On Thu, 12 Feb 1998, Ted Lemon wrote:

> Date: Thu, 12 Feb 1998 14:28:48 -0800
> From: Ted Lemon <mellon@hoffman.vix.com>
> To: Wolfgang Rupprecht <wolfgang@wsrcc.com>
> Cc: current-users@NetBSD.ORG
> Subject: Re: serial port control 
> 
> 
> > I was talking about a 60hz UPS, and the assumption is that the person
> > designing the UPS isn't a total dork and is limiting the rise time to
> > something reasonable.  Further I assumed that they did a good job and
> > didn't have horrible overshoot and ringing problems.  In short I'm
> > assuming that they didn't use the excuse of square wave output to hide
> > all sort of sins.
> 
> This is probably not a good assumption.   The watchword with the
> square-wave-output UPS is typically "cheap, cheap, cheap!".
> 
> Thanks for the explanation of switching power supplies, BTW - I had
> always wondered how they work.   Two of my machines downstairs use
> switching supplies; the other uses a linear supply, so I'm in a
> slightly different situation than the average user.   Also, I don't
> think CRTs use switching supplies, do they?
> 
> Is it possible that a typical (cheap!) switching supply makes
> assumptions about its supply that aren't valid if the supply produces
> a square wave, and that this could cause the linear supply to behave
> badly even though an ideal linear supply might love a square-wave
> input?  The reason I ask is that I have definitely heard of switching
> supplies being fried by square-wave UPS's.  Could be Urban Legend, I
> suppose.
> 
> 			       _MelloN_
> 
Most inexpensive inverters today use what is called a modified-square-wave
output, they try to approximate a sine wave with a 2 or 3 bit Digital
to analog converter, so more things will like it. For a given wattage
requirement, a true sine-wave converter will cost 3 to 4 times as much as
a modified-square-wave.
                               Dave Stanhope
                               dms@celtech.com