SMD) | This type of resistors have small dimensions. The resistors are surface mounted on the printed circuit board (PCB), this method is fast and requires small board area. |

Resistor network | Resistor network is a chip that contains several resistors with similar or different values. |

Carbon resistor | General types |

Trimmer or Trim pot.

A trimming potentiometer is used where the resistance must be adjustable but not continuously variable. The rage is Few ohms to 5m ohms and power rating is 1W.

**TOLERANCE**

The term tolerance denotes the acceptable deviation in the resistance value of a resistors, while precision resistors have a tolerance closed to 0.1%.

**What is resistor?**

Resistor is an electrical component that reduces the electrical current. The resistor’s ability to reduce current is called resistance and is measured in units of Ohms [Ω]. If we make an analogy to water flow in pipes, the resistor is a thin pipe that reduces the water flow.

Ohm’s law The resistor’s current (I) is equal to the resistor’s voltage (V) divided by the resistance (R):

The resistor’s power consumption (P) in watts [W] is equal to the resistor’s current (I) in amps [A] times the resistor’s voltage (V) in volts [V]: *P* = *I*·*V *The resistor’s power consumption (P) in watts [W] is equal to the square value of the resistor’s current (I) in amps [A] times the resistor’s resistance (R) in ohms [Ω]: *P* = *I*^{ 2}·*R *The resistor’s power consumption (P) in watts [W] is equal to the square value of the resistor’s voltage (V) in volts [V] divided by the resistor’s resistance (R) in ohms [Ω]: *P* = *V*^{ 2}/ *R *Resistors in parallel

*R*is given by:

_{Total }So when you add resistors in parallel, the total resistance decreases.

Resistors in series

The total equivalent resistance of resistors in series *R _{Total }*is the sum of the resistance values:

*R*=

_{Total}*R*

_{1}+

*R*

_{2}+

*R*

_{3}+… So when you add resistors in parallel, the total resistance increases.

Check your resistor by digital multi-meter. The actual resistor value is 1 k ohms. Reading shows .989 ohms

Resistor Colour code

How can the value of a resistor be worked out from the colours of the bands? Each colour represents a number according to the following scheme:

The first band on a resistor is interpreted as the FIRST DIGIT (Yellow) of the resistor value. For the resistor shown below, the first band is yellow, so the first digit is 4:

The second band gives the SECOND DIGIT. This is a violet band, making the second digit 7. The third band Red is called the MULTIPLIER and is not interpreted in quite the same way. The multiplier tells you how many zeros you should write after the digits you already have. A red band tells you to add 2 zeros . The value of this resistor is therefore 4 7 0 0 ohms, that is, 4 700 OHMS , or 4.7 K OHMS. Work through this example again to confirm that you understand how to apply the colour code given by the first three bands.

The remaining band is called the TOLERANCE band. This indicates the percentage accuracy of the resistor value. Most carbon film resistors have a gold-coloured tolerance band, indicating that the actual resistance value is with + or – 5% of the nominal value. Other tolerance colours are:

4 bands: digit, digit , multiplier, tolerance.When you want to read off a resistor value, look for the tolerance band, usually gold, and hold the resistor with the tolerance band at its right hand end. Reading resistor values quickly and accurately isn’t difficult, but it does take practice!

More Band Resistor color code

The resistance of the resistor and its tolerance are marked on the resistor with color code bands that denotes the resistance value. There are 3 types of color codes:

- 5 bands: digit, digit, digit , multiplier, tolerance.
- 6 bands: digit, digit, digit , multiplier, tolerance, temperature coefficient.

*maximum*rate of 0.5 W. In the example above, the calculated rate of heat loss was 0.6 W, so that a resistor with a higher power rating, 1 W or 2 W, would be needed. Some resistors are designed to pass very large currents and are cased in aluminium with fins to increase surface area and promote heat loss.