What is a Resistor? - A Comprehensive Guide to Resistors

Resistors are widely recognized and frequently utilized passive components within electronic design. Their significance in a variety of electronic devices and circuits is paramount. They assume the pivotal roles of regulating electric current, constraining current levels, partitioning voltages, and safeguarding components against undue current flow. This comprehensive guide is dedicated to delving into the realm of resistors, including their types, applications, and more details, to give you a deep understanding of these indispensable components.

What is Resistor?

A resistor, categorized as a passive electrical component, is essential to constrain the passage of electric current.

Resistor Definition

A resistor, classified as a passive electrical component, introduces resistance to the flow of electric current. It is a fundamental presence in nearly all electrical networks and electronic circuits. Quantified in ohms (Ω), Resistance defines the resistor's characteristic. An ohm signifies the resistance exhibited when a one-ampere (A) current traverses a resistor, resulting in a one-volt (V) drop across its terminals. Notably, the current remains directly proportional to the voltage across these terminals, encapsulated by Ohm's law in the equation: 

R=V/I.

Resistors find application in a multitude of scenarios. They are employed for diverse purposes, such as regulating electric current, dividing voltage, generating heat, matching and loading circuits, controlling gain, and setting time constants. These components are commercially accessible with a broad spectrum of resistance values spanning more than nine orders of magnitude. Their adaptability extends to acting as electric brakes to dissipate kinetic energy in large-scale applications like trains or scaling down to dimensions smaller than a square millimeter for intricate electronic systems.

Resistor Symbol

The globally recognized IEC symbol is rectangular, with leads positioned at both ends, depicted in the figure on the left. Conversely, the United States widely adopted the ANSI standard, portraying a fixed resistor with a zigzag line illustrated on the right.

What Do Resistors Do in a Circuit?

Resistors find various applications within electronic circuits, and many functions necessitate multiple resistors configured in series or parallel. Some typical roles that resistors serve include:

• Potential Dividers: When two or more resistors are connected in series, they create a voltage at their junction point, directly proportional to the ratio of their values. This principle is widely employed in circuits for generating intermediate voltages.

• Biasing Resistors: Transistors and numerous other devices require precise adjustment of their AC and DC operating characteristics and gain values for correct operation. Multiple resistors are used for this purpose, often referred to as biasing.

• Op-Amp Gain and Feedback: In most operational amplifier (op-amp) circuits, external resistors are essential for setting the gain and feedback configurations. Resistors play a pivotal role in achieving these functions.

• Current Limiting: Resistors effectively restrict the current flowing in a circuit element, contributing to safety measures. For example, they can limit the current directed to an LED to control its brightness.

• Impedance Matching: Achieving optimal power transmission at high frequencies necessitates aligning the impedance of a circuit's transmitting and receiving ends. Resistors can fulfill this impedance-matching requirement.

• Current Measurement: While many circuits must gauge the current flow, a voltage measurement is often more convenient. In such cases, inserting a resistor into the course to induce a voltage, as per Ohm's law, is a common technique for current measurement.

• Data and Address Bus Pull-Ups: Pull-up resistors are employed to mitigate noise issues in high-speed computer buses. They help establish a known state when a data bus is tri-stated or driven high, ensuring that its output high level remains well above the switching threshold of other logic components on the bus.

Types of Resistors

Resistors are offered in various shapes and sizes, with two prevalent types being through-hole and surface mount variants. Furthermore, resistors can be classified as static, standard, special, or even part of a collection of variable resistors.

Two fundamental categories of resistors are as follows:

• Linear resistors

• Non-linear resistors

Linear resistors

Resistors that exhibit variations in their values in response to alterations in temperature and voltage are termed linear resistors. Linear resistors can be categorized into two main types: fixed and variable resistors.

Fixed resistors

These resistors maintain a constant, unchangeable value. There are four distinct categories of such linear resistors:

• Carbon composition resistors

• Wire wound resistors

• Thin film resistors

• Thick film resistors

Variable resistors

Variable resistors lack a fixed value and can be adjusted using dials, knobs, or screws. They serve various purposes, including volume and tone control in radio receivers. There are three primary categories of variable resistors:

• Potentiometers

• Rheostats

• Trimmers

Non-linear resistors

Resistor values are influenced by both temperature and applied voltage and are not governed by Ohm's law. Among non-linear resistors, three prominent types are:

• Thermisters

• Varisters

• Photo resistor

Resistive Materials

After exploring the fundamental aspects of resistor design and the various configurations at our disposal, the next step is to delve into the materials employed in resistors and shed light on their respective merits and drawbacks.

Carbon Film Resistors

Carbon Film Resistors

Carbon film resistors have mostly given way to metal types for general-purpose applications due to their inferior tolerance and noise characteristics. Nevertheless, they continue to find utility in specialized applications.

The construction of these resistors involves the deposition of a carbon film onto a ceramic substrate, which offers several advantages:

1. The film is predominantly non-inductive.

2. The ceramic material serves as an excellent insulator against both heat and electricity.

3. The layout features a substantial cross-section. These factors contribute to their usefulness in high-frequency applications, owing to their low inductance, and make them well-suited for higher operating voltages.

4. Their robustness allows them to withstand transients more effectively than other resistor types.

Consequently, while their usage has diminished in some areas, carbon film resistors remain readily available and serve specific needs.

Metal Film Resistors

Metal Film Resistors

Metal film resistors are crafted by depositing layers of metals, like ruthenium, onto an insulating substrate. They come in a wide array of types and packages, and when compared to carbon film resistors, they boast a more straightforward manufacturing process and more compact physical dimensions.

In most standard applications, metal film resistors have replaced carbon film resistors. This shift is attributed to their advantages, including reduced noise, tighter tolerances, and generally improved temperature coefficients. What was once considered a conventional resistor has advanced significantly, offering high precision (0.1%) and low TCRs in the single-digit PPM/C range. Notably, metal film resistors are available in through-hole and surface mount (chip resistor) configurations.

Wirewound Resistors

Wirewound Resistors

Wirewound resistors possess appealing attributes suited for specialized applications, particularly in high-precision instrumentation where tolerances significantly surpassing 0.01% and extremely low-temperature coefficients (TCR) are imperative.

Additionally, they often emerge as the preferred resistor type for high-power applications, with examples rated up to several hundred watts readily accessible. It's crucial to remember that these resistors are constructed with wire wound around an insulated core, inherently forming an inductor. Therefore, they are not advisable for use in high-frequency applications and are typically unavailable in surface mount configurations.

Metal Oxide Resistors

Metal Oxide Resistors

These components resemble metal film resistors, with the distinction lying in the resistive element, which is typically an oxide, often tin. Notably, their performance exhibits subtle variations from metal film resistors, making them better suited for applications involving elevated voltage levels and increased power demands.

Nonetheless, their range of available options is usually more limited, and they may feature slightly reduced tolerances and temperature coefficients (TCRs) compared to their metal film counterparts. As with other resistor types, they are available in surface mount and through-hole configurations.

Metal Strip Resistors

Metal Strip Resistors

These specialized resistors are primarily employed in power supply applications for measuring current. They stand out with their frequently low ohmic values, and they are available with relatively modest temperature coefficients and moderate power dissipation to ensure minimal impact on the overall circuit's performance.

As the name implies, these resistors are precision laser-trimmed metal strips. Another noteworthy feature of these devices is their low thermal resistance, a critical attribute given their potential to carry high currents, necessitating effective heat dissipation during operation.

Resistor Specifications

What are the crucial attributes and specifications influencing the selection of a resistor? Factors to be considered encompass initial tolerance and value selection. It's essential to note that the tolerance or the deviation in the resistor's value is influenced by various parameters, as elaborated below.

Resistance

Resistance pertains to a resistor's capacity to impede the flow of current. Resistor resistance spans a broad spectrum, ranging from values as high as megaohms to less than 1 ohm. Standard resistor values are established by the Electronic Industries Association (EIA) and categorized into E3, E6, E12, E24, E48, E96, and E192 series, collectively known as E-series. The principles governing this classification will be elucidated in the following section, "tolerance."

Tolerance

The tolerance of a resistor establishes the range within which the resistance of the resistor may fluctuate. It signifies the degree to which the resistor's resistance may deviate from its nominal value, typically measured at room temperature (25°C) under no load. Tolerance is a critical factor impacting the accuracy and precision of a resistor. The ensuing discussion delves into the E-series, which is categorized by tolerance. The E-series numbers correspond to the number of resistors available in each decade; for instance, E6 encompasses 6 resistors within a single decade.

E-series	Tolerance (SIG FIGS)
E3	>20%
E6	0.2
E12	0.1
E24	5% ( also available in 2% )
E48	0.02
E96	0.01
E192	0.5%, 0.25% and higher tolerances

Temperature Coefficient

The resistance of a resistor experiences minor fluctuations in response to temperature variations. Typically, the resistance at room temperature, 25°C, serves as the reference point, and the unit for temperature coefficient is expressed in parts per million (ppm).

Power Rating

When a resistor operates within its specified power rating, it can guarantee a secure and dependable performance without generating excessive heat. The power rating typically spans from less than 1 watt (e.g., 1/16W) to several hundred watts, and it directly correlates with the resistor's physical dimensions. In essence, as the size of a resistor increases, so does its power rating.

Resistor Ageing or Stability

Aging and stability represent an intricate interplay of numerous factors influencing the resistance value over time. These factors include temperature fluctuations, prolonged exposure to high temperatures, humidity ingress, etc. Typically, this interplay results in a gradual rise in resistance over time, driven by the migration of conduction atoms within the device.

Resistor Noise

Any electronic component through which electrons flow introduces the potential for noise, and resistors are no exception. This consideration is particularly critical in high-gain amplifier systems or dealing with shallow voltage signals.

The primary source of noise within a resistor is thermal noise, stemming from the random fluctuation of electrons within the resistive material. This noise is typically modeled as white noise, characterized by a constant RMS voltage across a given frequency range. It can be quantified using the equation E = √4RkTΔF, where E represents the RMS noise voltage, R signifies the resistance value, k denotes Boltzmann's constant, T denotes the temperature, and Δf denotes the system's bandwidth.

System noise can be mitigated by reducing resistance, lowering operating temperatures, or narrowing the system's bandwidth. Additionally, another category of resistor noise, known as current noise, arises from the flow of electrons within the components. While it is infrequently specified, it can be compared if the manufacturer provides standard values under IEC60195.

High-Frequency Behaviour

The last aspect to consider is the specific resistor's high-frequency behavior. To put it simply, a resistor can be represented as a series inductor, followed by the resistor itself, and having a parasitic capacitor in parallel with it.

Even at frequencies as modest as 100 MHz (applicable even to surface mount resistors, which exhibit lower parasitic properties than their through-hole counterparts), the parallel capacitance can become a significant factor, causing the impedance to fall below its nominal level. The inductance may take precedence at higher frequencies, causing the impedance to rise above its minimum, potentially surpassing the minor value.

Resistor Standards

Numerous standards are in place for resistors, outlining methods for measuring and quantifying essential characteristics. Additionally, there are standards governing resistors' physical dimensions and resistance values. One of the most widely recognized standards is the color code marking system employed for axial leaded resistors.

Resistor Color Code

Resistor with a resistance of 5600 ohm with 2 % tolerance, according to the marking code IEC 60062.

Resistance value and tolerance are conveyed through colored bands encircling the component body. This method of marking electronic components was initially devised in the 1920s when printing technology was not yet advanced enough to imprint numerical codes onto diminutive components. Even today, the color code system remains the standard for most axial resistors up to one watt.

In the illustration above, a four-color band example is presented. In this instance, the initial two bands determine the significant digits of the resistance value, the third band signifies the multiplying factor, and the fourth band indicates the tolerance. Each color corresponds to a distinct numerical value, and this information can be readily referenced in a resistor color code chart or accessed via a resistor color code calculator. We also have more information on 4.7k resistor color code and 10k resistor color code waiting for you to explore!

Resistor Color Code Calculator

While resistors may not always display their values outwardly, their resistance can be determined by deciphering their color pattern. Plated-through-hole (PTH) resistors employ a color-coding system, adding a touch of visual distinction to circuits. On the other hand, surface-mount-device (SMD) resistors feature their unique value-marking system.

Below is a table presenting the color code for resistors:

Colour	Colour code
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Grey	8
White	9

Resistor Values (Preferred values)

During the 1950s, the growing production of resistors led to the necessity for standardized resistance values. These standardized values fall under the category of "preferred values." Preferred values are organized within the E-series, where each value is a specific percentage higher than the preceding one. Different E-series are established to accommodate various tolerance requirements.

Resistors in Series

Resistors are considered in series when the current passing through them is constant. They are linked end to end in a series configuration, and the total resistance in the circuit is the sum of the individual resistance values.

Resistors in Series Formula

Rtotal = R1 + R2 + R3 +……+Rn

In this equation,

Rtotal represents the total resistance, which is found by summing the reciprocals of all the individual resistances.

Resistors in Parallel

Resistors are considered parallel when their terminals are connected to the same two nodes. In a parallel configuration, these resistors share the same voltage at their terminals.

Resistors in Parallel Formula

1/Rtotal = 1/R1 + 1/R2 + ……+1/Rn

In this context:

1/Rtotal is the reciprocal of the sum of all the individual resistances.

Applications of Resistor

Here are some typical applications of resistors:

• Circuit functions

• Dividing voltage

• Heating

• Frequency and timing

• LEDs and transistor

Conclusion

In this comprehensive guide, we've explored the world of resistors, uncovering their types, applications, and their crucial role in electronics. You now understand more about "What is a Resistor?" and how these components shape our electronic world.

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