What is a power resistor?
Power resistors are used to dissipate energy by converting it to heat. They are designed to maintain stable performance under heavy loads and to handle thermal conditions safely.
Power resistors are usually physically larger than other resistors, and are designed for higher loads—typically over one watt. Calculating the power rating of these resistors follows the familiar formula P =I²R, where P is power, I is the current running through the resistor, and R is the resistor value in ohms. For resistor longevity in a power circuit application, the rated power should be more than 2x the actual power dissipated in the particular application.
Power Resistor Applications
Power resistors are used in a wide variety of applications, from power transmission and distribution networks to defibrillators. Some example applications are listed below:
High Current and Braking
High current power resistors can be used in load testing and dynamic braking in the transportation and semiconductor industries. These types of resistors use a modular and scalable design which enables designers to find the proper high current/braking resistor that will perform best in their designs, especially in motor deceleration.
High current resistors can improve applications that range from standard current loads to motor braking, such as trains and elevators.
Transportation and Automotive-Grade Power Resistors
Transportation solutions are typically customized to precise specifications; however, they are flexible in meeting most power needs. This class of resistors is designed to withstand harsh environments. Applications span from dynamic braking, custom controls, and lighting, as well as for military ground equipment, construction equipment, snowmobiles, dump trucks, and a wide variety of off-highway vehicles. Custom solutions also exist for agriculture and construction applications.
Types of power resistors
Power resistors are available in different types with diverse ratings, physical shapes, constructions, and sizes.
Some of the types of power resistors are listed below:
This class of resistors is available in many different wattage ratings, depending upon the application, mounting, and resistance range needed.
As the name implies, these resistors are made using an insulated wire wound around a core of non-conductive material like ceramic. The wires are typically poor conductors such as nickel-chromium and are constructed in different lengths and material compositions to control the value of the resistance.
Wirewound resistors are generally used when carbon resistors start to lose their effectiveness. Designers need to be aware that this class of resistors may affect the behavior pattern of the circuit at high frequencies due to their higher inductance characteristics compared to other classes of power resistors.
Ceramic-based power resistors can range from 1/2 watt to 1k watt. Some applications for ceramic power resistors are high power snubber circuits, motor controls, radar, defibrillators, circuit breakers, high voltage power supplies, medical devices, and more.
Non-inductive ceramic composition resistors can provide benefits beyond those of other classes of resistors such as wirewound, and film resistors. This is because they do not have a resistive film or wire that is vulnerable to malfunction. This makes composition resistors superior in high pulse applications where wire and film may fail. Ceramic resistors are also chemically inert and have excellent thermal stability, which leads to better durability and safety.
These two types of resistors look very similar, but thick film and thin film technologies are very different. Let’s look a little deeper.
The thickness of the resistive layer is the obvious difference between thick and thin film resistors. But there are also specification differences and process differences in their creation. The thin film uses sputtering, a physical vapor deposition (PVD) process, while the thick film uses screen printing.
Thanks to their sophisticated manufacturing process, thin film resistors have tightly controlled specifications. They have a thickness of typically 0.1 microns with tight tolerances with good stability along with lower capacitance.
Thick film resistors, in comparison, can be up to 1000 times thicker (100 microns) and will have high resistance values with high voltage capability with lower tolerance. Thick film resistors also have a higher capacitance than thin film resistors.
Metal oxide film versus carbon film resistors
As the name implies, a carbon film resistor places a layer of carbon film over a ceramic insulator substrate. It can endure high temperatures but has a negative temperature coefficient. In other words, it has less resistance capability over time with high temperatures.
Metal oxide film resistors instead use a tin oxide film layer over the insulated ceramic rod. This delivers superior voltage and temperature coefficients compared to carbon film resistors.
The thermal noise of a resistor depends on the type of resistor and how it interacts with resistance, bandwidth, and temperature. Wire wound and thin film resistors have low noise. Carbon film and metal oxide film are known for less noise, with metal oxide film having a lower noise design than carbon film.
For this reason, metal oxide film resistors are used in low-noise applications. Metal oxide film resistors have a better tolerance level—as low as 0.1%—while carbon film resistors have a minimum tolerance of 2%.
There are a few key factors that will need to be examined when choosing the right resistor type for an application. A single resistor may account for only a tiny percentage of the cost for a circuit, but a power resistor can become a source of damage for the entire circuit.
Quite frequently, resistors are used in higher-voltage circuit applications. It is important to verify the proper voltage rating of the resistor—especially in these higher-voltage applications—to ensure that it will not fail. Resistors are self-heating components and must be able to handle the power dissipation in their particular circuit.
What’s more, designers should take care to ensure that their resistors will continue to operate as expected as resistance drifts based on temperature. In addition to checking the temperature coefficient of resistance (TCR), also check the voltage coefficient of resistance (VCR). Resistance values are only fixed under stable voltage conditions. Although usually less significant than temperature-driven variation, voltage fluctuations can lead to unacceptable changes in resistance.
The resistor must also meet safety parameters and specifications for the part of the circuit in which it operates. It is good practice to select resistors that have a power rating greater than the power that it will handle. If a power resistor is operated above its specified power rating, its operating life may be reduced and short circuits may occur. A general guideline for power dissipation is to use a resistor that can withstand twice the maximum power it is expected to handle in the circuit.
Many kinds of resistors may look very similar on the outside. Some resistors may also be available with the same specifications—initial tolerance, noise, load life stability, etc. However, each type of resistor may be constructed with different resistive materials and may be produced differently. Designers need to beware because, in some cases, the resistor selected may exhibit a different performance characteristic in the circuit.
Resistors are not immune to combustion and may emit smoke, gas, and more. Also, resistors tend to lose their characteristics when immersed in a solvent for a long period of time. Designers need to keep these things in mind to avoid any damage to peripheral components and boards.
Some power resistors can be shielded –i.e., housed in metal or ceramic casings—or coated with resin as a part of their construction. This will help ensure that their performance, reliability, and stability are not affected.
If a protective coating is chipped or removed, the properties of the resistor may be impaired. To avoid damaging the coating, it is best not to use hard steel tools to place resistors.
Some applications may subject resistors to irregular current surges and spikes. Designers should also take care that the components selected can withstand an increased load for a brief period.
Even if the level of power applied to the resistor is within rated power, the temperature of the resistor may increase drastically due to such things as the wiring pattern, heat generated by adjacent components, circuit board design, etc.
Designers should also take care that any bending of printed circuit boards does not subject resistors to abnormal stress, which may damage the device or affect its performance.
Counterfeits vs. Originals
Like any other electronic/electrical component, resistors are subject to counterfeiting. An example is when carbon film resistors are sold as tight tolerance metal film resistors. This gives the user a lower performance resistance tolerance or poor temperature coefficient device in their circuit.
To avoid such problems, purchase components from an authorized distributor, or directly from the manufacturer using a contracted sales representative.
Everything You Need
Resistors can be one of the last considerations in a design, but as you can see, they are essential and will improve your design. Discover how Ohmite can provide you the appropriate power resistors for your electrical or electronic system. If you are looking for a resistor that can provide reliable performance, contact our experts to advise you on your application.