"Matt Mc_Carthy" bravely wrote to "MIKE ROSS" (26 Dec
03  05:26:17)
 --- on the heady topic of "Re: Knoppix"

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

 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.

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

BTW we were talking about PC psu's not m3's. The little transformer in
question looks like a small chromed E frame perhaps an inch wide or
less. This will supply between 9 to 15 volts for the TL494 pwm
controller ic. Sometimes there is no transformer and is instead derived
from the main rectified supply.

Which reminds me, another rather important reason why they need the AC
line is that most psu's rectifiers are designed as a half-wave voltage
doubler. The 120V/240V switch simply shorts out one of the doubler's
supply caps for 240V operation as used in Europe.


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

One may also find a voltage comparator to measure the various voltages
against a reference to generate the Power Good signal.


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

I won't dispute it but why am I getting the impression you are confusing
current and voltage? ;-)

 Mike
 ****

... Another good day; the computer is still working!
--- Blue Wave/DOS v2.30