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​Power resistor stability over time should be a key concern when choosing the correct resistor for a high-power application. If a resistor is to retain its resistance value over the long term, it is important to go further than a simple review of datasheet parameters.

Resistor Stability – A Definition

Resistor stability is expressed as a percentage variation from the stated resistor value at a defined temperature. It is a measure of the repeatability of a resistance measurement (the stated value) over time.

Power Resistor Stability Over Time

The three main issues that can affect the stability of a resistance value over time are thermal, electrical and mechanical stresses. Thick film power resistor stability is related to the choice of resistance film and the thick film power resistor manufacturing process.

Temperature Effects

The Ideal power resistor operating temperature is below 70C. At higher temperatures, appropriate thermal management is required, or the resistor performance will degrade. This will show as a change in the resistor value (its stability) over time.

It is important to understand the real resistor operating temperature. Directions of airflow and the proximity of heat-generating components can all have an impact.

The temperature Coefficient of Resistance (TCR) defines the resistive elements sensitivity to temperature change. TCR is related to the materials used and, to a lesser extent, to the power resistor design.

Electrical Stresses

One of the main reasons power resistors fail is the application of a single (or repetitive) surge or pulse condition. Pulse conditions can also degrade the resistor material and cause a permanent change in resistance without causing complete failure.

If the shape, duration and amplitude of the pulse are fed into the resistor design process at an early stage, its impact can be minimised. Survivability is improved by selecting an appropriate substrate and optimising the thick film firing process.

Mechanical Stresses

It is important to consider the damage to the power resistor while in service and its impact on resistor stability. Vibration or shock can damage the resistor substrate and resistive elements.

After manufacturing, film resistance depends on point-to-point contact of spheres of metal oxides within the resistor film. Mechanical stresses can disrupt the contact and change the resistance.

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.

Comparing the Stability Of Resistor Technologies

Wirewound resistor technologies can be more robust (and therefore stable) than thick film, but they have disadvantages, including size and inductance. MELF (Metal oxide leadless face) resistors also have superior stability to thick film power resistors but at a significantly higher cost.

Carbon film power resistors perform well when subjected to pulse conditions and have comparable stability to thick film. However, they have several disadvantages which often make thick film the preferred solution. Thin film resistors are not generally suitable for power applications.

Power Resistor Performance Trade-offs

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 power resistor could be a solution. But this can compromise heat dissipation, resistor functionality and long-term reliable performance. Choosing a specialist substrate material may maximise heat dissipation but only at an increased cost.

Resistor stability is an important issue to consider when choosing a power resistor. If in any doubt, it can be best to consult a specialist resistor manufacturer.