What You Should Know About Power Resistors

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ResistorsResistors are one of the most commonly used passive components 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.

By Potshangbam July

Resistors are of various types and are used extensively in electronic equipment. Each kind of resistor comes with different properties and configurations, and 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.

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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 are the oldest resistors, having properties like high power ratings and low resistive values. The price of these 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 are more or less similar to carbon film resistors. The resistivity is controlled by the thickness of the coating layer, and they can be made or calibrated to very specific 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 far 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 a lot 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, as well as meet safety parameters and specifications. Always look for a resistor that has a heat-sink.

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

There are different types of resistors 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 an ambient temperature exceeds the rated ambient 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. When these protective coatings are chipped or removed, the properties of the resistors can be impaired. 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 level of power applied, the temperature of the resistor can increase drastically due to, for example, the wiring pattern, heat generated by adjacent components, circuit boards, etc. Therefore, it is important to keep a check on all these to avoid any damage on peripheral components and boards.

One should also keep in mind that excessive bending of printed circuit boards in order to protect the resistors from abnormal stress is a harmful practice. Temperature coefficients must also be checked. This is very important when dealing with resistors that are used with high current or high temperatures, since the resistance drifts quite drastically. 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
Thick film high power resistorsDesigned for direct mounting on a heat-sink, these resistors deliver high power dissipation and pulse handling capabilities to help designers reduce component counts and lower costs in automotive applications. They serve as pre-charge or discharge resistors for inverters and converters in electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV). They are RoHS-compliant devices with a non-inductive design and tolerances down to ±1 per cent.
Contact: www.vishay.com
Product: CSS series; Manufacturer: Bourns
CSS seriesThese 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.
Contact: www.bourns.com
Product: ERJU resistor series; Manufacturer: Panasonic
ERJU resistor seriesThis 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.
Contact: www.panasonic.com
Product: TNPU e3 series; Manufacturer: Vishay Intertechnology
This series of automotive grade ultra-precision thin film flat chip resistors is available in low temperature coefficients down to ± 2 ppm/K in the 0603, 0805, and 1206 case sizes. The series has superior moisture resistivity (85 degrees Celsius; 85 per cent RH), sulfur resistance in accordance with ASTM B 809, and a wide temperature range of -55 degrees Celsius to +125 degrees Celsius.
These resistors are RoHS-compliant and AEC-Q200 qualified.
Contact: www.vishay.com
Product: LTR50 series; Manufacturer: ROHM
LTR50 seriesThis series of high power wide terminal thick-film chip resistors (10-910mΩ) is ideal for current detection in a variety of applications, including inverters, ACs, and energy-efficient appliances. Features include anti-surge and sulfuration resistance, and the resistors are available in 48 values.
Contact: www.rohm.com
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