RESISTORS

INTRODUCTION

A Resistor is a passive two terminal electrical component with the primary function to limit the flow of electric current, adjust signals level to divide voltage bias active elements. it is based on the working principle of electronic resistance. the resistors ability to produce the current is measured in unit of ohm. Resistors can also be used to provide a specific voltage for an active device such as transistor.

 In other words, the current through a resistor is inversely proportional to its resistance and directly poportional to the voltage across it. this is the well known ohm's law.

                                        

                                          V = IR

                                             or

                                           I = V/R

Resistors are used in virtually all electronic circuit and many electrical devices they are typically constructed of metal wire or carbon and engineer to maintain a stable resistance value over a wide range of enviormental condition.

Resistor schematic symbol and values:-

Two typical schematic symbols are as follows :-

1) The most common symbol for a resistor is 

2)  In keeping more with their physical appereance an alternative shematic symbol for the resistor looks like a small rectangular box .  In  USA rectangular box version is preffered over the zig zag version of resistor.

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RESISTORS IN SERIES AND PARALLEL                                                                                                                   

Simplest combination of resistors are the series and parallel connection . The total resistance of a combination of resistor on both their individual values and how they are connected .

SERIES COMBINATION OF RESISTOR                    

  Resistor are in series whenever a flow of charge called a current must flow so devices frequently. In a series circuit the output current of the first resistor flow into the input of the resistor and third resistor and so on. therefore , that is the same in each resistor. Resistors are set to be connected in series when they are changed together in a single line resulting in a common current flowing through them. then the amount of current flows through a set of resistors in series will be the same at all points in a series resistor network. for example, ir1 = ir2 = ir3 = iav = 1ma 

In the following example , the resistor r1, r2, r3 are all connected together in series between point a and b with the common current i flowing through them. 

The total resistance  Rt = R1 + R2+ R3

PARALLEL COMBINATION OF RESISTOR            

Resistors are in parallel when each resistor is connected directly to the voltage source by connecting wires having negligible resistance . In a parallel circuit all the resistor leads on one side of resistors are connected together and all the leads on the other side are connected together . Resistors are said to be connected together in parallel when both of thier terminals are respectively connected to each terminals of the resistors or resistors since there are multipple paths for the supply current to flow through, the current may not be the same through all the branches in a parallel network . The voltage drop accross all the resistors in a parallel resistive network is the same. Then, resistors in parallel have common voltage across them. for example, VR1 = VR2 = VR3 = Vab = 12V

In the following resistors in parallel circuit the resistor R1 , R2 and R3 are all connected together in parallel b/w the two points A and B as shows.

formula,  1/R1 = 1/R2 = 1/R3 = 1/Rn 

COMBINATION OF SERIES AND PARALLEL RESISTOR

Resistors can be connected together in an unlimited number of series and parallel combination to form complex resistive circuits.'

TYPES OF RESISTORS

There are various types of resistors but mainly two types of resistors:-

 FIXED RESISTORS

A fixed resistor can be used to limit the over all current flow b/w two discrete sub circuits. especially required dealing with the electronic components. Fixed resistors are by far the most widely used type of resistors. They are used in electronic circuits to set the right conditions in a circuit. There are values determine during the design phase of the circuit, and they should never need to be changed to adjust the circuit. They are maximum current ratings and tend to give the output. when these threshold are exceeded. There are many types fixed resistors which can be used in different circumstances. such as :-

1) wire bound resistor                                                                                                                                2) surface mount resistor 

3) carbon composition resistor

4) metal film resistor 

5) foil resistor0

VARIABLE RESISTOR

Variable resistor must have physical means of adjustment, either a rotating shaft or labelled that can be moved to vary the amount of electrical resistance. This gives a three connection to the component two connected to the fixed element and the third is the slider.

RESISTOR COLOUR CODE CHART