Power Resistors

Resistors are one of the passive components used in electronic devices. As the name suggests, they resist the flow of excessive electrical power or voltage passing through the circuit in a precise and controlled manner.

Resistors are of various types that are used extensively in electronic equipment. Each kind of resistor comes with different properties and configurations such that its usage is different from one circuit to another. Power resistors are critical components of the equipment used in power infrastructure, especially in the transmission and distribution networks.

Resistor types

Power resistors are available in different types and come with diverse ratings, physical shapes and sizes. Some of the types are listed below.

Wirewound resistors: These resistors are the oldest one, which has the properties of high power ratings and low resistance values. The price of the Wirewound resistors is comparatively higher than that of carbon resistors. Wirewound resistors are generally used when carbon resistors start to lose their effectiveness. The main disadvantage of these resistors is that they can affect the behaviour pattern of the circuit at high frequencies.

Carbon resistors: These resistors are the most commonly used in electronics manufacturing. They are temperature sensitive, long lasting, available in a wide range of values and are priced cheap.

Metal film resistors: These resistors are almost similar to carbon resistors. The thickness of the coating layer determines the resistivity and they can be made or calibrated to very precise dimensions. Metal film resistors are accurate as well as stable and reliable.

Thick and thin film resistors: These two resistors look very similar to the naked eye, but thick film and thin film technologies are very different. The resistive element of thick film resistors is much thicker in comparison with the thin film resistors.

Metal oxide resistors: These resistors are widely used these days because of their good performance. They can be used at high voltage and release a low level of operating noise. They come in a broad range of resistances with high temperature stability.

Things to keep in mind while selecting resistors

There are several factors that need to be examined while choosing the right resistor type for your application. A resistor accounts for a small percentage of the cost of the circuit, but could potentially be the source of lots of damage. Often, resistors are used in higher-voltage applications. One should ensure the resistor is suitable to use for the circuit voltage. It is important to check the voltage rating or it can cause sparks and break down the system. Resistors are self-heating components and a good resistor should be able to handle power dissipation, meet safety parameters and specifications. Always look for a resistor that has a heat-sink.

Source: https://www.circuitcrush.com/wp-content/uploads/Resistor-Characteristics.jpg

Also, avoid resistors that have a power rating less than the power it will be handled. It is preferred to go for a resistor with at least two times higher power levels. When power resistors are operated above their specified power rating, it can cause short circuits and reduce their lifespan.

Different types of resistors are available, but they look very similar on the outside. They may also be available with the same specifications—initial tolerance, noise, load life stability, etc. But each type of resistor is made of a different resistive material and produced differently, and therefore exhibits varied performances.

Safety tips

When the ambient temperature exceeds the specified standard temperature, it is not safe for resistors. Resistors are not resistant to combustion and emit smoke, red heat, gas, etc. They should be shielded or coated with resin, so that their performance, reliability and stability are not affected. The resistor properties can be affected, by chipping or removing such protective coatings. To sustain these protective coatings for a longer time, giving a shock to the resistors and handling them with hard tools like pliers and tweezers should be avoided.

In applications where resistors are subject to irregular current surges and spikes, care should be taken that the components used are capable of withstanding the increased load for a brief time. Regardless of the power level applied, due to the wiring pattern, heat generated by adjacent components, circuit boards, etc., the temperature of the resistor can increase drastically. Therefore, it is important to keep a check on all these to avoid any damage on peripheral components and boards.

One should also remember that excessive bending of printed circuit boards is a harmful practice to protect the resistors from abnormal stress. There is also a need to test the temperature coefficients. This is quite critical when dealing with high-current or high-temperature resistors, as the resistance drifts very dramatically. Lastly, resistors tend to lose their characteristics when immersed in a solvent for a long time.

Here are a few resistors from prominent players in the power resistor market:

Product: Thick film high power resistors; Manufacturer: Vishay Intertechnology

These resistors are designed for direct mounting on a heat sink and provide high-power dissipation and pulse handling capabilities to help designers minimize part counts and lower cost in automotive applications. For inverters and converters in electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV), they act as pre-charging or discharge resistors. They are RoHS-compliant devices with a non-inductive design and tolerances down to ±1 per cent.

Product: CSS series; Manufacturer: Bourns

These high power current sense resistors come in both 2-terminal and 4-terminal versions. They detect and convert current to an easily measured voltage, which is proportional to the current going through the device. The resistors provide long-term stability, low inductance, low thermal EMF, and are fully AEC-Q200 compliant.

Product: ERJU resistor series; Manufacturer: Panasonic

This series is made from an alloy of palladium and silver in the inner terminals, which prevents the resistors from sulfurisation and malfunctioning under harsh exposure. These resistors have high resistance to sulfurisation achieved by adopting an Ag-Pd-based inner electrode. They have a resistance tolerance of ±0.5 per cent. They are suitable for both reflow and flow soldering. Besides, they are AEC-Q200 compliant as well as RoHS compliant.

Product: LTR50 series; Manufacturer: ROHM

This series of high-power terminal-wide, thick-film chip resistors (10-910 mΩ) is suitable for current detection in a variety of applications, including inverters, ACs, and energy-efficient devices. Features provide resistance to surge and sulfuration and the resistors are available in 48 values.