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.

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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.

Resistors are commonplace power components in industrial buildings ' but their use is not limited to the factory floor. In many industries, resistors play a vital role in the power management of electrical equipment.

Resistance Beyond the Four Walls - Industrial Resistors Aren't Just for the Factory Floor

Simone Bruckner, Managing Director | Cressall

Resistors are passive electronic components that primarily create resistance to limit the flow of electric current. They also have many other uses, including adjusting signal levels, dividing voltages and handling unnecessary influxes of power, making them an essential piece of equipment in many electrical networks and electronic circuits.

In industrial buildings, resistors can take the shape of load banks, which test a back-up power source without connecting it to its normal operating load by simulating an electronic load. This is useful in industrial buildings to test back up generators or an uninterruptible power supply. 

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Without proper testing, an electrical fault could cause a building's power supply to cease and leave, for example, a production line at a halt. This demonstrates just one of the reasons why resistors are essential to industrial operations, but there are many other applications that require resistors.

 

Renewable energy 

As well as benefitting local industrial power supplies, resistors have a wider role in other power sectors ' including renewable energy. As countries across the globe strive to integrate more renewable energy into their power supply, resistors can help by increasing the efficiency of renewable energy generation equipment and protecting it from damage.

Solar power is an attractive renewable energy source as it's easy to install, scalable and can be implemented in a variety of locations, such as large solar farms or on residential or commercial buildings. However, efficiency is a limiting factor to its growth, as many panels barely surpass 20 per cent efficiency.

Many solar farms adopt solar tracking systems to improve efficiency, which use motors to move the panels so they're always directly orientated to the sun. Braking resistors can dissipate excess voltage generated by the decelerating motors to ensure the panels stop moving when required and land in the optimum position. As part of a regenerative braking system, resistors can help put any wasted braking energy back into the system to further increase efficiency.

Resistors also play an important role in wind power generation. Disturbances on the grid can cause high transient currents and voltages that can affect wind turbine generator rotors. There are many causes of grid disturbances, including power station faults and damage to electric transmission lines. With this in mind, it's important to protect the rotor from damage by short circuiting the rotor windings using a resistor during the period of disturbance. 

The uses of resistors in wind power are very similar to those in tidal power, as a tidal stream generator operates in much the same way as a wind turbine. Tidal power, although currently less popular, has great future potential as a reliable form of renewable energy as tides are more predictable than the wind and sun.

Like wind power, tidal power benefits from crowbar resistors, load banks and dynamic braking resistors. A power disturbance can cause a runaway condition that leads to overspeed, which can stress the turbine blade and eventually damage the mechanical structure. Here, a dynamic braking resistor can be implemented to prevent this by dissipating excess power.

 

Out at sea

Tidal power isn't a resistor's only marine application. Resistors can be found in a variety of offshore vessels, including ships, crane barges and oil rigs. It's becoming increasingly more common for offshore vessels to use electric drives in a range of powered applications, from cranes and propellers to cable laying and electric bow thrusters. 

One significant benefit of electric drives is the opportunity to replace mechanical braking with dynamic and regenerative braking systems. Electric braking systems can save weight, therefore reducing vessel fuel consumption. Energy efficiency can be enhanced with regenerative braking, which puts the wasted energy back into the system. Electric braking also offers greater control and reliability over mechanical braking.

Elsewhere on offshore platforms, neutral earthing resistors protect equipment from damage in the event of earth faults. They limit the current that flows through the neutral point of a transformer to a safe level that still allows operation of equipment. It's important to avoid damage to equipment in an offshore vessel, as the necessary replacement parts, or skilled engineers, may be back on land.

With over 100 years' experience in engineering, Cressall supplies a range of industrial resistors, from load bank and crowbar resistors to braking and neutral earthing. In marine applications, we can offer water cooled and air cooled resistors. Our advanced computer-aided design tools, highly skilled engineers and extensive library of data allow us to respond quickly and accurately to new product requests. 

While resistors are an essential power management mechanism in industrial buildings, their benefits extend far beyond keeping the production line moving. Resistors have claimed the land and sea through renewable energy and offshore operations ' protecting equipment and increasing efficiency across the globe.

 

The content & opinions in this article are the author's and do not necessarily represent the views of AltEnergyMag

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