A comprehensive guide on resistors
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A comprehensive guide on resistors

Are you planning to start working on your first PCB design? Then you will come across a variety of components, but none are as famous as the simple resistor. If you have ever glanced at a circuit board, you will notice that colorful resistors are placed everywhere. But what exactly is a resistor, and how does it function? Let us find out.

What is a resistor?

Resistors are simple electronic components that oppose the flow of either direct current (DC) or alternating current (AC). Circuits can be protected, operated, or controlled with them. They can also be used in conjunction with other components to mold electrical waves into shapes that meet the needs of the designer.

Most people are familiar with Ohm's law, which states that V=IR. The voltage across a resistor is V, the current flowing through it is I, and the resistance is R. Working with resistors is based on this equation, which relates current, voltage, and resistance (and other passive components).

What are the functions of resistors in a circuit?

Resistors are used in a variety of circuits. The functions listed below are the most common. Also, most applications require many resistors in serial or parallel arrangements.

  • Potential dividers:

    When two or more resistors are connected in series, the voltage at their junction point is proportional to their values. This feature is commonly used to generate intermediate voltages in circuits.

  • Current limiting:

    Resistors are used to control how much current passes through a circuit part. Many circuits benefit from this safety feature, such as limiting the amount of current that can go into an LED to control its brightness.

  • Op-amp circuits:

    Resistors are the principal element used for setting the gain and feedback functionality of most op-amp circuits.

  • Impedance matching:

    The receive and transmit ends of a circuit must have the same impedance to maximize power transmission at high frequencies. At least a portion of this requirement can be met using resistors.

  • Current measuring:

    Measuring the voltage of any circuit is an easier task than measuring current across a circuit. But thanks to resistors and Ohm's law, this can be easily calculated with the formula V=IR.

  • Transistor biasing:

    To make a significant voltage flow through its collector/emitter terminals, a transistor requires a small base voltage. However, because a transistor's base is sensitive to high currents, a resistor is used to control the current and give a safe biasing voltage.

  • Timing circuits:

    A resistor and a capacitor are always utilized as timing components in timer and oscillator circuits. The basic time pulse or trigger for the circuit is the time it takes to charge or discharge a capacitor. This charging and discharging process are efficiently controlled by a resistor, whose value is changed to get varying time intervals.

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Important parameters of a resistor:

When selecting resistors, remember that resistance is not the only factor to consider. There are several critical factors to keep in mind, just like any other component. More often, a resistor price varies depending on these factors. Here is a list of the most important parameters:

  • Power rating:

A resistor's power rating ( measured in watts) is a measurement of how much energy it can dissipate without harming or affecting its qualities. The nominal power rating can change depending on the operating conditions and surroundings. When the ambient temperature rises, the resistor, for example, gets de-rated to a lower value.

  • Thermal Resistance:

The thermal resistance of a resistor indicates how well it can dissipate energy into the environment. In reality, engineers utilize thermal resistance to model a system's heat dissipation. This is especially critical if the design requires the resistor to operate at or near its maximum value, as this might have a substantial impact on the system's long-term durability.

This parameter could be used, for example, to calculate the size of a PCB board required to keep the resistor's value and operating temperature within acceptable limits.

  • Tolerance:

The precision of a resistor is measured by its tolerance. Resistance with a smaller tolerance has a decreased chance of deviating from the indicated resistance value. Tolerances might range from 20% to a fraction of 1%. Assume you are using a 100 Ohm resistor with a 10% tolerance. This means that the resistors value can be as low as 90 Ohms or as high as 110 Ohms. This may or may not be a problem, depending on your circuit.

  • Resistor noise:

Resistor noise is an undesired phenomenon in particular circumstances, such as high gain amplifiers in audio applications. Some resistors are better at reducing noise than others. The noise qualities of metal foil are among the best, whereas the noise qualities of carbon composition are considered the worst.

There are mainly three types of resistor noise: thermal, contact noise, and shot noise.

Thermal noise is mostly affected by temperature, although it can also be affected by bandwidth and resistor resistance. Thermal noise that is temperature-dependent is alternatively called Johnson noise or white noise. White noise is a noise that has the same degree of intensity across all frequencies.

The amount of shot noise is determined by the bandwidth and average DC current. A higher average DC current indicates higher noise. To combat this form of noise, you can keep DC current levels low.

The average DC current, bandwidth, shape, and material type all affect contact noise. Contact noise is only present in resistors consisting of carbon particles; wirewound resistors do not exhibit this problem. Because contact noise increases as the current increases, keep the current low in low-noise circuits.

  • Stability:

Temperature variations, humidity, and other factors contribute to the aging and stability of a resistance value over time. Because external conditions can change a resistor's resistance, this specification is significant. Humidity, for example, can cause the resistor's insulation to swell, putting strain on the resistive element and causing fracture overtime when the resistor dries out.

  • Temperature coefficient of resistance (TCR):

A resistor's value is determined by the length, cross-sectional area, and resistivity of the resistive material used in its construction. TCR, which is measured in ppm/C (parts per million per degree centigrade), establishes the relationship between resistance and temperature. The resistive material's quality primarily determines it, but it is also affected by the resistor's structure.

A positive TCR indicates that as the temperature increases, the resistance increases as well. On the other hand, a negative TCR indicates that when the temperature rises, the resistance decreases.

  • Voltage:

The voltage that builds up across a resistor as current passes through it puts electrical stress on the materials that make up the resistor. A quick breakdown of the resistor and voltage fluctuation is possible if this voltage exceeds the permissible maximum.

The maximum voltage varies widely among different resistor types, ranging from a few volts for ordinary surface mount resistors to thousands of volts for some specialty high voltage resistors.

  • Frequency response:

In addition to resistance, resistors have inductive and capacitive properties. Even if they are minor, these properties can change the device's electrical impedance, especially at higher frequencies. As a result, a resistor can be used as an RC circuit, a filter, or an inductor.

Carbon composition resistors have several disadvantages compared to film-type resistors in most applications; however, they act as pure resistors at Megahertz frequencies. Wirewound resistors have the worst frequency response, as their design is essentially a coil of wire, similar to that of an inductor.

Conclusion:

So, this is all the essential information you need to know about resistors if you are designing your first PCB board. Resistors are incredibly versatile, and you will find yourself employing them in almost every electronic project you work on. If you are wondering where you can buy resistors of high-quality and in bulk quantities at the best prices, Moglix is the perfect online website for you.

Resistors: FAQs

Q. What are the 4 types of resistors?

A. There are many different varieties of resistors, each with its own set of applications, properties, and manufacture. However, the four common types of resistors include fixed, variable, thermistors, and varistors.

Q. How do resistors work?

A. They are made up of two wires or conductors connected at opposing ends or sides of a rather poor electrical conductor with a resistance measured in ohms. A resistor operates on the principle of Ohm's law, which states that the voltage across a resistor's terminals is proportional to the current flowing through it.

Q. What is the function of a resistor?

A. Resistors are mainly used to limit current, create a voltage drop in a circuit, divide voltages, polarise active elements, and can be considered a load in an electrical circuit.

Q. How do resistors adjust signals?

A. A resistor converts electrical energy into heat, which is then dissipated into the air. In electrical circuits, resistors add precisely regulated levels of resistance, thus adjusting the signals.