When trying to choose the right power resistor for a specialist application it is important to go further than a simple review of datasheet parameters. It may appear only resistance value, tolerance and voltage is relevant but if a resistor is to perform to specification over the long term there are a number of issues to consider.
Performance trade offs
A perfect resistor will maintain its resistance value over its entire service life regardless of any external stresses that may be applied. Unfortunately, the perfect resistor does not exist, which means there must be some level of compromise. Performance characteristics are often closely linked. Changing one parameter can have a detrimental effect on another.
Taking resistor value as an example. Choosing a relatively low resistance value will increase the heat generated by the resistor device. Ideal resistor operating temperature is below 70C. Hence heat must be dissipated or the resistor performance could be degraded. In extreme cases the power resistor component may fail.
A design compromise may be to increase the dimensions of the resistor to maximise heat dissipation and/or use a heat sink. This will reduce the board area available for other key components. The heat generated by the resistor may also impact on other sensitive components in close proximity. Above 150C there is an issue with solder temperatures and crystallisation.
One solution could be to choose a resistor substrate material with superior thermal properties. The challenge is to assess performance improvements both at the system and component level versus the increased costs.
The relationship between thick film resistor substrate material power rating and heat dissipation may be clear. What is less obvious is the relationship between choice of substrate and resistor tolerance, temperature coefficient of resistance and drift.
Resistor Stability Over Time
There are several factors that can degrade the resistance of a resistor component. These include thermal, mechanical and electrical stresses. Thick film resistor stability is directly related to the choice of resistance film and the thick film resistor manufacturing process.
When choosing a resistor it is crucial to consider what may impact on a resistor to change its value (and hence system performance) over the long term. Consider what may damage the resistor during its in service life and how may these threats be mitigated?
After manufacturing the resistance of the film is determined by point to point contact of spheres of metal oxides within the resistor film. The contact may be disrupted (and therefore the resistance changed) by a combination of the stresses outlined above.
These stresses can cause permanent changes in resistance value. The level of change is directly related to the level of stress with extreme stress causing complete failure. Minor stresses may cause negligible changes in resistance value but their impact can be cumulative over time.
A potential change in resistance value over time can be quantified to an extent by interpretation of published data. However, the impact of some stresses such as electrical transients will be unknown. Temperature Coefficient of Resistance (TCR) defines the resistive elements sensitivity to temperature change. TCR is largely determined by the materials used and to a lesser extent by the design.
Other resistor technologies such as wirewound may be more robust (and therefore stable) than thick film but they have disadvantages including size and inductance. There are many issues to consider when choosing a power resistor. If in any doubt it can be best to consult a specialist resistor manufacturer.