Locked in resistance: Exploring the world of fixed value resistors

Fixed value resistors are electronic components that provide a specific amount of resistance to the flow of electric current

As the name suggests, fixed value resistors have a predetermined resistance value that is set during their manufacturing process and cannot be changed or adjusted by the user.

The resistance value of a fixed value resistor is measured in ohms, this and the power rating of the resistor is determined by the physical properties of the resistor, such as its size, material, and shape. Fixed value resistors come in a variety of types, including carbon composition, film, metal oxide, and wire wound resistors, each with their own unique properties and characteristics.

This guide covers topics as below

Types of fixed value resistors
Carbon composition resistors
Film resistors
Properties of fixed value resistors
Applications of fixed value resistors
Selection and installation guide
Steps for soldering and mounting a resistor
Maintenance and troubleshoot

Types of fixed value resistors

There are several types of fixed value resistors available, each with their own advantages and disadvantages. Some of the most common types include:

Carbon composition resistors

Carbon composition resistors are fixed value resistors made from a blend of carbon powder and an insulating binder material, such as ceramic or plastic. The mixture is fashioned into a solid cylindrical rod or tube and marked with a colored band to indicate the resistance value after being cut to the correct length.

Figure 1: Carbon composition resistor

Carbon composition resistors have several distinct advantages, including a wide range of resistance values, ranging from a few ohms to several megaohms, making them a flexible option for various circuit designs. These resistors also have a high tolerance, which means that their actual resistance value can deviate by up to 10% or more from the nominal value, making them ideal for circuits where precise resistance values are not essential. Additionally, carbon composition resistors offer excellent long-term stability, meaning that their resistance value remains relatively constant over time, even under varying operating conditions.

However, it's important to note that these resistors have a high temperature coefficient, meaning that their resistance value changes considerably with changes in temperature. This can be advantageous in certain applications where temperature changes are anticipated, but it may be detrimental in others where temperature stability is crucial.

Film resistors

Film resistors are a type of fixed value resistor that are made from a thin film of resistive material, such as carbon or metal, deposited onto a ceramic or plastic substrate. The resistive film is then coated with a protective layer, such as epoxy or lacquer, to prevent damage and improve long-term stability. Film resistors are a popular choice in many electronic applications due to their unique characteristics. One of the main advantages of film resistors is their high precision. They can be manufactured with very tight tolerances of 1% or better, ensuring accurate and consistent resistance values. This makes them ideal for use in circuits that require precise and stable resistance values. Film resistors have low noise generation and low temperature coefficient. They produce very little electrical noise, which makes them well-suited for use in audio amplifiers and other applications where noise interference can be a problem.

Figure 2: Film resistor

Film resistors also offer good long-term stability, meaning that the resistance value remains relatively constant over time, even under varying operating conditions. This ensures that the circuit remains accurate and reliable over a long period of time.

There are several types of film resistors, each with its own unique characteristics that make them suitable for different applications.

One type of film resistor is the carbon film resistor, which is made by depositing a layer of carbon onto a ceramic substrate. Carbon film resistors are inexpensive and have a low noise generation, making them popular in audio applications. They also have a high temperature coefficient, which can be beneficial in circuits that require automatic temperature compensation.

Metal film resistors are another type of film resistor, which are made by depositing a thin layer of metal onto a ceramic substrate. Metal film resistors offer a high degree of precision, typically with tolerances of 1% or better. They also have a low noise generation and a low temperature coefficient, making them suitable for a variety of applications, including precision measurement and control circuits.

Thick film resistors are made by depositing a thick layer of resistive material onto a ceramic substrate. Thick film resistors are less precise than other types of film resistors, but they are highly durable and can handle high power applications. They also have a low noise generation and are less expensive than other types of film resistors.

Figure 3: Thich film resistor

Another type of film resistor is the foil resistor, which is made by winding a thin resistive foil around a ceramic core. Foil resistors are highly precise and can be manufactured with very tight tolerances. They also have a low temperature coefficient and are highly stable over time, making them ideal for use in precision measurement applications.

Figure 4: Foil resistor

Wirewound resistors

Wire wound film resistors are a type of film resistor that is made by winding a wire around a ceramic core. The wire is coated with a thin layer of resistive material, such as nickel chromium, to provide the desired resistance value. Wire wound film resistors are often used in applications that require high power handling capability and precision.

Figure 5: Wire wound resistor

Properties of fixed value resistors

Fixed value resistors have several important properties that determine their performance and suitability for different applications. Some of the most important properties of fixed value resistors include:

Resistance value: This is the most fundamental property of a resistor and refers to the measure of its opposition to the flow of electrical current, measured in ohms (Ω). Fixed value resistors have a specific resistance value that is predetermined during manufacturing.
Tolerance: This refers to the degree of variation in resistance value from the nominal value. Fixed value resistors typically have a tolerance of a few percent, but high-precision resistors with tolerances as low as 0.1% are also available.
Power rating: This is the maximum amount of power that a resistor can safely dissipate without damage, measured in watts (W). Fixed value resistors have different power ratings depending on their construction and size.
Temperature coefficient of resistance (TCR): This is the rate at which the resistance value of the resistor changes with temperature. Fixed value resistors typically have a TCR of a few hundred parts per million per degree Celsius (ppm/°C).
Stability: This refers to the ability of a resistor to maintain its resistance value over time and under different operating conditions. Some types of fixed value resistors, such as wirewound and metal film resistors, have high stability, while others, such as carbon composition and thick film resistors, are less stable.
Noise: This refers to the random fluctuations in the resistance value of a resistor caused by thermal or electrical effects. Carbon composition and thick film resistors tend to have higher noise levels compared to other types of fixed value resistors.

Applications of fixed value resistors

Fixed value resistors are widely used in electronic circuits and devices. Here are some of the common applications of fixed value resistors:

Voltage division: Fixed resistors are used to divide voltage in a circuit. By placing a fixed resistor in series with another resistor, a voltage divider circuit can be created. This can be used for applications such as level shifting, biasing circuits, and signal conditioning.
Current limiting: Fixed resistors can be used to limit current in a circuit. By placing a resistor in series with a load, the current flowing through the load can be limited to a specific value. This can be used for applications such as LED current limiting and motor control.
Signal conditioning: Signal conditioning involves modifying an input signal to suit the requirements of a specific application. One common use of fixed value resistors in signal conditioning is to create a voltage divider circuit, which can be used to reduce the amplitude of a signal, adjust its offset, or provide a reference voltage.

For example, in sensor applications, the output voltage of a sensor may not be in the range that is required for the rest of the circuit. By using a voltage divider circuit with two fixed resistors, the output voltage of the sensor can be divided down to the appropriate range. The ratio of the two resistors determines the output voltage of the divider circuit.

In addition to voltage division, fixed resistors can also be used to provide a bias voltage to a circuit. For instance, in an amplifier circuit, a fixed resistor can be used to bias the input transistor to operate in the linear region, which results in better linearity and reduced distortion.
Timing circuits: Fixed value resistors are commonly used in timing circuits to set the timing characteristics of the circuit. Timing circuits are used to generate precise delays, oscillations, or pulses, which are used in a wide range of applications such as digital systems, communications, and control systems. One of the most common timing circuits is the RC circuit, which consists of a resistor and capacitor in series. The resistor sets the charging and discharging time of the capacitor, which determines the overall timing of the circuit. The time constant of the RC circuit is given by the product of the resistance and capacitance, and it determines the rate of charge or discharge of the capacitor. In addition to RC circuits, fixed value resistors are also used in other types of timing circuits, such as the 555-timer circuit, which is a popular integrated circuit used to generate precise timing signals. In the 555-timer circuit, fixed resistors are used to set the charging and discharging time of the timing capacitor, which determines the overall timing characteristics of the circuit.
Voltage regulation: Fixed resistors are used in voltage regulator circuits to provide a stable output voltage. By placing a fixed resistor in the feedback circuit of a voltage regulator, the output voltage can be adjusted to a specific value.

Selection and installation guide

The selection and installation of fixed value resistors depend on the specific application requirements and the characteristics of the resistor itself. Here are some general guidelines for selecting and installing fixed value resistors:

Selection:

Determine the resistance value required for the specific application. This may involve calculations or measurements based on the circuit requirements.
Choose the appropriate resistor type based on the application requirements. Fixed value resistors come in a variety of types, such as carbon composition, metal film, metal oxide, and wire wound resistors, each with different properties and characteristics.
Consider the tolerance and power rating of the resistor. The tolerance indicates the maximum deviation from the nominal resistance value, and the power rating indicates the maximum power that the resistor can safely dissipate without overheating.
Consider the environmental and operating conditions, such as temperature, humidity, vibration, and noise, and choose a resistor that can withstand those conditions.

Installation:

Ensure that the resistor is securely mounted and properly spaced to avoid contact with other components or surfaces.
Consider the thermal management of the resistor, especially for high-power applications. Provide adequate ventilation and heat sinks to dissipate the heat generated by the resistor.
Test the circuit and the resistor to ensure that the resistance value and other characteristics are within the expected range and that the resistor is functioning properly.

Steps for soldering and mounting a resistor

Introduction: Soldering and mounting a resistor is a common task in electronics. By following these simple steps, you can ensure that the resistor is securely mounted, and the solder joint is of high quality.

Steps:

Gather tools and materials: Collect all the necessary tools and materials for the task. You will need a soldering iron, solder wire, flux, needle-nose pliers, wire cutters, and the resistor itself.
Cut the resistor leads: Use wire cutters to trim the resistor leads to the appropriate length, leaving enough length for proper spacing and mounting.
Apply flux: Apply a small amount of flux to the leads of the resistor. Flux helps to improve the wetting and adhesion of the solder to the leads. Most Solder types include flux in their composition, so this step may only be necessary for larger resistors.
Position the resistor: Position the resistor in the desired location on the circuit board, ensuring that the leads are aligned with the appropriate pads or holes.
Hold the resistor in place: Use needle-nose pliers to hold the resistor in place while soldering the leads.
Heat the soldering iron: Turn on the soldering iron and let it heat up to the appropriate temperature. The temperature should be hot enough to melt the solder, but not so hot that it damages the resistor or other components.
Apply solder: Touch the solder wire to the heated tip of the soldering iron to melt a small amount of solder onto the tip. Then, touch the tip of the iron to the junction of the resistor lead and pad, allowing the solder to flow onto the junction.
Remove the iron: Remove the soldering iron from the junction but hold the resistor and lead steady until the solder cools and solidifies.
Inspect the joint: Inspect the joint to ensure that the solder has flowed smoothly and that there are no cold solder joints or solder bridges.
Trim excess leads: Use wire cutters to trim the excess leads of the resistor to the appropriate length.
Repeat: Repeat the process for each lead of the resistor, making sure to solder them in the correct positions.

Maintenance and troubleshoot

Maintenance of fixed value resistors is generally not required, as they are passive components that do not have any moving parts or wear out over time. However, there are a few things you can do to ensure their proper functioning:

Check for physical damage: Inspect the resistor for any physical damage, such as cracks or breaks. If the resistor is damaged, it should be replaced.
Check for discoloration: If the resistor is discolored, it may have been subjected to excessive heat or voltage. Check the circuit to ensure that the resistor is not being overloaded, and replace the resistor if necessary.
Measure the resistance: Use a multimeter to measure the resistance of the resistor. Compare the measured value to the nominal value to ensure that the resistor is within tolerance.

Troubleshooting fixed value resistors can be done by checking the circuit and the resistor itself. Some common issues and their solutions include:

Open circuit: If there is no resistance reading on the multimeter, the resistor may be open. Check the circuit for any breaks or discontinuities and replace the resistor if necessary.
Short circuit: If the resistance reading is zero or close to zero, the resistor may be shorted. Check the circuit for any unintended connections or solder bridges and replace the resistor if necessary.
Drift in resistance: If the resistance value of the resistor is changing over time, it may be due to a change in temperature or other environmental factors. Check the circuit to ensure that it is operating within the expected temperature range and replace the resistor if necessary.

Farnell has partnered with many different suppliers with fixed resistors portfolio includes resistors - fixed value, chip SMD resistors, current sense SMD resistors, fixed network resistors, fixed precision resistor networks, MELF SMD resistors, miscellaneous fixed value, panel / chassis mount resistors, RF resistors, resistor accessories, resistor kits & assortments, through hole resistors, zero ohm network resistors, that can be used for different kind of applications in the industrial environment.