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Resistors: A Detailed Guide

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Written by :

Dr. Arvind Kulkarni

Dr. Arvind Kulkarni is a seasoned expert in electrical instrumentation with over a decade of experience. Holding a Ph.D. from one of India’s top institutes in Electrical Engineering, he combines academic excellence with practical expertise to deliver precise instruments like Ammeters, Voltmeters, Panel meters, and Current Transformers that you can trust.

In addition to his Ph.D., Dr. Kulkarni also holds prestigious electrical instrumentation and measurement certifications, making him a trusted authority in the industry.

Resistors are characterized by two attributes: precision and power. Precision resistors frequently have restricted operating temperature constraints and power dissipation ratings, and are designed for applications needing precise resistance tolerance and stability. 

Resistors might seem like tiny, unassuming parts, but there are many resistor advantages since they manage the flow of electrical current when using gadgets. But what exactly do they do, and how do you pick the right one for your project? Let’s break it down!

Function of a Resistor

A resistor is a simple two-terminal electrical component that opposes the flow of current. We calculate its resistance in ohms () using Ohm’s law. The flow of electrical current is restricted as the resistance value increases. This is significant because we frequently need to control or reduce the current flowing through specific parts of electronic circuits.

For example, if an LED receives too much current, it will quickly burn out. By connecting a resistor to the LED, you can limit the current flow and prevent damage. Resistors are also employed in transistor and amplifier circuits to set biasing points, control voltage levels, and protect sensitive components from damage caused by excessive voltage.

Types of Resistors 

A resistor is a passive part of a circuit that resists the flow of current. Resistors come in a variety of forms. These resistors differ in terms of construction, power dissipation capabilities, and tolerance to different factors (like light and temperature). Some of the most common types of resistors include:

  1. Carbon Composition Resistor

A carbon composition resistor (also known as a carbon resistor) is a type of resistor that is widely used. These resistors are inexpensive and simple to build. The main benefits of these resistors are that they are widely available, inexpensive, and extremely durable.

  1. Wire Wound Resistor

A wire wound resistor is made by wrapping an insulating cylinder around a wire made of manganin or constantan. The temperature coefficient of resistance for both constantan and manganin is nearly zero. As a result, the resistance variation of these resistors with temperature is minimal.

Over the damaged wire, an insulating covering, such as baked enamel, is applied. This insulating, heat-resistant material cover reduces the impact of changes in outside temperature.

  1. Variable Resistor

As the name suggests, a variable resistor can change its resistance value, much like a potentiometer. A wiping contact and a revolving shaft are present. Wiping the contact essentially modifies the effective length of the resistive element, which in turn modifies the resistance of a resistive semicircular bar or coil. A resistor of this type is a rheostat.

  1. Light Dependent Resistor

An LDR, or light-dependent resistor, has resistance that varies with the quantity of light that strikes it. The cadmium sulfide used to manufacture this has a tiny quantity of electrons when not lit.

When a laser beam contacts a substance, electrons are liberated, increasing its conductivity. As a result, it has low resistance when exposed to light and high resistance when not exposed to light.

Resistor Color Code

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Resistors are necessary parts of electrical and electronic circuits because they produce voltage dips and regulate current flow. To function correctly, resistors need a specific “resistivity” or “resistance” value, which can vary from millions of Ohms to fractions of an Ohm.

Making resistors in every possible value is not practical. Instead, resistors are created using “preferred values,” which are represented by colorful bands on their bodies.

The color coding method is used to determine each resistor’s unique resistor color code. Four color bands are generally used to determine the precise value of resistors, as seen below. Every color has a numerical value between 0 and 9. For resistor color labeling, there is a reasonably easy formula.

  • International Color coding System

A standardized resistor color coding system was devised to determine a resistor’s ohmic value easily. The colored bands indicate numbers and multipliers, each representing a different value.

  • Tolerance Indication

It is simple to decipher when you read the color bands—such as yellow, violet, and red—from left to right. The resistor in this instance is valued at 4.7 kΩ. A resistor’s tolerance is indicated by the fourth and fifth bands, which express the variance from the prescribed value. Tolerances often range from 1% to 20%, with specific color codes such as brown (1%), red (2%), gold (5%), and silver (10%).

  • Larger Power Resistors with the British Standard scheme

The color coding scheme may not be practicable on larger power resistors. Instead, the British Standard (BS 1852) approach represents the multiplier value with letters such as “K” for thousands, “M” for millions, and “R” for quantities smaller than one.

Resistor Ratings and Power Dissipation

Resistors are important in circuits, but not all resistors work for every job. We choose resistors based on factors like color codes, which help us understand voltage, tolerance, and resistance. There’s also one more important thing to consider: the power rating.

The power rating of a resistor tells us the maximum power it can handle without getting damaged. Resistors produce heat, and the power rating shows how much heat they can handle safely. If we go over this limit, the resistor can burn and harm the circuit.

Watts, or units of power, are used to rate the resistor power. That’s why it’s also called wattage.

Bigger resistors usually have the ability to handle more power. However, keep in mind that as the wattage (or power) of a resistor goes up, so does its cost. Resistors come in a range of sizes, from as low as 1/8th watt to several kilowatts. You can often figure out a resistor’s wattage by looking at its size

Applications of Resistors in Industries

  1. Heating Applications

Industrial resistors and heat have an inverse relationship, according to Joule’s law of heating. Temperature can be adjusted by adjusting the current and time variables. Temperature sensors frequently employ thermocouples. The resistance values of these resistors vary with temperature. Thermistors are made of metal oxides.

Due to the high temperature, appliances such as toasters, electric stoves, microwaves, heaters, and toasters glow. This is because when current flows through the metal filament, it acts as a resistor and generates heat.

  • Automatic External Defibrillators

External defibrillators and automated defibrillators both work on the same principles. They treat patients with life-threatening cardiac arrhythmias, or irregular heartbeats, which can end in cardiac arrest, by administering an electrical shock to reestablish a normal heart rhythm. 

Keep in mind that internal defibrillators, which are normally only used in operating rooms, use electrodes in direct contact with the heart, whereas external defibrillators deliver the charge through paddles placed on the chest.

  • Blood Glucose Meters

People with diabetes can keep an eye on their health and react appropriately to elevated or decreased blood sugar levels by using blood glucose monitoring. Knowing how and why these levels fluctuate enables impacted individuals to control their medication, diet, and activity schedules better. 

For those with Type 1 diabetes (and some Type 2 diabetes) whose treatment calls for insulin injections, this is especially crucial because routine blood glucose testing enables them to assess the efficacy of their most recent insulin dose and schedule the next one. A computerized gadget called a blood glucose meter offers a comparatively easy way to check blood glucose levels. 

  1. Voltage Dividers

Resistors are voltage dividers in electrical circuits. A voltage divider divides the source voltage among the various electrical circuit components to provide the required operating voltage at the load terminal or output.

  1. LEDs and Transistors

Transistors and LEDs are examples of semiconductor devices that are sensitive to input current values. As a result, resistors are utilized in the circuit to moderate the input current value.

As electricity goes through an LED (Light Emitting Diode), it begins to emit light. The most basic circuit for powering an LED is a voltage source connected in series with a resistor and an LED. This type of resistor is known as a ballast resistor. 

The ballast resistor controls the current flowing through the LED. It keeps too much electricity from accumulating and damaging the LED. When a transistor is turned on, the base resistor must allow enough current to travel through it for the transistor to be fully saturated.

Tips for Choosing and Using Resistors

There are a number of factors to take into account when choosing a resistor for your project:

  • Power rating: Since resistors carry electricity and emit heat, choose one with an appropriate power rating to prevent overheating. Tolerance relates to two values: the genuine resistance value and its rated value, which is typically stated as a percentage.
  • Resistance value: Select a resistor with the appropriate resistance value for your circuit.
  • Temperature coefficient: Since certain resistors alter resistance values in response to temperature changes, be sure the resistor you select has a coefficient that is appropriate for your application.

You may choose the right resistors application for your project and calculate these values with the help of internet tools.

Common Mistakes and Troubleshooting

Anybody who works with electronics must comprehend common errors and troubleshooting solutions related to resistors, which are essential parts of electronic circuits. The following are common resistor issues and solutions:

  1. Resistance change fault repair

These kinds of defects are more prevalent. The resistance value varies when the circuit modifications, temperature, and voltage exceed the limit. Using a multimeter reveals that there is a significant discrepancy between the nominal and real resistance values. 

The value increases in magnitude. Installing a replacement resistor is necessary to fix a fault brought on by a shift in resistance. The resistance to resistance change is shown in the figure. To solve the issue, replace the resistor. 

  1. Fixing the internal contact failure

Inadequate internal contact between resistors is the main cause of noise, intermittent noise, and other noise in household electrical equipment. Resistors can also occasionally operate with a minor flashover.

Deterioration of the carbon film causes potentiometers to commonly have inadequate contact. The TV will be quiet or dull with no picture, and vice versa, if the radio sounds soft at first, stops when it gets loud, and so on.

  1. Fixing of open circuits

In an open circuit, the circuit cannot be reached and the resistance is infinite. Sometimes the open circuit issue can be seen visibly, like in cases where something is detached, loosened, or broken. Occasionally, a multimeter must be used to measure it. When measuring, the pointer indicates infinity. 

Conclusion

Resistors used in an electronic device regulates or restricts the amount of electrical current that can pass through a circuit. Resistors can provide a certain voltage to an active component, such as a transistor. 

They are crucial in electronic devices because they control or limit the flow of electrical current. There are various varieties available, including fixed, variable, thermistors, photoresistors, and varistors, each with unique qualities and applications.

FAQs

  1. Does resistor reduce current or voltage?

A resistor limits the amount of electricity that can pass through it, thereby reducing current flow in a circuit. However, if the resistance is too high, it can also lower voltage levels.

  1. What are the 3 main purposes of using a resistor?

A resistor serves several functions, the most frequent of which are to regulate current flow, manage voltage levels, and protect sensitive components from damage.

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