The Ultimate Guide to Resistors

Resistors are one of the most basic components used in almost every electronic circuit today. Therefore, it is important that we understand their characteristics and how they are used in circuits. In this article, we will explore the fundamental concepts of resistors, including their working principle, types, properties, and how to use them in integrated circuits (ICs). So, without further ado, let's get started. 

 

<h2>What is a Resistor?</h2>

A resistor is a passive component in the form of lengths of wire or pieces of carbon, of which the resistance is accurately known. This means that a resistor cannot generate on its own; it simply exhibits resistance to the current flowing through it. Every resistor has a specific resistance value that is measured in Ohms, and the amount of resistance offered by a resistor depends on the quantity and type of materials used in it. 

<h2>How do Resistors Work?</h2>

 

We can understand the working process of a resistor by using the I-V characteristics graph above. The graph shows that a resistor is a linear device since it obeys Ohms' law and the I-V nature has a scaling property. The 10K or 10,000Ω resistor has a steeper gradient than the 1K or 1,000Ω resistor because the 10K resistor allows less current compared to the 1K resistor. 

 

The current flowing through the 10K resistor encounters more resistance due to an increase in the material’s opposition to electron flow. As a result, the electrons collide with atoms to transfer energy. This energy is then converted to heat, which is dissipated in the resistor. The reduction in energy level results in a lower voltage and current, which excites the resistor. We can use the Ohms' law equation V=IR to calculate the amount of current flow in the resistor.

Parameters Affecting Electrical Resistance

There are basically four factors that affect the resistance of a conductor: 

 

1. Length of the conductor 

2. Area of the conductor 

3. Temperature of the conductor 

4. Substance of material

 

The resistance of a conductor of uniform cross-sectional area is directly proportional to its length and inversely proportional to its area. This means that the longer the wire, the higher the resistance, and the thicker the wire, the less the resistance. On the other hand, the temperature of a conductor affects its resistance based on the material of the conductor. 

 

For example, in metallic conductors such as gold, silver, and copper, the higher the temperature, the higher the resistance. However, in non-metallic conductors such as diamond and graphite, the higher the temperature, the less the resistance. Materials like constantan and manganin are slightly affected by temperature changes. Therefore, such materials are used for making standard resistors. 

 

Experiments show that the resistance of a wire or metallic conductor at a constant temperature is related to its length and cross-sectional area by the expression: R=ρL/A. Where ρ is a constant of proportionality known as resistivity. This means that the resistance of a material is also directly proportional to its resistivity. 

Properties of Resistors 

Before choosing a resistor for a project, we need to consider the characteristics of the resistor in order to select the most suitable one. Here is a list of the different properties of resistors that we need to consider: 

 

• Power rating 

• Tolerance 

• Temperature coefficient

• Frequency response 

• Noise and stability 

Power Rating

The power rating of a resistor is defined as the maximum power that can be handled by the resistor. This power is given by the expression P = IV or I2R. For example, if a voltage of 5V is applied to a resistor and 1A of current is flowing to the resistor, then the power dissipated across the resistor = 5×1 = 5W, meaning that the power rating of the resistor should be more than 5W.  

 

A useful rule of thumb states that a resistor should have a power rating that is at least 2 to 4 times greater than the maximum power dissipated across it. The commercially available resistors have a power rating as low as 1/16W or as high as 300W. 

 

Sometimes the manufacturer of the resistor also provides the power derating curve, which basically defines how the maximum power dissipated across the resistor changes with temperature. It also shows the ambient temperature of the resistor. 

Tolerance

The tolerance of a resistor is the extent to which its resistance deviates from its nominal value. For example, if a resistor of 100Ω has a tolerance of 1%, it means that the actual value of the resistance is between 101Ω and 99Ω. 

 

The commercially available resistors have a tolerance value as low as 0.1% or as high as 20%, but they find it easier to achieve the tolerance of 0.1%. To maintain the tolerance of your resistor, you need to select a resistor with built-in temperature coefficient compensation. 

Temperature Coefficient

The temperature coefficient of a resistor is defined as the extent to which its resistance value changes with temperature. This temperature coefficient can be either positive or negative, and its general unit is ppm/°C. 

 

For example, if we have a resistor of 100Ω, that is operated at an initial temperature of 25°C and a final temperature of 29°C with a temperature coefficient of 50 ppm/°C. The value of the resistance is 100+[0.000050×(29-25)×100] = 100.02Ω. The temperature coefficient is particularly critical when the resistor is operated at a high temperature. 

Frequency Response

The frequency response of a resistor refers to how its resistive impedance changes with the frequency of the electrical signal passing through it. This is because the resistor is usually constructed with some inductance and capacitance, which limit its maximum frequency of operation. Therefore, while selecting a resistor for high-frequency applications, we need to make sure its frequency range is between 50 and 100 MHz for efficient operation. 

Categories of Resistors

Resistors can be classified into two categories: 

1. Fixed resistor: this resistor has two terminals, and its value cannot be changed once fabricated.

2. Variable resistor: this resistor has three terminals, and its value can be changed by adjusting the knob in the second terminal. 

Types of Fixed Resistor

The following are the main types of fixed resistors available today: 

 

• Carbon composition resistor 

• Carbon film resistor 

• Metal film resistor 

• Wire wound resistor 

• Metal oxide film resistor

• Surface mount (SMD) resistor 

Carbon Composition Resistor

 

The carbon composition resistors are composed of carbon particles and a binder, which binds the embedded leads and caps together. Their resistive component is created from a mixture of carbon powder and ceramics. This makes them useful for current limiting and circuit protection in applications with high-energy pulses. However, their tolerance value of 5-20% indicates that the resistors have low accuracy and are susceptible to systematic errors. 

Carbon Film Resistor

As illustrated above, the thin layer of the carbon film resistor is deposited on the ceramic substrate in the form of a helix, and the resistance value automatically changes when the pitch of the helix is adjusted. The carbon film resistors are low-cost resistors, and they produce less noise compared to the carbon composition resistors. The available tolerance values for these resistors are 2, 5, 10, and 20%. In addition, the ideal temperature coefficient of these resistors lies between 2×10-4Ω/°C and -8×10-4Ω/°C, making them useful in high-voltage and high-temperature applications. 

Metal Film Resistor

The metal film resistors are very similar to the carbon film resistors, but instead of a carbon film, a thin layer of metal film is  deposited on the ceramic substance. They produce less noise than the carbon film resistors, and they are basically used by many designers in bridge circuits and active filters because of their good stability and high temperature coefficient of resistance (50-100 ppm/°C). Furthermore, their minimum and maximum tolerance values are 0.1% and 2%, respectively, making them less susceptible to systematic errors. 

Metal Oxide Film Resistor

Metal oxide film resistors are the third type of film resistor. They also share some similar features with the carbon and metal film resistor, but in this case, a metal oxide film is deposited on the ceramic substrate, and tin oxide is used as the metal oxide layer. The metal oxide film resistor is also a budget-friendly resistor, but in terms of noise level, stability, and tolerance value, their performance is less efficient than the metal film resistors. 

Wire Wound Resistor 

In this resistor, the metallic resistive wire is wound around the ceramic material. Therefore, the thickness or gauge of the metallic wire determines the resistance of the resistor. Metal alloys like copper and silver alloys are used for this resistive wire because of their low temperature coefficients. As a result, the wire-wound resistors are quite suitable for high precision as well as high power applications, but they are not suitable for high-frequency applications. 

Surface Mount Resistors (SMD)

 

Surface-mount resistors (SMD) are the tiny resistors in various PCBs and motherboards. In the internal structure of this resistor, there is a thin layer of resistive film on its ceramic body. This resistive film acts as the resistive element of the resistor, and it can either be a metal film or a metal oxide film. The SMD resistor is soldered to the PCB with the help of the metal contacts on both sides of the resistor. They are basically used to achieve very high accuracy and very low tolerance values in high-frequency applications.  

How to use Resistors in Circuits?

Here are some key points on the applications of resistors in circuits: 

 

1. Current limiting: resistors can be used to limit the flow of currents in circuits so as to prevent the electronic components from being damaged. Current-limiting resistors are commonly used in light-emitting diodes (LEDs), motors, and batteries. The general formula for calculating the resistance of current-limiting resistors in LEDs is R= Vtotal-VLED/ILED. 

 

2. Voltage dividers: potentiometers or voltage dividers are widely used in circuits to divide the input voltage into some reasonable fraction. They are extremely useful in comparators and sensor circuits to feed reference voltage input. The output of the voltage divider is given by Vout = (R1/R1+R2) Vin. If R1=R2, then Vout=½ Vin. 

 

3. Feedback elements: feedback is a concept used in operational amplifiers, popularly known as op-amps. To get an amplified signal from an op-amp, we need to control its gain by feeding a portion of the output to the inverting input using a resistor. 

 

4. Filter circuits: resistors form an integral part of passive filters, along with capacitors and inductors. The major types of filters that can be constructed using resistors include low-pass filters, high-pass filters, and band-pass filters. 

Summary

Resistors are the backbone of automatic current control in electronic circuits, and understanding their applications is crucial for designing batteries, motors, amplifiers, and filters. In this article, we have discussed the definitions, characteristics, types, and applications of resistors. The next article will focus on resistor color codes, but before then, kindly explore our range of resistors today and find the perfect fit for your next project!