Variable Resistors

 

 Variable Resistors :-

• There are two general ways in which variable 

    resistors are used.

• One is the variable resistor whose value is easily changed.

• The other is semi-fixed resistor that is not meant to be adjusted 

  by anyone but a technician. It is used to adjust the operating 

  condition of the circuit by the technician.

• Semi-fixed resistors are used to compensate for the inaccuracies 

  of the Variable Resistorsresistors, and to fine-tune a circuit. The 

  rotation angle of the variable resistor is usually about 300 

  degrees. 

  Some variable resistors must be turned many times( multi-turn 

 Pot) to use the whole range of resistance they offer. This allows 

 for very precise adjustments of their value.These are called 

“Potentiometers” or “Trimmer Potentiometers” or “presets”.

  

 • The four resistors at the center are the semi-fixed type. The  two resistors on the left are the trimmer potentiometers

 • There are three ways in which a variable resistor’s value can 

     change according to the rotation angle of its axis. When type 

    “A” rotates clockwise, at first, the resistance value changes 

     slowly and then in the second half of its axis, it changes very 

     quickly. It is well suited to adjust a low sound subtly. They are 

     sometimes called “audio taper” potentiometers.

                                     

     Variable Resistors:-

                                   

         

  

                   

 • In type “B” the rotation of the axis and the change of the resistance value are directly related. The rate of change is the same, or linear, throughout the sweep of the axis. This type suits a resistance value adjustment in a circuit, a balance circuit and so on. They are sometimes called “linear taper” potentiometers.

• Type “C” changes exactly the opposite way to type “A”. In the  

early stages of the rotation of the axis, the resistance value 

changes rapidly, and in the second half, the change occurs more 

slowly. As for the variable resistor, most are type “A” or type “B”.

                                

Resistor Color Coding :-

           

• This is an international and universally accepted 

  method, developed many years ago as a simple and quick way of 

  identifying a resistors ohmic value no matter what its size or 

  condition.

• It consists of a set of individual colored rings or bands in spectral 

   order representing each digit of the resistors value.

• The resistor color code markings are always read one band at a 

   time starting from the left to the right, with the larger width 

   tolerance band oriented to the right side indicating its tolerance.

• By matching the color of the first band with its associated 

    number in the digit column of the color chart below the first 

    digit is identified and this represents the first digit of the 

    resistance value.

• Again, by matching the color of the second band with its 

   associated number in the digit column of the color chart we 

   get the second digit of the resistance value and so on.

 Standard Resistor Color Code Chart :-

 

 Resistor Color Code Table :-

 

Calculating Resistor Values :-

 • In order to calculate the correct value of resistance, the 

     following method is used in a color code system :

 • The left-hand or the most significant colored band is the band 

     which is nearest to a connecting lead with the color coded 

     bands being read from left-to-right as follows; 

 • Digit, Digit, Multiplier = Color, Color x 10 color  in Ohm’s (Ω’s)

Example 1 :-

   
Let us take a resistor which has the following colored markings;

• Resistance = Yellow, Violet, Black, Red = 4,  7 ,0,2 = 4 70 x 102 = 

4700oΩ or 47  k Ω

• The  fifth bands is used to determine the percentage tolerance of 

   the resistor.

• Hence, Tolerance (Brown)= ±1%

• Tolerance of the resistor is also an important property to 

  consider.A 100 Ω resistor with 10% tolerance, means that its 

  value can be any fixed value between 90 to 110 ohms.

• Hence the resistor tolerance can be defined as  a measure of 

   the resistors variation from the specified resistive value and is 

  a consequence of the manufacturing process and is expressed 

  as a percentage of its “nominal” or preferred value.

• If resistor has no fourth tolerance band then the default 

   tolerance would be at 20%.

Example 2 :-

    

 Resistance = Brown, Black, Orange = 1, 0 ,3 = 10 x 103 = 10kΩ

    Tolerance(Gold) = ±5%

Resistors in Series and Parallel 

Combinations :-

• In our previous post about resistors , we studied about 

different types of resistors.

• In some cases when we do not get the desires or specific 

resistor values we have to either use variable resistors such as 

potentiometers or presets to obtain such precise values. 

However,  such pots are too expensive to use for every case.

• Another method to do this,  is to combine two or more 

resistors to obtain the necessary precise values.Such resistor 

combinations cost very less.

• Now the question arises as to how one should combine these 

 resistors.

• Now the question arises as to how one should combine 

 these resistors.

• The resistors  can be combined in two different ways such 

  as:

• Series Combinations.

• Parallel combinations.


Resistors in Series :-

 • Resistors are said to be connected in “Series“, when they 

   are daisy chained together in a single line.

• Calculating values for two or more resistors in series is  

  simple, just add all the values up.

• The  series connection ensures that the SAME current flows  

  through all resistors.

• In this type of connection R Total  will always be GREATER 

  than any of the included resistors.

• The total resistance is the sum of all the resistors connected 

   in series and is given by the expression  :

• R Total = R1 + R2 + R3 +…………

Resistors in Series :-

• As the resistors are connected together in series,  the same 

current passes through each resistor in the chain and the  

total resistance, RTotal of the circuit must be equal to the 

sum of all the individual resistors added together. That is

• R Total = R1 + R2


•This total resistance is generally known as the Equivalent 

Resistance and can be defined as; “a single value of 

resistance that can replace any number of resistors in 

series 

without altering the values of the current or the voltage in 

the circuit“.

• The series connection can be characterized by the  

following points:

1.The same current flows through all the resistors 

     connected in series.

2.The resultant resistor is the SUM of all resistors in series.

3. Series resistors divide the total applied voltage 

    proportional to their magnitude.

Parallel Combination :-

•The fig. below shows the circuit of resistors in parallel 

combination where two resistors R1 and R2  are connected 

in parallel across the supply voltage E .

• As we can see from the fig. above :

• There are two paths available for Current. Hence current 

   divides.

• But voltage across the resistors are the same.

• If the two resistors are equal the current will divide 

  equally and the RTotal will be exactly half of either 

  resistor or exactly one third if there are three equal 

  resistors.