The capacitance and voltage rating of an electrolytic capacitor is
set by the thickness of an aluminum oxide layer deposited on the
metal foil "plates" of the capacitor. Of course, the bulk amount of
square inches of aluminum plate material also determines the range
of capacitance that is possible with varying thicknesses of oxide.

The electrolyte serves the purpose of making an intimate electrical
connection to the plate. The problem is the electrolyte shows some
tendancy to dissolve the oxide layer. When this happens, the
capacitor's value increases, and its safe voltage rating decreases.
This is why a long disused electrolytic capacitor tends to blow up
when it is abruptly put back into service.

Electrolytic capacitors are self adjusting for working voltage (to
some degree). If they are operated for a long time at 50% of their
rating, the oxide reduces in thickness, and they become higher
capacitance, and lower working voltage. If you try to increase their
operating voltage, they will draw too much current. They will either
adapt to the new higher voltage, or they will blow up from the heat.

The tolerance has nothing to do with manufacturing capabilities, or
price, and everything to do with the highly variable nature of the
oxide layer.

-Chuck, WA3UQV




Mike Knudsen wrote:
In article , "Frank
Dresser" writes:


Just as speculation, let's say cap manufacturers have learned to make
electrolytic capacitors with good precision at little extra cost. And let's
imagine that setting the target capacitance to 5% - 10% low reduces the cost
of the "active ingredients" by 5% -10%. Well, that would be a nice reward
for knowing how to do the job!


This makes very good sense.
I suspect that back in the old days, manufacturers would throw in up to 100%
extra foil plates area just to make sure they at least met the rated
capacitance. So you would get caps well over the ratings.

But yes, once they got the process down really tight, why toss in extra
material. In fact, shaving it on the low side is just what the front-office
bean coutners probably tell them to do nowadays! --Mike K.

Oscar loves trash, but hates Spam! Delete him to reply to me.