When choosing the correct power resistor for a specialist application it is important to go further than a simple review of datasheet parameters. If a resistor is to perform to specification over the long term, there is more to consider than resistance value, tolerance and voltage. Power resistor stability over time should be a key concern.
Resistor Performance Tradeoffs
A perfect resistor will maintain its resistance value over its service life regardless of any external stresses 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.
For example, if the system board area is a concern reducing the size of the resistor could be a solution. But this can compromise heat dissipation, resistor functionality and long term reliable performance. Choosing a specialist resistor substrate material may maximise heat dissipation but only at an increased cost.
Power Resistor Stability Over Time
When choosing a power resistor it is important to consider factors that can affect the stability of the resistance value over time. These include thermal, electrical and mechanical stresses.
The Ideal resistor operating temperature is below 70C. Heat must be dissipated or the resistor performance could be degraded. In extreme cases, the power resistor component may fail.
The heat generated by a resistor can impact other sensitive components nearby. Above 150C there is an issue with solder temperatures and crystallisation.
It is important to understand the real operating temperature. Directions of airflow and the proximity of heat-generating components can all have an impact.
The relationship between 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.
Temperature Coefficient of Resistance (TCR) defines the resistive elements sensitivity to temperature change. TCR is determined by the materials used and, to a lesser extent, by the design.
One of the main reasons power resistors fail is the application of a single (or repetitive) surge or pulse condition. If the shape, duration and amplitude of the pulse are quantified and fed into the design process at an early stage its impact can be minimised. Survivability can be improved by selecting an appropriate substrate and optimising the thick film firing process.
It is important to consider what may damage the resistor while in service. Inappropriate handling can damage the substrate material and/or the interface with the system board. Vibration or shock can damage the resistor substrate and resistive elements.
Mechanical stresses can cause permanent changes in resistance value. The level of change is directly related to the stress level, with extreme stress causing complete failure. Minor stresses may cause negligible changes in resistance value, but their impact can be cumulative over time.
Thick film power resistor stability is related to the choice of resistance film and the thick film power resistor manufacturing process. After manufacturing film resistance is determined by point to point contact of spheres of metal oxides within the resistor film. The contact may be disrupted (and the resistance changed) by a combination of the stresses outlined above.
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.