Resistor
failure rates vary considerably. This is true not only between resistor styles,
but also between the quality levels of a single style. Figure 2 shows the relative
difference in failure rate between each resistor style addressed in this
section.
As a rule, fixed value resistors should be used
whenever practical due to their higher reliability and lower cost. A fixed value
resistor style will typically be an order of magnitude more reliable than an
equivalent variable resistor in the same application.
The difference between reliability of fixed value
film and wirewound resistors is not significant. It is more dependent upon the environment in
which it is used (particularly temperature) and power dissipation requirements
than on the construction method of the resistor. However, as a rule, film styles
are more reliable than wirewounds.
The difference between reliability of variable
resistors is more pronounced than it is for fixed resistors, as Figure 2 shows. This is
primarily because the unenclosed styles are more susceptible to damage from
environmental factors such as salt/fog, dust and dirt, humidity, etc. In
addition, the very high power ratings of some variable resistors increase stress
factors, thermal expansion/contraction, and thus naturally cause higher failure
rates.
Figure 2. Relative Failure Rates for
Resistors
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