How To Choose The Right Resistor - 3 Things To Consider
When trying to choose the right resistor for a particular application it may appear only resistance value, tolerance and voltage is relevant. These parameters are important but if a resistor is to perform to specification over the long term there are three key issues to consider.
Performance trade offs
There is no such thing as a perfect resistor, which means there must be some level of compromise. Performance characteristics are often closely linked. Therefore, fixing 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. This heat must be disipated or the resistor performance could be degraded or the 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.
If thick film technology resistor technology is chosen one solution may 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.
There are several factors that can degrade the resistance of a resistor component. These include environmental and electrical stresses. Thick film resistor stability is directly related to the choice of resistance film and the thick film resistor manufacturing process.
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 environmental effect such as temperature, mechanical stress and/or electrical stress.
Various electrical and environmental 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 (see below). 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. Power Coefficient of Resistance (PCR) quantifies the resistance change due to self-heating when power is applied.
Other resistor technologies such as wirewound may be more robust (and therefore stable) than thick film but they have disadvantages including size and inductance.
What Makes A Resistor Fail
The four key elements that may cause resistor failure are:
Transient electrical conditions (pulse)
A resistor may fail as a result of the application of one of these stresses or a combination. One or more stresses may damage the resistor and impact on stability (see above) without causing complete failure.
Mechanical damage can be caused by shock and/or vibration. It may be caused by poor handling or by stressing the device during manufacture. Poor consideration of the resistor mounting method can cause long term low level mechanical stress.
Unlike mechanical stresses thermal stress tend to be longer term. A resistor may operate within an environment where its specified temperature limits are exceeded. If the operating temperature is well understood resistor damage can be minimised by derating the resistor appropriately. However, this approach will not work for any sudden unexpected rises in temperature.
Sometimes potential electrical stresses are known but they can be difficult to quantify. More often electrical stresses are transient in nature and difficult to understand. Resistors are therefore often over designed to withstand electrical stresses that may occur. Resistor damage by electrical stresses tends to be more common than damage by mechanical or thermal effects.
Like most electronic components ESD can cause catastrophic damage to resistor devices although wirewound devices tend to be more robust than the film types of resistor. The impact of ESD is similar to that of a transient or pulse described above but more severe and unpredictable. The only way to protect the resistor device is to have appropriate ESD management processes in place.
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?