TU-56 capacitor repair (part numbers)

Eric Smith eric at brouhaha.com
Mon Mar 21 19:15:33 CST 2005


Tony wrote:
> This is not really my field, but I would think you could always replace
> an electrolytic by a non-electrolytic (such as an oil-filed capacitor),
> but not necessarily the reverse.

Assuming the capacitance and voltage ratings are the same, that would
probably be true unless the circuit required a minimum or maximum ESR.
Electrolytic capacitors generally have a much higher ESR than
non-electrolytic types, though there are some new (expensive) low-ESR
electrolytics.

Most of the time it is OK to replace a capacitor with one that has
lower ESR.  An example where that is not true is filter capacitors on
low-dropout (LDO) linear voltage regulators; LDOs have both minimum and
maximum ESR requirements for the output filter capacitor, and in some
cases the input filter as well, and exhibit oscillation and other
instabilities outside the specified ESR range.  That's why I still
prefer normal (non-LDO) linear regulators wherever LDO is not required.

Typically where moderately high capacitance is required with low ESR,
solid tantalum capacitors are used, however there are some potential
problems with tantalums so they are best avoided when not necessary.
(And they are relatively expensive.)  Tantalum capacitors will fail
in a particularly spectacular fashion if reverse-biased.  One day
when we arrived at work, there was a 3/4-inch hole burned through our
multi-thousand-dollar prototype PCB, and a charred spot on the
workbench below it.  Although there was not enough evidence left to
prove it, we believe that a tantalum capacitor had been installed
backwards.

Eric




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