Resistor

A resistor is an electronic component that is used to resist or oppose the flow of electric current in a circuit. It is a passive component, which means that it does not require any external power source to function.

Resistors are typically made of materials such as carbon, metal, or wire-wound materials. They come in a variety of shapes and sizes, and are marked with a color code or numerical value that indicates their resistance. The unit of resistance is ohms, symbolized by the Greek letter omega (Ω).

Resistors are commonly used in electronic circuits to control the flow of current, limit the amount of current that flows through a circuit, and provide a specific voltage drop. They can also be used to divide voltage, generate heat, and perform other functions.

Overall, resistors are essential components in electronics and electrical engineering, and are used in a wide range of applications in devices such as computers, televisions, radios, and more.

Types of resistors

There are several different types of resistors, each with their own unique characteristics and applications. The most common types of resistors are:

1. Carbon composition resistors: These are the oldest and most basic type of resistor, and are made by mixing carbon powder with a binding agent and shaping it into a cylindrical form. They are inexpensive and widely available, but have poor tolerance and stability compared to other types.
2. Carbon film resistors: These are similar to carbon composition resistors, but have a thin film of carbon deposited on a ceramic substrate. They are more stable and have better tolerance than carbon composition resistors.
3. Metal film resistors: These are similar to carbon film resistors, but use a thin film of metal instead of carbon. They have better stability, accuracy, and temperature coefficient than carbon film resistors.
4. Wire-wound resistors: These are made by winding a wire around a ceramic or fiberglass core. They are used in high-power applications because they can handle higher currents and dissipate more heat than other types of resistors.
5. Thick film resistors: These are made by depositing a thick film of resistive material onto a ceramic substrate. They have good stability, accuracy, and tolerance, and are commonly used in surface mount technology (SMT) applications.
6. Thin film resistors: These are similar to thick film resistors, but use a much thinner film of resistive material. They have better stability, accuracy, and tolerance than thick film resistors, but are more expensive.
7. Variable resistors: These are resistors that can be adjusted to vary their resistance. They are commonly used as volume controls, tone controls, and variable voltage dividers.

See also: Potentiometer

See also: Thermistor

See also: Photoresistor

The most common type of resistor is the carbon film resistor. This type of resistor is widely used in electronic circuits because of its low cost and availability, and it is suitable for a wide range of applications. Carbon film resistors have a thin layer of carbon deposited onto a ceramic substrate, which provides the resistive element. They are typically available in a range of resistance values and power ratings, and can be found in through-hole and surface mount packages. However, other types of resistors, such as metal film and thick film resistors, are becoming increasingly common due to their improved stability, accuracy, and tolerance.

Resistor Color Code 

The resistor color code is a system of markings used to indicate the resistance value of a resistor. Each color represents a different digit, and the colors are arranged in a specific order to determine the value of the resistor. The color code is typically found on the body of the resistor, and consists of bands of different colors.

The standard color code for four-band resistors is as follows:

• The first band indicates the first digit of the resistance value (the significant digit) and is usually a color from the series: black, brown, red, orange, yellow, green, blue, violet, gray, or white.
• The second band indicates the second digit of the resistance value (the multiplier) and is usually a color from the series: black, brown, red, orange, yellow, green, blue, violet, gray, or white.
• The third band indicates the tolerance of the resistor (the maximum deviation from the nominal resistance value) and is usually a color from the series: gold (5%), silver (10%), or no band (20%).
• The fourth band (if present) indicates the temperature coefficient of the resistor (the rate of change of resistance with temperature) and is usually a color from the series: brown (100 ppm/°C), red (50 ppm/°C), orange (15 ppm/°C), yellow (25 ppm/°C), blue (10 ppm/°C), violet (5 ppm/°C), or gray (1 ppm/°C).

For example, a resistor with the colors brown, black, red, and gold would have a resistance value of 1,000 ohms (10 x 100 = 1,000 ohms) with a tolerance of 5%.

Application of Resistors

Resistors have a wide range of applications in electronics, including:

1. Voltage division: Resistors can be used to divide voltage in a circuit by creating a voltage drop across the resistor. This technique is used in voltage dividers, which are used to measure voltage, control the gain of an amplifier, or create reference voltages.
2. Current limiting: Resistors can be used to limit current in a circuit by creating a resistance that limits the flow of current. This technique is used to protect components from excessive current and to control the brightness of LEDs.
3. Biasing: Resistors can be used to bias a transistor or other semiconductor device, setting the operating point of the device and allowing it to function as an amplifier or switch.
4. Timing: Resistors can be used in conjunction with capacitors to create timing circuits, such as oscillators and time delays.
5. Temperature sensing: Certain types of resistors, such as thermistors and RTDs (resistance temperature detectors), change their resistance in response to changes in temperature. These components are used for temperature sensing and control.
6. Signal conditioning: Resistors can be used for signal conditioning, such as impedance matching, filtering, and attenuating.
7. Power dissipation: Resistors can be used to dissipate power in a circuit, such as in power supplies, voltage regulators, and load resistors.

Overall, resistors are one of the most fundamental components in electronic circuits, used in a variety of applications to control current, voltage, and power.

Series and parallel resistors

Resistors can be connected in two ways – in series and in parallel.

In a series connection, the resistors are connected end to end such that the current flows through each resistor in turn. The total resistance of the series connection is equal to the sum of the individual resistances. In other words, the total resistance is greater than the resistance of any single resistor. The current through each resistor is the same, but the voltage across each resistor is different, with the voltage dropping across each resistor in proportion to its resistance.

When resistors are connected in series, the total resistance (R_total) is equal to the sum of the individual resistances (R1, R2, R3, etc.):

R_total = R1 + R2 + R3 + …

For example, if two resistors of 10 ohms and 20 ohms are connected in series, the total resistance would be:

R_total = 10 ohms + 20 ohms = 30 ohms

In a parallel connection, the resistors are connected side by side such that the current splits and flows through each resistor simultaneously. The total resistance of the parallel connection is less than the resistance of any single resistor. In other words, the reciprocal of the total resistance is equal to the sum of the reciprocals of the individual resistances. The voltage across each resistor is the same, but the current through each resistor is different, with the current through each resistor proportional to its conductance (the reciprocal of its resistance).

When resistors are connected in parallel, the reciprocal of the total resistance (1/R_total) is equal to the sum of the reciprocals of the individual resistances:

1/R_total = 1/R1 + 1/R2 + 1/R3 + …

The total resistance is then calculated as the reciprocal of this sum:

R_total = 1 / (1/R1 + 1/R2 + 1/R3 + …)

For example, if two resistors of 10 ohms and 20 ohms are connected in parallel, the total resistance would be:

1/R_total = 1/10 ohms + 1/20 ohms = 0.1 + 0.05 = 0.15 R_total = 1 / 0.15 = 6.67 ohms

Series and parallel resistor connections have different effects on the overall resistance and current of the circuit. By understanding how to calculate the total resistance and current in each type of connection, one can design circuits with the desired electrical characteristics.

Resistor Manufacturing

Resistors can be manufactured using a variety of techniques, including:

1. Film deposition: Metal film and thin film resistors are made by depositing a thin layer of resistive material on a ceramic substrate using a vacuum deposition process.
2. Wire winding: Wirewound resistors are made by winding a resistive wire around a ceramic or fiberglass core using automated winding machines.
3. Carbon composition: Carbon composition resistors are made by mixing carbon powder and a binder material, which is then molded into a cylindrical shape and baked.
4. Thick film: Thick film resistors are made by screen-printing a thick layer of resistive material on a ceramic substrate and then firing it in a high-temperature furnace.

Once the resistors are manufactured, they may be trimmed or adjusted to achieve a precise resistance value using laser trimming or mechanical trimming techniques. The resistors are then typically coated with a protective layer to prevent damage from environmental factors such as humidity, temperature, and vibration.

Resistivity and Resistance

Resistivity and resistance are related but distinct concepts in electrical circuits.

Resistance is a measure of how difficult it is for electrical current to flow through a material, and it is measured in ohms (Ω). The resistance of a material depends on its geometry (length, cross-sectional area, etc.) and its resistivity (ρ), which is a fundamental property of the material.

Resistivity (ρ) is the intrinsic property of a material that describes how much resistance it offers to the flow of electrical current, and it is measured in ohm-meters (Ω·m). Resistivity is a measure of the material’s ability to conduct electricity and is dependent on factors such as temperature, composition, impurities, and pressure.

The relationship between resistance (R), resistivity (ρ), and geometry (l, A) of a conductor is given by the following equation:

R = ρ (l/A)

where l is the length of the conductor and A is its cross-sectional area. This equation shows that the resistance of a conductor increases with length and decreases with increasing cross-sectional area, while the resistivity of the material remains constant.

In summary, resistance is a measure of how much a material resists electrical current, while resistivity is an intrinsic property of a material that describes its ability to conduct electricity.