Resistors are everywhere in electrical devices. These simple passive components have a huge impact when it comes to circuit design. They come in many different varieties and have a wide range of functions. As apparent from their names, the resistor’s primary function is to provide resistance against an electrical current. Other functions of resistors include:
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- Dividing voltage
- Generating heat
- Powering LEDs
- Matching and loading circuits
- Controlling gain
- Fixing time constraints
Determining the right type of resistor for a project or design depends on multiple factors that need to be planned ahead of time before sourcing resistors for large-scale production. An engineer needs to be aware of the following factors when selecting a type of resistor:
- Resistance
- Tolerance
- Rated power dissipation
- Package and mounting
- Voltage rating
- Material construction
- Inductance and capacitance
- Thermal range
- Operating noise
Resistors come in a variety of different types with their own ratings and sizes. When designing a circuit, it will help to know the advantages and unique functions of each variety of resistors.
Common Types of Linear Resistors
Linear resistors react in accordance with Ohm’s Law. These resistors will change value through direct proportion to the applied voltage and temperature. Typically, linear resistors are broken down into two categories: fixed resistors and variable resistors.
Fixed Resistors
These resistors provide consistent resistance across a circuit. These types of resistors are most commonly used on printed circuit boards and in electronics. Fixed resistors can be sourced with a variety of sizes and material compositions. The most common fixed resistors are as follows:
Carbon Composition Resistors: This type of resistor is one of the oldest types of components on the market. These were commonly used up until the 1960s and are characteristically made from a mixture of powdered carbon and ceramic. Although there are still carbon composition resistors available on the market, they are generally more expensive and less used because other types of fixed resistors have more efficient specs like tolerance, voltage dependence, and stress thresholds.
Wire Wound Resistors: These resistors are constructed with an insulated metal wire wound around a core of non-conductive material like ceramic, plastic, or glass. The metal wires usually consist of highly resistant alloys like nichrome or manganin. These resistors also date back to the turn of the century, but unlike carbon composition resistors they are still widely used today. They are able to resist high power applications, are stable under high temperatures, and provide long-term stability. However, they tend to be more costly and can’t be applied in high-frequency devices.
Thin Film Resistors: These come in two varieties, carbon film resistors and metal film resistors, but have nearly identical constructions. They consist of a ceramic core surrounded by a thin resistive layer of carbon or metal film. Thin film resistors are ideal for use in applications that need high stability, high precision, and low noise like use in medical devices, audio equipment, and testing and measuring devices
Thick Film Resistors: These fixed resistors are most commonly used in consumer devices. They are constructed like thin-film resistors but, as the name indicates, use thick films of metal oxides or cermet oxides. These types of resistors are the lowest cost and most available to source. Typically they are used in any electrical device that uses a battery or AC power source.
Fusible Resistors: These resistors serve two distinct functions: providing resistance against an electrical current and acting as a fuse to break a current if it overloads. Fusible resistors work to not only regulate current, but also act as a failsafe in the case of a power surge. They are constructed similarly to wire wound resistors and are commonly used in expensive electronic devices like TV sets, amplifiers, and safety monitoring and managing equipment.
Variable Resistors
Unlike fixed resistors, the resistance values of these components can be manipulated through a dial, knob, or screw. Since they are able to control voltage and currents, they’re commonly used in radio and audio equipment. The common types of variable resistors include:
Potentiometers: These resistors are commonly controlled through a dial or knob. They consist of three terminals, with the resistance value regulated by a moving contact (aka a wiper) that is connected to a control shaft. Twisting the control shaft will increase or decrease the voltage across the resistor. They are commonly used in audio/visual equipment and in transducers.
Rheostats: Also known as tapped resistors or variable wire wound resistors, these variable resistors use a sliding contact to regulate voltage. The core of the resistor is constructed similarly to wire wound resistors. Like potentiometers, these resistors are used to control voltage in audio/visual equipment and in transducers.
Types of Non-Linear Resistors
Non-linear resistors differ from linear resistors in that their resistance value changes in accordance with temperature, light, or voltage, as opposed to following Ohm’s Law like linear resistors. They can also be used to manipulate the voltage of a current, therefore, are also types of variable resistors. Common types of non-linear resistors include:
Thermistors: This type of variable resistor regulates voltage in proportion to temperature changes. Thermistors have applications in consumer appliances, automobiles, thermometers, and rechargeable batteries.
Varistor Resistors: These types of resistors are made of semiconductor materials like silicon and ceramic metal-oxides. The resistance value of these resistors varies along with the applied voltage of a circuit. Varistors are able to withstand high applications of DC voltage and are often used as a transient voltage suppressor in telecommunication lines, radio communication devices, and in power strips.
Photo Resistor or LDR (Light Dependent Resistors): As the name indicates, the resistance value of these resistors varies with exposure to light. These resistors are used in light sensors and measurement equipment, in consumer appliances, and in photography equipment.
Surface Mount Resistors (SMDs): Also referred to as chip resistors, these resistors are mounted directly into PCBs, unlike other types of resistors that are usually mounted with the through-hole method. This allows for speedier production and saves space on a circuit board. They are used mostly in the production of computing equipment as well as other technology.
Resistors 101: Types of Resistors and their Functions
Resistors are everywhere in electrical devices. These simple passive components have a huge impact when it comes to circuit design. They come in many different varieties and have a wide range of functions. As apparent from their names, the resistor’s primary function is to provide resistance against an electrical current. Other functions of resistors include:
- Dividing voltage
- Generating heat
- Powering LEDs
- Matching and loading circuits
- Controlling gain
- Fixing time constraints
Determining the right type of resistor for a project or design depends on multiple factors that need to be planned ahead of time before sourcing resistors for large-scale production. An engineer needs to be aware of the following factors when selecting a type of resistor:
- Resistance
- Tolerance
- Rated power dissipation
- Package and mounting
- Voltage rating
- Material construction
- Inductance and capacitance
- Thermal range
- Operating noise
Resistors come in a variety of different types with their own ratings and sizes. When designing a circuit, it will help to know the advantages and unique functions of each variety of resistors.
Common Types of Linear Resistors
Linear resistors react in accordance with Ohm’s Law. These resistors will change value through direct proportion to the applied voltage and temperature. Typically, linear resistors are broken down into two categories: fixed resistors and variable resistors.
Fixed Resistors
These resistors provide consistent resistance across a circuit. These types of resistors are most commonly used on printed circuit boards and in electronics. Fixed resistors can be sourced with a variety of sizes and material compositions. The most common fixed resistors are as follows:
Carbon Composition Resistors: This type of resistor is one of the oldest types of components on the market. These were commonly used up until the 1960s and are characteristically made from a mixture of powdered carbon and ceramic. Although there are still carbon composition resistors available on the market, they are generally more expensive and less used because other types of fixed resistorstypes of fixed resistors have more efficient specs like tolerance, voltage dependence, and stress thresholds.
Wire Wound Resistors: These resistors are constructed with an insulated metal wire wound around a core of non-conductive material like ceramic, plastic, or glass. The metal wires usually consist of highly resistant alloys like nichrome or manganin. These resistors also date back to the turn of the century, but unlike carbon composition resistors they are still widely used today. They are able to resist high power applications, are stable under high temperatures, and provide long-term stability. However, they tend to be more costly and can’t be applied in high-frequency devices.
Thin Film Resistors: These come in two varieties, carbon film resistors and metal film resistors, but have nearly identical constructions. They consist of a ceramic core surrounded by a thin resistive layer of carbon or metal film. Thin film resistors are ideal for use in applications that need high stability, high precision, and low noise like use in medical devices, audio equipment, and testing and measuring devices
Thick Film Resistors: These fixed resistors are most commonly used in consumer devices. They are constructed like thin-film resistors but, as the name indicates, use thick films of metal oxides or cermet oxides. These types of resistors are the lowest cost and most available to source. Typically they are used in any electrical device that uses a battery or AC power source.
Fusible Resistors: These resistors serve two distinct functions: providing resistance against an electrical current and acting as a fuse to break a current if it overloads. Fusible resistors work to not only regulate current, but also act as a failsafe in the case of a power surge. They are constructed similarly to wire wound resistors and are commonly used in expensive electronic devices like TV sets, amplifiers, and safety monitoring and managing equipment.
Variable Resistors
Unlike fixed resistors, the resistance values of these components can be manipulated through a dial, knob, or screw. Since they are able to control voltage and currents, they’re commonly used in radio and audio equipment. The common types of variable resistors include:
Potentiometers: These resistors are commonly controlled through a dial or knob. They consist of three terminals, with the resistance value regulated by a moving contact (aka a wiper) that is connected to a control shaft. Twisting the control shaft will increase or decrease the voltage across the resistor. They are commonly used in audio/visual equipment and in transducers.
Rheostats: Also known as tapped resistors or variable wire wound resistors, these variable resistors use a sliding contact to regulate voltage. The core of the resistor is constructed similarly to wire wound resistors. Like potentiometers, these resistors are used to control voltage in audio/visual equipment and in transducers.
Types of Non-Linear Resistors
Non-linear resistors differ from linear resistors in that their resistance value changes in accordance with temperature, light, or voltage, as opposed to following Ohm’s Law like linear resistors. They can also be used to manipulate the voltage of a current, therefore, are also types of variable resistors. Common types of non-linear resistors include:
Thermistors: This type of variable resistor regulates voltage in proportion to temperature changes. Thermistors have applications in consumer appliances, automobiles, thermometers, and rechargeable batteries.
Varistor Resistors: These types of resistors are made of semiconductor materials like silicon and ceramic metal-oxides. The resistance value of these resistors varies along with the applied voltage of a circuit. Varistors are able to withstand high applications of DC voltage and are often used as a transient voltage suppressor in telecommunication lines, radio communication devices, and in power strips.
Photo Resistor or LDR (Light Dependent Resistors): As the name indicates, the resistance value of these resistors varies with exposure to light. These resistors are used in light sensors and measurement equipment, in consumer appliances, and in photography equipment.
Surface Mount Resistors (SMDs): Also referred to as chip resistors, these resistors are mounted directly into PCBs, unlike other types of resistors that are usually mounted with the through-hole method. This allows for speedier production and saves space on a circuit board. They are used mostly in the production of computing equipment as well as other technology.
With a wide range of types and applications, when companies need a steady source of resistors, it is best to go with a company that can stock and schedule delivery. Sensible Micro has access to a reliable network of suppliers for micro components, including all types of resistors. We pride ourselves on securing our customers high quality components as well as minimizing lead times on all our orders. Our stocked inventory is kept in a temperature-controlled warehouse and each outgoing shipment is inspected in our in-house inspection and testing lab for quality assurance. Need resistors? Contact one of our sourcing experts today.
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A Complete Guide to Resistors
What are the key characteristics and specifications that affect the choice of resistor? Factors that should be taken into consideration include initial tolerance and value selection. However, the tolerance or variation of the value of a resistor is affected by multiple parameters, as explained below.
Temperature Coefficient
This is a measure of the variation of the nominal value as a result of temperature changes. Generally quoted as a single value in parts per million per degree centigrade (or Kelvin), it can be positive or negative. The equation for calculating the resistance at a given temperature is:
Rt=Ro[1+α(T-To)]
Where Ro is nominal value for room temperature resistance, To is the temperature at which the nominal resistance is given, T is operating temperature and α is the TCR.
Put simply, a 1 MΩ resistor with a TCR of 50ppm/K will change by 50Ω per 1 degree of temperature rise or fall. This may not sound like much but consider if you were using this resistor as the gain resistor in a x10 non-inverting amplifier circuit with 0.3v on the + input. The worst-case change in output could be as much as 7.5mv which is equivalent to about 5LSBs in a 5v 12-bit ADC circuit. This kind of change can be quite noticeable in precision design. Remember also that the TCR is quoted as ±x ppm/C so it is feasible, although unlikely, that the second resistor in the circuit could change in the opposite direction hence double the possible error. Finally, it’s worth noting that some precision resistors quote variable TCRs over the temperature range the circuit is operating in, and this can complicate the design process significantly.
Resistor Ageing or Stability
Ageing and stability are a complex amalgam of multiple changes to the value of a resistance value over time and are the result of temperature cycling, high-temperature operation, humidity ingress and so on. Typically, the value will lead to an increase in resistance over time as conduction atoms migrate within the device.
Thermal Resistance
The thermal resistance is a measure of how well the resistor can dissipate power into the environment. In practice, engineers use thermal resistance to model the heat dissipation for a system – it is thought of as a set of series ‘thermal resistors’, each representing one element of the heat dissipation of the system.
This is mainly important if the design means the resistor is running at or near its maximum value and can significantly affect the long-term reliability of the system. An example of where this parameter could be used is to calculate the size of a PCB pad or ground plane requirement that would be used to keep the resistor’s value and operating temperature within acceptable limits.
Thermal and Power Rating
All resistors come with a maximum power rating, specified in watts. This can be anything from 1/8th watt right up to 10s of watts for power resistors. In a first pass analysis, the engineer would check that the resistor is operating within its rated value. The equation for calculating this is P=I² R, where p is the power dissipated in the resistor, i is the current flowing and R is the resistance. Sadly, things can be more complicated than this; for exact work, the engineer needs to take account of the thermal derating curve for the resistor. This specifies the amount by which the designer needs to de-rate the maximum power dissipation above a given temperature.
This might seem theoretical as often the de-rating kicks in at quite high temperatures, but a power circuit in an enclosed housing in a hot region can often exceed the cut in point and the maximum power dissipation will need to be reduced appropriately. It’s also worth noting that the maximum operating voltage of a resistor is de-rated with power dissipation.
Resistor Noise
Any electronic component that has flowing electrons is going to be a source of noise, and resistors are no different in this respect. In high gain amplifier systems or when dealing with very low voltage signals, it needs to be considered.
The major contributor to noise in a resistor is thermal noise caused by the random fluctuation of electrons in the resistive material. It is generally modelled as white noise (i.e. a constant RMS voltage over the frequency range) and is given by the equation E=√4RkT∆F where E is the RMS noise voltage, R is the resistance value, k is Boltzmann’s constant, T is the temperature and Δf is the bandwidth of the system.
It is possible to lessen system noise by reducing the resistance, the operating temperature or the system’s bandwidth. Additionally, there is another type of resistor noise called current noise which is a result of the electron flow in devices. It is rarely specified but can be compared if the standard numbers using IEC60195 are available from the manufacturer.
High-Frequency Behaviour
The final challenge to consider is the high-frequency performance of the particular resistor. In simple terms, you can model a resistor as a series inductor, feeding the resistor which has a parasitic capacitor in parallel with it.
At frequencies as low as 100Mhz (even for surface mount resistors which have lower parasitic values than through-hole parts) the parallel capacitance can start to dominate, and the impedance will drop below nominal. At a higher frequency still, the inductance may predominate, and the impedance will start to increase from its minima and may well end up above the nominal value.
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What are the key characteristics and specifications that affect the choice of resistor? Factors that should be taken into consideration include initial tolerance and value selection. However, the tolerance or variation of the value of a resistor is affected by multiple parameters, as explained below.
Temperature Coefficient
This is a measure of the variation of the nominal value as a result of temperature changes. Generally quoted as a single value in parts per million per degree centigrade (or Kelvin), it can be positive or negative. The equation for calculating the resistance at a given temperature is:
Rt=Ro[1+α(T-To)]
Where Ro is nominal value for room temperature resistance, To is the temperature at which the nominal resistance is given, T is operating temperature and α is the TCR.
Put simply, a 1 MΩ resistor with a TCR of 50ppm/K will change by 50Ω per 1 degree of temperature rise or fall. This may not sound like much but consider if you were using this resistor as the gain resistor in a x10 non-inverting amplifier circuit with 0.3v on the + input. The worst-case change in output could be as much as 7.5mv which is equivalent to about 5LSBs in a 5v 12-bit ADC circuit. This kind of change can be quite noticeable in precision design. Remember also that the TCR is quoted as ±x ppm/C so it is feasible, although unlikely, that the second resistor in the circuit could change in the opposite direction hence double the possible error. Finally, it’s worth noting that some precision resistors quote variable TCRs over the temperature range the circuit is operating in, and this can complicate the design process significantly.
Resistor Ageing or Stability
Ageing and stability are a complex amalgam of multiple changes to the value of a resistance value over time and are the result of temperature cycling, high-temperature operation, humidity ingress and so on. Typically, the value will lead to an increase in resistance over time as conduction atoms migrate within the device.
Thermal Resistance
The thermal resistance is a measure of how well the resistor can dissipate power into the environment. In practice, engineers use thermal resistance to model the heat dissipation for a system – it is thought of as a set of series ‘thermal resistors’, each representing one element of the heat dissipation of the system.
This is mainly important if the design means the resistor is running at or near its maximum value and can significantly affect the long-term reliability of the system. An example of where this parameter could be used is to calculate the size of a PCB pad or ground plane requirement that would be used to keep the resistor’s value and operating temperature within acceptable limits.
Thermal and Power Rating
All resistors come with a maximum power rating, specified in watts. This can be anything from 1/8th watt right up to 10s of watts for power resistors. In a first pass analysis, the engineer would check that the resistor is operating within its rated value. The equation for calculating this is P=I² R, where p is the power dissipated in the resistor, i is the current flowing and R is the resistance. Sadly, things can be more complicated than this; for exact work, the engineer needs to take account of the thermal derating curve for the resistor. This specifies the amount by which the designer needs to de-rate the maximum power dissipation above a given temperature.
This might seem theoretical as often the de-rating kicks in at quite high temperatures, but a power circuit in an enclosed housing in a hot region can often exceed the cut in point and the maximum power dissipation will need to be reduced appropriately. It’s also worth noting that the maximum operating voltage of a resistor is de-rated with power dissipation.
Resistor Noise
Any electronic component that has flowing electrons is going to be a source of noise, and resistors are no different in this respect. In high gain amplifier systems or when dealing with very low voltage signals, it needs to be considered.
The major contributor to noise in a resistor is thermal noise caused by the random fluctuation of electrons in the resistive material. It is generally modelled as white noise (i.e. a constant RMS voltage over the frequency range) and is given by the equation E=√4RkT∆F where E is the RMS noise voltage, R is the resistance value, k is Boltzmann’s constant, T is the temperature and Δf is the bandwidth of the system.
It is possible to lessen system noise by reducing the resistance, the operating temperature or the system’s bandwidth. Additionally, there is another type of resistor noise called current noise which is a result of the electron flow in devices. It is rarely specified but can be compared if the standard numbers using IEC60195 are available from the manufacturer.
High-Frequency Behaviour
The final challenge to consider is the high-frequency performance of the particular resistor. In simple terms, you can model a resistor as a series inductor, feeding the resistor which has a parasitic capacitor in parallel with it.
At frequencies as low as 100Mhz (even for surface mount resistors which have lower parasitic values than through-hole parts) the parallel capacitance can start to dominate, and the impedance will drop below nominal. At a higher frequency still, the inductance may predominate, and the impedance will start to increase from its minima and may well end up above the nominal value.
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