In some applications, selecting the correct high-power resistor is critical. Equipment can be damaged, or operator safety compromised if a high-power resistor fails in service.
High power resistors are generally defined as having a power rating of more than 5W. They are often found in motor braking applications, load banks, power supplies, amplifiers and power conversion applications.
When comparing resistors, their performance may appear to be similar but it is important to understand the advantages, disadvantages and limitations of each resistor technology.
No material or manufacturing method is perfect. Ambient temperature, moisture, transient circuit conditions and mechanical stress all have an impact on performance.
Depending on the voltage applied, the resistor material and the manufacturing method of a resistor it will be subject to some self-heating. This must be accounted for during the selection process.
When comparing resistor technologies consider:
- The required resistance value.
- The resistor tolerance.
- Power handling capability.
- Changes in resistance value over time.
- Changes in resistance value with temperature.
Types of high power resistor
There are several types of high-power resistors. A brief description of the construction of each type follows:
The resistive element is a wire wound around a non-conductive core. Resistor performance depends on the wire specification and the core material.
Conductive particles are distributed in a ceramic paste and then fired. Ceramic resistors have excellent pulse handling capabilities.
Metal film resistors
Metal film resistors are available to approximately 35 watts. However, resistance options begin to diminish above 1 or 2 watts.
Similar in construction to carbon film resistors, the main difference is the use of a metal alloy (typically nickel-chromium) as the resistive material rather than carbon.
Thick film resistors
Thick film high power resistors are made by a screen printing process using a conductive ceramic-and-glass-mixture composite suspended in a paste. After the resistor has been screen printed, it is baked at high temperatures to remove the paste solvents and fuse the ceramic-and-glass composite.
Various substrate materials are available. Alumina is the most common substrate material, but other materials are used in very high power applications.
Thin film resistor devices are generally not suitable for high-power applications.
Comparing high power resistor technologies
High power thick film resistors have the highest resistance values, followed by Wirewound. The maximum resistance value of carbon resistors is poor in comparison. As stated above the resistance range of metal film resistors is limited at high power ratings.
Power handling capability
When power handling capability is a key concern it is important to consider both the power rating of the device and heat dissipation (see below). Wirewound resistors have the highest power handling capability followed by thick film. Again, ceramic resistors are in last place.
Surge and ESD
Ceramic resistors tend to have the best surge and ESD pulse performance. Wirewound resistors also perform well, closely followed by thick film.
There is little point in choosing a resistor with a high power rating if it is impossible to dissipate the heat generated. Overheating will compromise the performance of the resistor. In extreme cases, it can cause resistor failure.
The power watt density (and hence its ability to dissipate heat) of thick film is superior to Wirewound and ceramic resistors. This is one of the major advantages of thick film resistor technology.
The TCR of Wirewound resistors depends on the metal alloy used. A pure wire may have TCR of several thousand ppm /C but alloys perform much better. They can have TCR values comparable with thick film but over a limited temperature range.
Thick film resistors have low TCR in the 100ppm to 50ppm /C range. Lower TCR is available if specialist materials are used. The TCR of ceramic resistors is poor in comparison.
If a resistor is electrically, thermally or mechanically stressed it can change its resistance value. Stresses are a key consideration in high-power applications. They can be instantaneous or cumulative over time.
Ceramic resistors have excellent electrical pulse survivability, but generally, Wirewound resistors and metal film resistors are the most stable over time.
For a given power rating, Wirewound resistors are much larger than their ceramic or thick film resistor equivalents.
Closely related to size are resistor mounting issues. As heat dissipation is a major issue, positioning the resistor relative to heat-sensitive components is an important consideration. As are heat sinking, airflow and cooling.
The bulk of Wirewound resistors can be an issue. Positioning ceramic and thick film resistors in a system is often easier to implement.
Based on the above, a cost-benefit analysis should lead to selecting the most appropriate high-power resistor for the application. The prime consideration should be the cost of resistor failure. This could cause expensive damage to equipment and/or a threat to personnel. Saving a little on the cost of the resistor could be a false economy.
When comparing resistor technologies, each has its advantages and disadvantages and none is perfect. It is therefore essential to decide on the most important parameters for the application and select accordingly.
For many applications, the relatively small size, low cost and heat dissipation properties of thick film make it the preferred choice. In very high power applications, and where high energy electrical pulse is a concern, other resistor technologies may be best.
The choice of ceramic resistors can be limited, but a wide range of Wirewound and thick film resistors are available from the major resistor manufacturers. Where a standard device does not meet the requirements of the specification, custom high-power resistor manufacturers can often deliver a solution.