I was an electronics technician in the U.S. Navy.
Most household appliances, such as computers and stereos, use a
power supply to convert the AC into DC, and, yes, capacitors form
a part of the circuitry that accomplishes this.
If a circuit runs on AC, such as an incandescent light fixture,
or perhaps more noticeably, with a fluorescent fixture, you will
be able to see the light flicker at the zero voltage points, 120
times a second for a 60Hz AC current, as noted on this page by
Joe Wolfe, of the School of Physics at The University of New
One solution to this, in AC circuits, as he notes, is to use
3-phase AC, which produces overlapping waveforms as illustrated
in the figure in the middle of the page.
But, as I said, in most equipment, the AC is converted to 12 or
24 volts AC by using a transformer, and is then converted to DC
voltage by a process called rectification. The voltage is often
rectified with the use of diodes, which only allow current to
flow in one direction. By "bridging" these diodes in a circuit,
full use is made of both the negative and positive AC cycles,
and a constant DC voltage is produced.
A very good illustration of this is given on the Wikipedia
Newer power supplies, such as the one in your computer, use
a technique called "switching" to convert 60Hz AC to a much
higher frequency. This way, smaller transformers can be used
to convert the AC to a lower voltage before it is rectified
to the voltages used by your computer hardware...usually 12,
5 and 3 volts. Smaller transformers mean smaller and lighter
power supplies which will better fit in your computer case.
A good explanation of this is found in this article on the
The mildly fluctuating DC voltage is then subjected to what's
known as "voltage regulation", which produces a very constant
DC voltage for sensitive equipment like your computer.
You can read about voltage regulators and stabilisers on
I hope that clears things up!
Additional information may be found from an exploration of
the links resulting from the Google searches outlined below.
Searches done, via Google:
AC "zero volts" current
"a power supply works"
rectification diodes bridge