All about Resistors - Symbol, Types, Colour Code and Applications

Definition of Resistor

Resistor is a passive element which is used in electrical and electronics circuit & equipment for controlling voltage and current according to the requirements. It is also called a power dissipative element because it dissipate power in the form of heat or I2R loss. In electrical Locomotives resistors are mainly used to bypass earth fault current and controlling over current.

Figure - Practical Resistor

Types of  Resistors 

Resistors can be classified in terms of composition

1. Carbon Film resistor
2. Metal Film Resistor
3. Wire wound resistor

These names are given according to the material used in resistor film. Carbon film resistors are used for small circuits and equipments from quarter watts to few watts. Metal film resistors are used for medium watt and wire wound for medium to high wattage.

Colour Coding of Resistor

Value of resistance and watt are mentioned on the resistor one can select the value according to voltage and current requirements.

Some resistors like Carbon resistors have colour coding on it, to know the value of resistor following method is used:-

It has four to five layers first & second layer tells about digit and third layer value written as 10 to the power and the fourth one tells about the tolerance means accuracy of resistor i.e. how much percent the value of resistance offered can deviate. In case of five layers starting three layers tells about digit 4th about 10 to the power and 5th about tolerance.

Lets take a example to understand it more clearly.

I use the trick BB ROY of Great Britain has Very Good Wife to remember the colour code.

1. In five layer resistor with colour code  brown black black brown red we will write as 100×10^1=1K ohm , +/- 2 % tolerance.  Three starting layer colour code written as it is, forth layer used for multiplier and last one tells about tolerance.
2. Colour code and resistor tolerance is given in table.
3. In four layer resistor we will write as 10×10^2=1K ohm , +/- 5% tolerance

Few resistors like wire wound have rating and value written on it while SMD type resistors have coding on it. To know the value of resistor write the value in same manner as followed in colour coding and you will get the value of resistor.

For example let the code is 120 then value of resistor will be 12×10^0=12 ohm , if there are four digit code on smd resistor then write first three digit as it is and use fourth digit as multiplier. If code uses R , K or M then it indicates ohm, kilo ohm and Mega ohm value of resistor and replace the alphabet by dot as written below

Applications of Resistor in electric circuits.

Resistors are available in limited value so to get the desired value of resistance we use parallel, series, star and delta combination of resistances.

1. Parallel Combination:- parallel combination is used to produce the value of two or more resistances. The generated value will always be less than the smallest one. Practically voltage remains same and current get reduced, we use this combination to speed up DC series motor in Trains, cranes, current divider circuits etc.
2. Series Combination:- This combination is used increase the equivalent resistance value. The generated value will always be greater than the largest one. The current remains same but voltage get decreased, we use this combination in motor starters, voltage divider circuits, AC induction motors etc.
3. Star-delta Combination:- In star delta combination resistance value can be reduced or increased three times if we use all three resistances with same value. This combination is used in industries to start 3 phase induction motor which limits the current to a safe value.

Fig-1 shows a simple resistors network in which three resistors are connected in parallel. Voltage across each parallel connected resistors remains the same whereas current get reduced. 

The equivalent resistance of network will be 

1/RAB =1/R1 + 1/R2 + 1/R3

RAB = 60/13 Ohm

Which is less than smallest value resistance 10 ohm. So this is why we reduced the resistance by using parallel combination of three resistors.

Fig-2 shows three combination connected in series which are being to used the equivalent resistance of circuit. Current in all series connected resistors remain the same whereas voltage get reduced. The equivalent resistance of the circuit will be

RAB = R1 + R2 + R3

RAB = 45 Ohm

Fig-3 shows a simple voltage divider circuit which is used to get desired voltage at required place. We know that current remains same when resistors connected in series but voltage varies lets check voltage across both 30 ohm and 70 ohm resistors.

V30 = V×R1 / (R1 + R2)

V30 = 30 Volt

Similarly,

 V70 = V×R2 / (R1 + R2)

V70 = 70 Volt

Fig-4 shows simple current divider circuit which is used to get desired current at required point. We know that in parallel combination voltage remains same but current get reduced lets check current through both 3 ohm and 7 ohm resistors.

IR1= I×R2 / (R1 + R2)

IR1 = 7 Ampere

IR2= I × R1 / (R1 + R2)

IR2 =3 Ampere

Fig-5 shows three resistors connected in star ,the equivalent resistance from any of two end can be find by following method.

Lets find out equivalent resistance at terminal A and B

RAB = R1 + R2 +R1×R2/R3

RAB = 27 ohm

It shows that if all three resistances are of same value then in star Combination resistance at each terminal get reduced thee times. These terminal resistances are also called delta resistances.

Fig-6 shows three resistors connected in delta ,the equivalent resistance from any of two end can be find by following method.

Lets point O is the centre point of delta circuit. find out equivalent resistance at terminal O and A

ROA= (R1 × R2 ) / ( R1 + R2 + R3 )

RAB = 3 ohm

From above fig-5 and fig-6 it is clear that in star -delta conversion equivalent resistance get tripled and in delta to star conversion it becomes one third of previous value if we use all resistance of same value.

Practical uses of Resistors

All three type of resistors are used for different purposes like

1. Carbon and metal film resistors are used in electronic cards of different electronic equipment like battery charger, Bluetooth speaker, TV,LED bulbs and mobiles etc.

Values of these resistors are determined by colour coding and wattage are determined by size quarter watt, half watt ,one watt and 2 watt carbon resistors with colour coding are available in market. Basically carbon resistors are used for light loads to limit voltage and current. Lets take a very basic practical example of indicator LED which is used in our 5 Ampere switch board which tells about the presence of current. So to find out the value of resistor which will be placed in series to LED to limit the current to a safe value or operating value of LED, normally LED operates 2 to 3 Volts and 10 to 20 mA. Let LED operating voltage is 3v and current is 10 mA then

Value of resistor = (Source voltage - Voltage drop across LED OR operating Voltage of LED) / LED operating current)

R = (230 V - 3V) / 10 mA

R = 22.7 K ohm

2. Wire Wound Resistors are used for heavy loads like in converters and inverters to limit higher values of current required to drive AC or DC motors for specific applications through IGBT and MOSFET. Practically You can see it in your home inverter and electrical ladders, lifts and trains etc. They can withstand high value of current Because nichrome or other high melting point wires are used in these type of resistors with insulation.

3. Variable Resistor are used where variation in resistance is the basic need to control some mechanism, practical example of this is a fan regulator in which when you reduce resistance value speed of fan increases as current through it get increases and vice versa.

Points you must know about 

1. Resistance of any material depends on type of material, length of material , area of material and atmospheric conditions. 
2. R = ρ ×l/A  formula shows that resistance of any material is directly proportional to type of material i.e. resistivity and length of material where as inversely proportional to area of material.
3. It indicates that we can increase resistance by increasing length and decreasing area where as we have to decrease length and increase area to reduce the resistance.
4. Silver, copper and aluminium are best conductors with least resistance.
5. Mica, Bakelite and porcelain (Silicon+Felspar+Clay) are best insulators with highest resistance.
6. Nichrome (Nickel+chromium), Eureka (Nickel+Copper) or Constantan, Kanthal (Iron+Chromium +Aluminium) and silicon carbide are used where working temperature is very high.
   

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