Jonathan said:
In virtually every schematic I've seen for regulated power supplies, the
storage capacitor that is used after rectifying the current is a polarized
capacitor. Why is a polarized capacitor always specified? What would
happen if I used a non-polarized electrolytic capacitor?
The power supply would be twice as big and cost ten times as much.
Also, I've read that electrolytic capacitors are fairly short lived. Is
there any downside to using a longer lived type of capacitor in this
function, aside from the size advantages of the electrolytic?
The two I already mentioned.
A capacitor must contain two conductive surfaces and an insulation
layer that separates them and stores energy as an electric field is
applied to it. Most non electrolytic capacitors have flat plates of
metal with flat a layers of some dielectric between them. In film
capacitors, this structure is often rolled up to save space.
Electrolytic capacitors have one metal plate and one conductive liquid
plate with only the oxide layer on the metal as a dielectric
separator. Even with a flat plate, this way of coating a metal
surface with a uniform layer of insulation with the other plate
intimately enclosing the metal plate is pretty space efficient. But
the metal plate in the electrolytic is electrochemically etched so
that it is essentially a metal sponge with a hugh surface area
compared to the original metal foil. The nit is oxidized
electrolytically and covered with the conductive liquid that forms the
second plate. This process multiplies the capacitance per volume by a
huge factor. Unfortunately, you cannot reverse the voltage or the
oxide will be removed as atomic hydrogen is produced at the surface of
the metal, which combines with the oxygen to form water. And you have
to live with the difficulties of a component that contains a liquid
component. The liquid also is not as conductive as metal, so
capacitive current heats it quite a bit. But if you can keep it cool,
and not expose it to reverse voltage or excess voltage it can have a
very useful life. Something like 10 years for good quality examples.
The short life given in the specs assumes the internal temperature is
constantly at the rated maximum.