Technical Data Sheet - Octopart

12
Mounting continued....
Board mounting types are designed to be mounted by their
terminals alone. Larger types may have dummy pins for
extra rigidity. Screw terminal and tag ended types may be
fixed with a base stud or suitable mounting clamp.
Adequate space should be allowed between components
for cooling air to circulate, particularly when high ripple
currents are being applied.
Altitude and Low Air Pressure
All capacitors manufactured by BHC are hermetically
sealed and should therefore suffer no electrolyte seepage
even under vacuum conditions. Additionally the electrical
parameters of capacitance, esr, impedance and leakage
current will be unaffected.
If a capacitor is operated at altitude, however, the life
will be affected slightly for two reasons. Convected heat
loss will be reduced as the air density falls resulting in the
capacitor running hotter with a consequent reduction in life.
As the air pressure drops the differential between the
internal case pressure and external pressure increases.
A complete vacuum would cause the internal pressure to
rise by 15 psi (approx.). If maintained this would lead to
increased electrolyte vapour loss and give a slight reduction
in life expectancy.
LIFE EXPECTANCY
The life expectancy represents the typical period of
time until the end of life is reached, which in this case is
characterised as follows:
CATASTROPHIC FAILURE
- open or short circuit
MECHANICAL FAILURE
- operation of safety vent, split sleeving, etc
PARAMETRIC FAILURE
- capacitance change > ± 10%
- esr > 2 x initial value
- impedance > 3 x initial value
- leakage current > specified limit
Some circuits may be able to tolerate larger parametric
variations than shown above, in which case the life of the
component will be extended beyond the figures quoted.
The life expectancy data is statistically derived from
extensive endurance testing of standard production
components and data gathered from components in the
field. It does not guarantee the performance and BHC
Components cannot assume responsibility for it’s use.
Reducing the stress level on the capacitor (i.e. lower
voltage/current/temperature) will increase the life
expectancy, as will improved cooling.
Application & Operation
Life Expectancy
Alcohols
Component cleaning using solvents such as isopropanol,
methanol, ethanol, and propanol would not normally have
any detrimental effects and therefore do not require any
special precautions.
Aqueous cleaning methods
Aqueous cleaning methods in conjunction with saponification
may be used. However, it is recommended that immediate
drying of the component in hot air at approximately 85°C for
at least 5 minutes is carried out.
Water can become entrapped beneath the sleeve and unlike
the solvents used above may not be adequately dispelled
by evaporation at room temperature. Trapped water can
cause the hydration and discolouration of the surface of the
aluminium can, however this is in no way detrimental to the
functioning of the capacitor.
Halogenated Hydrocarbons
Halogenated Hydrocarbons contain CFC’s and as such are
ozone depleting chemicals (ODC’s). It is not recommended
that they are used as cleaning solvents. In addition these
solvents can be injurious to electrolytic capacitors by
absorption into the rubber seals followed by subsequent
diffusion into the case, and attack of the winding, leading to
premature failure.
BHC will calculate the life expectancy of a capacitor under
other sets of conditions provided as much as possible of
the following data is supplied:
Operating voltage - this should be taken as the sum of the
nominal d.c. voltage and the peak of the a.c. ripple voltage.
Ripple current - the rms values should be given at each
frequency.
Air temperature - the temperature of the air surrounding or
flowing over the capacitors
Thermal aspects - type of cooling, i.e. natural convection or
rate of forced air flow (m/s). Thermal resistance of heat sink
or chassis.
End of life criteria - any special end of life conditions if
different from those stated above.
Life Expectancy and Thermal Characteristics
A key aspect of the life expectancy calculation is the core
temperature of the capacitor. It is essential to determine
this operating core temperature either by calculation or by
measurement.
Heat is generated inside the capacitor by the effect of
ripple current which raises the core or hot-spot temperature
above that of the ambient air. Heat is also generated by the
leakage current, however this is normally small enough to be
ignored.
Other circuit components in close proximity will also
contribute to the heating of the capacitor. As will any
mechanical connections to the capacitor, such as the
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