23 Dec 2003, 23:11, MIKE ROSS (1:167/133), wrote to Tom Walker:

Hi MIKE.

 ->> 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 
 TW>> designed for operation from DC, be it 12/120/220 would need it's 
 TW>> own internal Frequency reference.

 MR> You are right about needing the 60 Hz powerline in most cases 
 MR> because the typical psu has a startup circuit that requires a separate
 MR> little transformer to supply the switchmode controller IC. 

Wonder where they hide that 'startup circuit' and 'little transformer'? 
Nothing like that in ANY I've seen from TRS-80 Model III through ATX.

 MR> However, I've seen the rare psu circuits designed slightly 
 MR> differently that don't need this initial standby supply and thus will 
 MR> operate even off straight DC but this type of psu is not typically 
 MR> found in pc's. I've seen this notably in an Amdek monitor which used a 
 MR> circuit based, IIRC, around a TDA4301. I think that with relatively 
 MR> little modification this type of circuit may even work from a 12V 
 MR> battery but I'm not completely sure.

Thre only IC's in PC PSs is the optoisolator which is fed from the
overvoltage crowbar to kill the switcher.

There were a few in the middle years of the AT PS which used a PWM chip in
an attempt at current regulation, but that trend died pretty quick.
.......................

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

Same here!


     Good luck...  M.

--- Msged/386 TE 06 (pre)