"Tom Walker" bravely wrote to "David Drummond" (23 Dec
03  10:36:10)
 --- on the heady topic of "Re: Knoppix"

 -> Does a switchmode supply not need an AC source (as in is it somehow
 -> locked to the frequency of the mains)?

 TW> Yes it does need the 60 Hertz line frequency to operate. One designed
 TW> for operation from DC, be it 12/120/220 would need it's own internal
 TW> Frequency reference.

You are right about needing the 60 Hz powerline in most cases because
the typical psu has a startup circuit that requires a separate little
transformer to supply the switchmode controller IC. However, I've seen
the rare psu circuits designed slightly differently that don't need this
initial standby supply and thus will operate even off straight DC but
this type of psu is not typically found in pc's. I've seen this notably
in an Amdek monitor which used a circuit based, IIRC, around a TDA4301.
I think that with relatively little modification this type of circuit
may even work from a 12V battery but I'm not completely sure.

 Mike
 ****

P.S.   A VERY HAPPY HOLIDAY SEASON TO ALL!!!

... Nobody born after 1970 can appreciate the greatness of PONG.
--- Blue Wave/DOS v2.30