Resistors and Types of resistors

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In this article, we are going deep into resistors and different types of resistors. A resistor is one component that we use in every electronic circuit.

Whats is resistance?

“Resistance” by definition is the opposition to free flow or motion of any substance. In our case, the exact term is “electrical resistance” – which we commonly refer to as “resistance”. The electrical resistance of a material is the opposition to free flow of electrons inside that material. Electrical resistance vary with different materials and it is measured in units of Ohms.

So that’s some refreshment on your school physics. If you need further reference, go to Electrical Resistance and Conductivity. Now lets get into our topic of interest resistors.

What is a Resistor ?

A resistor is a passive electronic component that we use in electronic circuits either to limit current through an active component or to reduce voltage in the circuit (by dropping voltage across the resistor). In other words, a resistor installs electrical resistance into an electronic circuit. Resistors are 2 terminal passive components. They are basically of 2 types – 1) Fixed Resistor and 2) Variable Resistors

Note:- Current through a resistor is directly proportional to the voltage applied across the resistor. This is derived from Ohms Law and is represented by equation I = V/R – where I=current through resistor (in amperes), V = voltage across resistor (in volts) and R = resistance of the resistor (in ohms)

The photographic image of a resistor is given below. It is a 4 band – axial lead resistor.

Resistance - 4 Band Axial Lead

Photograph of a Resistor (Carbon Film)

Resistor Symbol and Notation

Symbols used to represent resistors vary with different standards. Different countries have set different standards for themselves. However the most popular and commonly used are American Standards. Another commonly used standard is IEC (International Electrotechnical Commission). We have added a graphic of resistor symbols used in both standards. In addition, you can see the popular notations used to represent different resistor values; say 1 Ohm, 1 Kilo Ohm etc.

Resistance - Standard Symbol and Notation of unit Ohm

Resistor - Symbol Standards

Power Dissipation in Resistors

Resistor are used in circuits made for different applications. A basic classification is low power application and high power applications.  When current flows through a resistor, energy will be dissipated in the form of heat. This heat energy dissipation is measured in terms of power. Every resistor available in market comes with a power rating, say 0.5 watts, 1 watts, 10 watts etc. If the heat dissipated in the resistor exceeds this maximum power rating, the resistor will get damaged. Based on the maximum power rating given in the resistor, it can be classified as low power resistor and high power resistor. Low power resistor have maximum power rating less than 5 watts where as high power resistors are those with maximum power rating higher than 5 watts.  Low power resistors are usually cylindrical in shape,with its connecting leads (the two terminals to connect the resistor to a circuit) coming out axially from the two end points of the resistor. High power resistor always comes with a heat sink to withstand the high amount of heat energy produced in these resistors. Connecting terminals of high power resistors comes out through the heat sink in most cases.

Power Dissipation Calculation in Resistors

Power dissipated in a resistor can be calculated by knowing either voltage across the resistor or current through the resistor. We can use the following equations to calculate power dissipated in a resistor:-

 1.    P = V2/R

2.     P = I2R

Where P = power dissipated, V = voltage across resistor in volts, I = current through resistor in amperes and R = Resistance value in ohms.

A 3rd equation -> P = V.I can also be used to calculate power dissipated across a resistor.

How to choose power rating of a resistor ?

A circuit designer must pre calculate the maximum expected power that might dissipate across the resistor. By pre calculating this power, the designer can choose the right power rating for the resistor used in the circuit.  If the resistance to be used in the circuit is 330 Ohms, then the maximum power that possibly could dissipate in this resistor can be calculated by knowing the supply voltage. If the supply voltage is 12 volts, then maximum possible power is calculated using   V2/R  - which [ (12 * 12 )/330 ]= 0.435 watts. Now the designer can choose a 330 Ohms resistor with 0.5 watts power rating.

 

Classification of Resistors:

From operating conditions point of view, resistors can be classified into two.

1) Fixed resistors

fixed-resistors

2) Adjustable/ variable resistors

1) Fixed resistors are further classified into:

a) Carbon composition type resistors b) Metalized type resistors c) Wire wound type resistors

a) Carbon composition type resistors:

This is the most common type of low wattage resistor. The resistive material is of carbon-clay composition and the leads are made of tinned copper. These resistors are cheap and reliable and stability is high.

b)Wire wound resistors:

These resistors are a length of wire wound an insulating cylindrical core. Usually wires of material such as constantan (60% copper and 40% nickel) and manganin which have high resistivities and low temperature coefficients are employed. The completed wire wound resistor is coated with an insulating material such as baked enamel.

c) Metalized resistors

It is constructed using film deposition techniques of depositing a thick film of resistive material onto an insulating substrate.Only approximate values of resistance can be had by this method.

2) Variable resistors

For circuits requiring a resistance that can be adjusted while it remains connected in the circuit(for eg: volume control on radio), variable resistors are required. They usually have 3 lead two fixed and one movable.

To know about the color coding of resistors and more basics click here:- Working of Resistors

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