Important facts of Resistor:




A resistor is a passive component. It is used to oppose the flow of current. The behaviour of resistor is specified by Ohm’s law. Its unit is Ohm (Ω).

Ohm’s law:

V= I*R.


Fig 1 shows the symbol of resistor.

Various types of resistors:

  1. Fixed

  2. Variable

  3. Dependent on physical quantity

Variable resistor:

  • In this the resistance can be varied from low to high value.

  • They are classified as trimmers, rheostat, and potentiometer.

  • They are used to control brightness or volume in TV and radios.

  • This is three terminal components with one fixed terminal at each end and third is movable which is move from low to high value.

 Fig 2 shows symbol of variable resister.

Types of variable resistor:

  1. Potentiometer:

These are available in carbon or wire wound type.

They are used to control voltage and current in circuit.

It may be linear or non-linear.


Fig 3 shows the potentiometer, from fig we can see that there are three terminal namely input, output, and ground.

  1. Trimmer or Preset:

Trimmers are used in electronic equipment to make minor adjustment.

They are small in size.


Fig 4 shows preset.

  1. Rheostats:

It is high wattage variable resister.

It is used to limit the flow of current.

And also used for voltage dividing.


Fig 4 shows actual rheostats

Depend on physical quantity resisters (Varistors):

The non ohmic variable resisters are called as varistors.

The resistivity of material is depending on the change in light, voltage and temperature.

Types of varistors:

    1. LDR(Light Dependant Resister):

LDR is a photo resister.

The light sensitive-photo conductive material is used for the construction like cadmium sulfide, and lead sulfide.

The resistance is increases when LDR is in dark.

These resisters are used in photo electric relay circuit.


Fig 5 shows LDR.

    1. VDR(Voltage dependent resister):

It is made up of silicon carbide.

The resistance varies with respect to applied electric strength. If voltage increases resistance increases if voltage decreases resistance decreases.

They are used in TV, stabiliser, frequency multiplier etc.

Fig 6 shows VDR.

    1. Thermistors:

They are temperature dependent resister. The resistance varies with resistance.

 Fig 7 shows thermistor.

There are two different types:

      1. Negative temperature coefficient

      2. Positive temperature coefficient

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