Introduction: Resistors: the Ultimate Guide

About: Guzel Sans did bachelor in Power Electrical Engineering. His areas of interest are HF Modelling, Power Systems Protection, and Electronics design Engineering. Guzel Sans loves to play football. He also spends …

The resistor is the basic Electrical component which controls the flow of current in circuits. In circuits subscript R represents it.

This Ultimate Guide provides an introduction to resistors, its types, symbol, common prefixes and wattage rating of resistors.

Step 1: Types of Resistors

Simple Resistor: It is a simple two terminal device which provides the fixed value of resistance to the connected circuit.

Variable Resistor: As the name shows this resistor provides a variable resistance to the output circuit.

Photoresistor (LDR): LDR stands for the Light dependent resistor. Its resistance changes with the amount of light falling on it.

Adjustable: A variable resistance whose value can be adjusted within certain limits.

Tapped: Resistor having some taps which can be used to vary the value of resistance.

Thermistor: Temperature-dependent resistor.

Trimpot: A type of variable resistor which uses a screw to change the value of resistance.

Step 2: Resistance

Resistance is the extent of resistance offered to the circuit by any resistor.SI unit of resistance is ohms (Ω). A resistor of 100 ohms will offer more resistance compared to 10-ohm resistor.

Step 3: Resistor Configurations

Very often two or more resistors connected in the circuit to achieve another value of resistance.These connections can be divided into three configurations:

  1. Series
  2. Parallel
  3. Series-Parallel

Series Combination of circuits

In series configuration head of one resistor joins with the tail of another one and there is no combination in between them. The equivalent resistance of series combination is: R_equivalent = R_1 + R_2 + R_3

Parallel Combination

In this combination heads of all components share a common node and tails of all components share a common node too. The equivalent resistance of parallel circuits is: R_equivalent = 1/(1/R_1 + 1/R_2). Alternatively, you can use a calculator like this to find the equivalent resistance online.

Step 4: How to Select the Right Resistor for Your Project

Different circuits require different values of resistance for proper functionality. The choice of any resistance is governed by Ohm's Law whose basic mathematical statement is: V = IR

Using this equation we can find R = V/I

In circuit often V and I are known. For example, we know that a 12 V input source is at our disposal and current allowed is 5mA. Using R = 12V/5mA = 2.4 k ohm.

Step 5: Resistor Sizing for Your Project

The size of the resistor is directly related to its power handling capability. Selecting proper wattage rating of a resistor is known as its sizing.

Formula P = (V^2)/R is used for finding power dissipation.

Other modifications to this equation can be:

P = VI

P = (I^2).R


V = Voltage applied to the input terminals

I = Current passing through the resistor

R = Resistance of the resistor

P = Wattage rating

Remember that size of resistor depends on wattage rating. The more the watts, the greater the size.

Example: A 5-ohm resistor connects to 12 V source. Calculate its minimum wattage rating for proper working.

Solution: P = (V^2)/R = 28.79 watt

Step 6: Winding Up

Winding up the above discussions I hope all above guide is enough for kickstarting your projects. Please inform me in comments about your projects and if you find this guide a useful source. Best of luck to you.