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.

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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 M&#; resistor with a TCR of 50ppm/K will change by 50&#; 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 IEC 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.

How To Choose the Right Resistor(s) for Your Project | Quest Components

26 Sep, | Quest Components

How to Choose the Right Resistor(s) for Your Project

Resistors are among the most common of all electronic components, but not all resistors are the same. Resistors are made of different materials and come in different types. Each has unique properties that makes it better suited for specific applications and less than ideal for other applications. Here is what you need to know to choose the right resistor(s) for your project.

Resistance

The basic criterion for selecting your resistor is its resistance value. Resistors are sold in standardized value ranges set by the IEC (International Electrotechnical Commission). The values in each range follow an exponential curve, keeping the tolerance within a designated percentage. Custom resistance values are available, but they are special order items. Let us know if you have any unusual resistance values you need and we can provide with a quote for supplying the parts you need.

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Tolerance

Tolerance is the amount that the resistance of a specified resistor can vary from its target value. Most resistors have a 5% tolerance, though 1% tolerances are readily available. Large &#;power&#; resistors tend to have a tolerance of 10% or even 20%, though precision models are available. High-precision resistors, with tolerances of 0.1% to 0.01% and lower are available, but tend to be a little more pricey when compared to the basic 5% resistor. The resistors with high-precision tolerances are highly useful for instrumentation, precision measuring devices, and reference applications to name a few.

Package and Mounting

Resistors are packaged in different ways and have different mounting styles. For one-off, hand-soldered applications, this is not necessarily a big concern. If you are mass producing computer chips, the packaging and mounting style could become a primary consideration.

Some common packages are:

• Axial leaded
• Surface mount
• Radial leaded
• Wire mounted

Power Dissipation Rating

Since the function of a resistor is to impede current flow, some power is dissipated as heat. Whether this matters depends on the size of the resistor, the size of the device in which it is placed, and the heat tolerance of the device. A tiny single resistor in an analog device is unlikely to dissipate enough power to be noticeable, while a bank of large resistors working at their maximum capacity can put out significant heat.

Voltage Rating

In physically small devices, the voltage ratings tend to be low. In large, high voltage systems, it is generally better and safer to raise the voltage of the circuit by connecting multiple resistors in series rather than using a single resistor at its max voltage rating.

Resistive Material

Not counting semiconductors, there are three basic types of resistive materials: composition, metal film, and wire-wound. Each has its own unique properties:

Film Resistors are made of conductive metal oxide paste on a ceramic substrate, and are laser cut to create tight tolerances. Due to their low noise and temperature stability, film resistors are ideal for radio frequency or high frequency applications.

Some common types of film resistors are

Wire-wound resistors are made by winding a wire of thin metal alloy onto an insulating ceramic. With high power ratings and precision low ohmic value, these resistors are a great choice for measuring circuits and heat sinks. In addition to resistance, some also have inductance, producing a combination effect known as impedance.

Some common types of wire wound resistors are

Composition resistors are made of graphite or carbon dust bound to non-conductive ceramic clay. They are inexpensive, low- to medium-power, low-inductance, and good for a variety of applications. However, noise and stability become problematic when these resistors get hot.

Some common types of composition resistors are

Temperature Range

In normal ambient temperatures, checking the power dissipation of the resistor is fine. If the resistor will operate in significantly elevated temperatures, though, it is important to look at the power dissipation derating curve. As the resistor gets closer to its maximum allowable temperature, the less power can be dissipated. This puts the resistor, and ultimate the entire device, at risk for overheating and failure.

Noise

Resistors can put out three types of noise: shot noise, flicker noise, and thermal noise. Shot noise sounds something like a rushing river, but it is generally an extremely low level of not-unpleasant white noise. Flicker noise is more random and can be far more annoying. Composition resistors have the most flicker noise, and larger resistors have less than smaller ones of the same type. Thermal noise becomes a problem at higher temperatures, and metal film resistors tend to have the least. Overall, lower-value resistors create less noise than higher-value resistors.

Choosing just the right resistor(s) for your project can be complex. It is best to work with a professional who can provide the guidance you need to ensure that you truly get what you need.

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