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.
In reality, 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 manufacturing method a resistor will be subject to some self-heating and this must be accounted for during the selection process.
When comparing resistor technologies it is important to consider:
- The required resistance value.
- The resistor tolerance.
- Power handling capability.
- Changes in resistance value over time.
- Changes in resistance value with temperature.
The key measure for temperature effects is the resistor TCR (temperature coefficient of resistance). The lower a TCR the better a resistor will be at maintaining its resistance regardless of ambient temperature variations and self-heating effects.
The four major resistor technologies to compare are Wirewound, Thin Film, Thick Film resistors and metal foil. Each technology has performance and long term stability issues depending on the external temperature, electrical or mechanical stresses applied.
Of the four main resistor technologies, Wirewounds are the most mechanically robust. They have high tolerance to electrical surge or pulse events and good ESD performance. Wirewound resistors have varying TCR dependant on the metal alloy used. 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.
Wirewound resistor technology is relatively low cost (with only thick film cheaper) and is available with low tolerances. However, wirewound devices tend to be large and their frequency performance is poor. The construction method makes wirewound resistors the most likely to be damaged by poor handling. Packaging, insertion, and lead forming processes can all impact on performance.
Thin Film Resistor Technology
Thin Film resistors consist of a thin layer of resistive material (typically 50 to 250 Angstroms) deposited on a ceramic or silicon substrate. They tend to be used in precision applications where specific resistor values are required. Thin film resistors are available with tight tolerances.
The manufacturing method makes thin film resistors expensive compared to thick film and wirewound. The technology also makes the resistor vulnerable to ESD, surge and pulse events. Thin film technology is also vulnerable to moisture in the environment and to damage via mishandling.
Thin Film resistor technology tends to be restricted to low power applications. However, the technology does have low TCR, good high frequency performance and relatively small mechanical dimensions.
Thick Film Resistor Technology
Thick film resistor technology has high power handling capability, good thermal performance (particularly when coupled with a heat sink) and high surge, pulse survivability. Thick Film is also excellent in application where ESD is an issue and is the preferred resistor technology in AntiStatic Charge or Discharge Applications.
Thick film resistors are more stable than wirewound and have low TCR in the 100ppm to 50ppm /C range. Lower TCR are available if specialist materials are used.
If very high precision is required thick film may not be the best choice. It is also susceptible to thermal shock but, in general, it is a good all rounder. It can handle high power, provide a wider range of resistance values and withstand high surge conditions. Assuming all application parameters are correctly specified properly designed Thick Film resistors will outlive the applications in which they are designed to be used.
Foil resistors deliver high precision and stability. They have higher power handling capability, improved tolerance and lower TCR than thick film. However, they are the highest cost of the resistor technologies discussed in this post.
A metal foil is mounted on a ceramic carrier. The desired resistance value is achieved by a photoetched resistive pattern in the foil. The foil generally has a thickness of several micrometers. The ceramic carrier delivers high power and thermal performance.
When comparing resistor technologies each has its advantages and disadvantages and none is perfect. It is therefore important to decide on the parameters that are most important for the application and select accordingly.