Failure can be catastrophic, so electrolytic
capacitors should always be selected and
used with care. The following precautions
are recommended:
Always ensure that a capacitor is
correctly rated in terms of value,
working voltage, ripple current,
dissipation factor and ESR.
Never exceed the working voltage and
ripple current ratings of a capacitor.
There is a very risk of overheating and
explosion if electrolytic components
are operated beyond their ratings!
Get it Right! – Selecting and using reservoir capacitors
- In critical applications, always use
‘low-ESR’ capacitors. If in doubt,
use a larger value of capacitor with a
lower value of ESR in order to improve
performance and reduce internal
temperature rise. - Remember, ESR increases with
temperature, so power supply cooling
is important. In order to permit
convection cooling check that there is
sufficient space around large capacitors
and avoid placing them in close
proximity to heatsinks and dissipators. - Large electrolytic capacitors can retain
their charge for long periods after
the supply has been disconnected.
It is essential to ensure that such
components are fully discharged before
attempting to work on circuits. To
ensure that this is the case, a high-value
fixed resistor can be connected across
the capacitor’s terminals, as shown in
Fig.6.16(a). Suitable values for resistor
R range from around 47kΩ to 1MΩ. - Electrolytic capacitors that have
been kept in store for long periods
will invariably be less reliable than
new-stock components. Where
performance and reliability are
important, you should avoid using
surplus and old-stock components.- Where electrolytic capacitors are
connected in series it is advisable
to connect fixed voltage equalising
resistors in parallel with each
component, as shown in Fig.6.16(b).
Suitable values for R usually
range from around 10kΩ to 100kΩ,
depending upon working voltage. - A simple LED indicator (see Fig.6.16(c)
can be fitted to indicate that charge
is present in large electrolytic
components. This also provides a
discharge path (see point 5). Suitable
values for R range from 1kΩ to 47kΩ,
depending upon working voltage. - Electrolytic capacitors should be
disposed of with care, particularly
in cases where the seal may have
ruptured and chemical material may
have become exposed.
Fig.6.13. Stripboard layout of the 9V-to-15V CMOS Fig.6.14. The finished 9V-to-15V CMOS Logic Supply Converter.
Logic Supply Converter.
Fig.6.15. Load regulation
characteristic for the 9V-to-15V
CMOS Logic Supply Converter.Fig.6.16. Reservoir capacitor
arrangements.
A K U B L V C M W D N X E O Y F P G Q H R I S J TA K U B L V C M W D N X E O Y F P G Q H R I S J TA K U B L V C M W D N X E O Y F P G Q H R I S J T1 2 3 4 5 6 7 8 9 9 8 7 6 5 4 3 2 1ST2 12
ST1 12
++breaks on the underside of the board and
12 links on the upper side. As always,
once assembly is complete it is well worth
carrying out a careful inspection of the
circuit board, particularly checking the
off-board wiring and links to ST1 and ST2.
The off-load output voltage of the 9V-to-
15V CMOS supply converter is just less
than 17V. Due to its inherent ability to
operate over a wide range of supply
voltages, this is not usually a problem with
CMOS logic. However, if it is necessary
to maintain the output much closer to
the nominal 15V then a 15V shunt Zener
diode can simply be connected in parallel
with C5, as shown in red in Fig.6.12. The
off-load output voltage will then be nearer
to 15.1V, falling to 14.9V at the nominal
15mA load current (see Fig.6.15).Next month
In next month’s Teach-In 2019 we
will be introducing negative voltage
converters and taking a detailed look at
thermal design considerations for power
supplies. Our Practical Project will take
the form of a simple low-current +12V
to –9V converter.