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Q: When alternating current reverses.... ( Answered 5 out of 5 stars,   2 Comments )
Subject: When alternating current reverses....
Category: Science > Technology
Asked by: electricquestion-ga
List Price: $10.00
Posted: 16 Jul 2006 13:21 PDT
Expires: 15 Aug 2006 13:21 PDT
Question ID: 746849
I know that AC electricity reverses direction after falling to zero.My
question is: given that the current will be stopped while at zero, how
does equipment continue to operate? I think I read it may be by using
capacitors to store current, and that in light bulbs it was too fast
for the human eye to notice the changes of direction.

The kind of equipment I'm talking about is normal electrical equipment
-computers,stereos etc.

Subject: Re: When alternating current reverses....
Answered By: sublime1-ga on 16 Jul 2006 15:25 PDT
Rated:5 out of 5 stars

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
South Wales:

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
HowStuffWorks website:

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

"voltage regulation"
electricquestion-ga rated this answer:5 out of 5 stars and gave an additional tip of: $2.00
Very helpful,sublime-1.Thanks very much.

Subject: Re: When alternating current reverses....
From: kottekoe-ga on 16 Jul 2006 20:27 PDT
Good question. Let me give a simpler answer than the official one.
There are several ways to deal with this:

1) It doesn't matter: 
  * A flourescent light flickers too fast (120 Hz) to notice unless
you wave your hand in front of it.
  * An incandescent light hardly flickers at all since it takes a
while for the filament to cool off

2) Use multiple phases:
  * Industrial motors typically run off three phases that overlap.
This was Nikola Tesla's great invention

3) Use some device to store energy to get you through the zeros:
  * Single phase motors store energy in the inertia of the rotor, so
they coast through the zero
  * AC to DC power supplies store energy in a capacitor when the
supply voltage exceeds the desired DC voltage and drain energy from
the capacitor when the supply voltage dips too low.
  * Some circuits store magnetic energy in an inductor
Subject: Re: When alternating current reverses....
From: sublime1-ga on 17 Jul 2006 19:21 PDT

Thanks very much for the 5 stars and the tip!


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