Variable resistors

Variables resistors

Variable resistors are those resistors there value of resistance is adjustable. Examples are given below

Potentiometer

1. Potentiometers 

Potentiometer is a resistor with three terminal, it consist of a rotating or sliding contact. Resistance value can be increase or decrease by moving the rotary or sling adjustment. Potentiometers are commonly used for controlling volume, bass. Trouble,brightness contrast in audio and video equipmentsets.Preset is a miniature potentiometer, which are used to regulate signals Or voltages in various electronic circuits. 

Definition of potentiometer is,  an instrument used to measure electromotive force by altering voltage. 

2.Rheostat

Rheostat
Image credit goes to flickr.com

Rheostat is used to control current. Two terminal used for it's operation and third one for sliding. Rheo stat can handle higher current and voltages. So it can be used for controlling motors, light etc A rheostat cannot be used as potentiometer but a high wattage potentiometer can be used as rheostat. Definition of rheostat is, an instrument used for controlling electrical current by altering resistance

3.Thermistors

Thermistor

                              Image credit  Wikimedia

Thermistors show resistance variations when Temparature varies. There are two types of thermistors, the NTC (negative temparature coefficient ) and PTC (positive temparature coefficient) . In the case of  ntc thermistors the value of resistance decrease when temparature

Increase.but the resistance value increase in ptc thermistors when temparature increase. 

4.Photo resistors

Image credit goes to Wikimedia commons

When light fall on photo resistors the conductivity also increases this type of resistors are knon as LDR ( Light dependable  resistors) 

There are three type of photo resistors, visible light sensitive type, ultraviolet light sensitive type and infra red sensitive  type. They are made by using some chemicals, such as lead sulphide. 

Cadmium sulphide, indium antimonide, cadmium selenide, lead selenide etc

5.Digital variable resistors. 

Digital variable resistors can be adjusted by electric signals, you cannot control it mechanically. It is available in the form of integrated circuit ( ic) . These resistors are also called as digi pots. In future digi pot may completely replace the normal mechanical potentiometers. 

 In a digital circuit by the use of digi pot a processor and a software can set voltage divider ratio, or vary the resistance value. 

Digital potentiometer a test circuit

   Image credit ; Wikimedia commons

The small 8 pin ic is working as potentiometer which is connected to a micro controller. 

6. Varistors

Varistors are voltage depend resistors (VDR) generally used for surge protection

Varistor

                     Image credit; flikr.Com

Varistor consist of two terminals, When voltage increases the resistance between the terminals decreases because of this property it can suppress high voltage surges. Now a days MOVs

(Metal oxide varistors) are commonly used. When the voltage across the terminals are very high it will short, this protect the other components from being damaged. 

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