Path: utzoo!attcan!uunet!samsung!usc!apple!bionet!hayes.ims.alaska.edu!acad3.fai.alaska.edu!ftpam1 From: ftpam1@acad3.fai.alaska.edu (MUNTS PHILLIP A) Newsgroups: sci.electronics Subject: Re: Switched power supplies, how do they actually work ? Message-ID: <1990Oct21.034846.4812@hayes.ims.alaska.edu> Date: 21 Oct 90 03:48:46 GMT References: <17660122@hpfcdj.HP.COM> Sender: usenet@hayes.ims.alaska.edu (J Random USENET) Reply-To: ftpam1@acad3.fai.alaska.edu Organization: University of Alaska Fairbanks Lines: 23 News-Software: VAX/VMS VNEWS 1.3-4 Nntp-Posting-Host: acad3.fai.alaska.edu In article <17660122@hpfcdj.HP.COM>, myers@hpfcdj.HP.COM (Bob Myers) writes... >One other *disadvantage of switchers that I believe Henry forgot to mention >is a typically atrocious power factor, as the switching action plays merry >hell with the input current waveform. This has become quite a concern at >sites which have a large number of independent switchers (such as a bunch >of PCs or workstations) on their AC lines. They also have a negative resistance characteristic. If the AC mains sag (brown out) the switching supply will simply adjust its duty cycle and draw more current. Imagine a whole city of switchers during a brown out...as the voltage drops each power supply draws more current which makes the voltage drop some more... A couple of years ago I went to Britain on business with a microprocessor emulator with a Panasonic 5V switching power supply. The company provided me with one of the rinky dink shaver adapters to plug it into the 220V over there. Just before the fuse popped, that switcher was providing 5V out with less than 20VAC in! (An intermittent 5V, which is why I happened to be measuring the input.) Philip Munts N7AHL NRA Extremist, etc. University of Alaska, Fairbanks