Resistance is the property of a conductor which oppose the flow of current .
There are many types of resistor described below
Wire wound resistors are manufactured by using resistance wire (nicrome) wrapped around the insulating core , such as ceramic, porcelain , bakelite pressed paper.
- it is used for high current application.
- wattage range from 1 to 100 watt or more .
- the resistance can be less then 1 ohm & go up to few thousand ohm .
These are made of fine carbon or graphite mixed with power insulating material as a binder in the proportion needed for the desired resistance value ,
- carbon resistor are available in value of 1 ohm to 22 mega ohm .
- power rating 0.1 to 2 watt.
The film resistor are processed by depositing a metal vapour on a ceramic base .
- 1 ohm to 10 Mega ohm
- upto 1 watt power rating
- metal film resistor can work from 120⁰ C to 175⁰ C.
-These resistors have the highest value of temperature coefficient.
Carbon film resistor:-
- Carbon film resistor are available from 1 ohm to 10 mega ohm
- power rating 1 watt
- temprature range 85⁰ to 155⁰ C
Resistance depends upon temprature, voltage, light.
PTC resistors (sensistor)
- Positive temprature coefficient resistor.
- in PTC resistors as the temperature increase the resistance value increases non linearly.
NTC resistor (thermistor)
- negative temperature coefficient resistor.
- in case of NTC resistor as the temperature increase the value of resistance decreases non linearly.
- the PTC and NTC resistor can perform switching operation at specific temprature they are used for measurement & temp. Compensation.
VDR (Varistors) :-
- voltage dependent resistor
- it is used in voltage stabilisation , arc quenching & over voltage protection .
- if voltage is increased its resistance decreases .
LDR :-
- Light dependent resistor .
- it is also known as photo conductor.
- LDR shall have exposed surface to sence the light.
- these are used for light barriers in operating relays.
- these are also used for measuring the intensity of light.
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Range of resistance :-
LOW RESISTANCE - one ohm to below one ohm
Ex - armature resistance , shunt of ammeter , cable length .
MEDIUM RESISTANCE- >1 to 100000 (100kohm)
Ex- bulb , heater , relay , motor stator .
HIGH RESISTANCE - Above 100 k ohm
Ex - insulation resistance , Carbon composition resistance .
Method of measuring low resistance ;-
• voltmeter & ammeter method
• comparison of unknown with standard using potentiometer
• shunt type ohm meter.
Medium resistance :-
• series type ohm meter
• voltmeter & ammeter method
• substitution method
High resistance :-
• meggar or mego ohm meter
- there are two types of ohm meter
- series type ohmmeter is used for measuring medium resistance
- shunt type ohmmeter is used for measuring low & medium resistance.
** The accuracy of the ohmmeter depends greatly upon the condition of the battery .
** The meter scale will be marked zero ohm at the right end & infinity ohm at the left end.
** The ohmmeter has a non linear scale because of the inverse relationship between resistance & current .
** The wheatstone bridge is used for measurement in the range about 1 ohm to 1 mega ohm .
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