On Sat, 04 Feb 2006 23:20:01 +0000 (GMT), "Dave Plowman (News)"
wrote:

In article ,
Gerard Bok wrote:
You can substitute a DC adaptor for an AC one, but not the other way
around.

No. You cannot!
(Only in rather rare situations this is possible.)

Please explain why not.

Well, at first: if DC adapters were for 'universal use' no one
would bother to design / produce / stock / sell AC types :-)

If you supply AC to a device, you can do all sorts of nice things
with the input power:
- rectify it
- rectify it with a doubler (giving + and - for the same voltage)
- rectify it and pull some mains frequency for timing. (Old times
clocks used to do that. Before crystals and digital deviders
became cheap.)
- some loads do not allow dc at all. As several switches don't.
Some motors don't either. In general: if the device is some kind
of coil, don't even think of replacing an AC adapter with a DC
type.

What also counts is the voltage. If an adapter is labeled 10 Volt
AC it is quite easy to pull some 14 volts out of it. (Rectify it,
which yields root 2 times the ac voltage.) You cannot do the same
with DC. (And you cannot replace a 10 volt AC type by a 14 volt
DC type either. :-)
(Please note, that also the load applied may play a rather
important role in the actual voltage an adapter supplies!)

James Sweet mentioned a special case: if the only thing done on
the input is a rectifier, you can connect a DC adapter.
That is: if the device is designed to handle a rather big range
of input voltages and if the rectifier is overdesigned, as in the
DC situation the current through half the rectifier is more than
double that of the AC situation. (Again: with cheap diodes
plentiful in supply, a modern device is likely to handle this
situation. Old devices maybe not. I've seen many 1N4148 'bridge
rectifiers :-)

--
Kind regards,
Gerard Bok