Resistor

What is a resistor?

A resistor is a two terminal passive component which is used to limit the current flow through an electric circuit. In other words it apposes the flow of free electrons in a net. This property of the resistor is called resistance.

The SI unit of resistance is ohm. To refer that we use the Greek letter Ω.

Symbol for Fixed resistor:

Symbol for Variable resistor:

I'm telling pro's and cons before getting the formula and types of resistor because if you understand why do we use this, and it will give you the general idea about resistor.

Why do we use resistor?

• In most of the circuits, resistor is used for voltage dividing, current limiting and signal adjusting purpose.
• And If do not limit the current flow through the circuit, then the excess current flow can cause serious damages to our equipment. Meanwhile if the current is not that much excessive then it may survive.

Advantage of using a resistor:

1. It is the cheapest way to limit the current and dividing voltage. If you have 9V battery and you only want 5V out of it, You can use voltage divider using a resistor at the least cost. (if you know the trick)
2. It can enhance the voltmeter range. And also it is used in DSO probes (10X and 100X).
3. It is used as a bleeder, based on the application.

Disadvantage of resistor:

1. I²R loss. The resistor converts the electrical energy into heat energy even if you don't want it. 
2. As the power rating increases, it occupies more space in PCB. 
3. In some high frequency application it may produce unwanted audio-able noice

Formula and calculation:

We have 2 methods to calculate the resistance.

Method 1:

A resistance of a material is directly proportional to the length (L), and inversely proportional to that area(A) of that particular material. 

So, Resistance R α L/A 

To get rid of the proportional symbol which is only a theory that cannot be used for a real life math, we should add a constant on the right side.

Here we are, R= ρL/A 

ρ is called resistivity. As we said earlier, this is a constant with respect to the material.

Example- Resistivity for copper at 20°C = 1.68X10^-8 Ωm (ohm-meter).

SI unit of resistivity is ohm-meter.

Method 2: 

In first method, we saw how to derive the resistance of the material by it's construction. In this method we are going to see, how to derive a materials resistance by it's reaction towards the voltage and current.

Generally, the potential difference (V) between to point of the conductor will be directly proportional to the current (I) flow through the conductor.

So,  V α I

Getting rid of the α, 

Now, V = IR

R is constant and called as a resistance of that particular conductor. 

R= V/I

TYPES OF RESISTOR:

There are plenty of types of resistors are available in market in these recent generation. I simplified as much as I can. 

 

I'll simply  from first image to second image

Type of resistor by it's package:

1. Surface mount type (SMD)
2. Through hole type (TH)

Before getting into the SMD type (only few types are there according to no. of bands), we'll see the types of through hole resistors.

Types of resistor by it's resistance value:

1. Fixed resistors
2. Variable resistor (example- potentiometer, rheostat).

Types in fixed resistors:

1. Carbon film resistor (CFR)
2. Metal film resistor (MFR)
3. Metal Oxide resistor (MOR)
4. Wire wound resistor (WWR)
• Inductive wire wound resistor 
• Non inductive wire wound resistor
5. Thick film and thin fit resistor
6. Special resistor

I can explain all of the types in this blog only but it will make this blog even longer and complicated, I'll explain all about these types in my next blog.

Thank you and please share if you liked and found this informative, and if not, please comment your opinion it'll help for my next blog.

Photo credits: pixabay.com