Theory- 19 :- Variable resistors, pots, presets, types and application. Log and Linear resistors.

Variable resistors, pots, presets, types and application. Log and Linear resistors.

 

VARIABLE RESISTORS In fixed value resistor the value of resistance is fixed, at the time of manufacture, whereas, variable resistors are manufactured such that its resistance value can be changed by the user. A typical variable resistor, commonly known as potentiometer is shown in fig 1.

For example, in a variable resistor of 10 KΩ, It is possible to set the resistance of the variable resistor, to any value between 0 Ω TO 10 KΩ.

Any variable resistor will have a minimum of three terminals. Referring to Fig 1, the maximum resistance exits across the end terminals X & Z. The resistance across terminals X & Y or across Z & Y can be varied by rotating the shaft.  

Different symbols used to represent variable resistors are shown fig 2.

  Variable resistors are also known as Potentiometers or Pots. The term potentiometer is used for variable resistors because a variable resistor cann be used to adjust the potential difference [voltage] applied to a device or a circuit as shown in Fig 3.

Resistance values of Potentiometers are printed on their body either directly in ohms or in a coded form. The value printed always refers to the maximum value that  Potentiometer can have. Hence the minimum value to which it can be set is always 0 Ω..   TYPES OF VARIABLE RESISTORS: Variable resistor different types are available in market. These types can be categorized as follows: 

Based on the types of control as shown in fig 4.  

z     In rotary types of variable resistors, the resistance across the terminals x-y increases as the shaft is rotated anti clock –wise whereas in sliding type, resistance across x-y increases as the slider is moved from A to B

2.Based ON  the resistive material used as shown in fig 5.

      Fig 5 illustrates the brief constructional detail of carbon type and wire wound type variable resistors. In carbon type, the resistive material used is carbon or graphite. Wire wound type variable resistors uses several turns of resistive turns of resistive wire such as tungsten or manganin.