Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!lll-winken!uunet!tektronix!midas!jeffw
From: jeffw@midas.STS.TEK.COM (Jeff Winslow)
Newsgroups: sci.electronics
Subject: Re: Running a Mac/SE from batteries and inverter
Keywords: Mac, batteries, inverter
Message-ID: <4224@midas.STS.TEK.COM>
Date: 7 Apr 89 19:22:28 GMT
References: <13671@jumbo.dec.com> <2446@iscuva.ISCS.COM>
Reply-To: jeffw@midas.STS.TEK.COM (Jeff Winslow)
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 26

In article <2446@iscuva.ISCS.COM> jimc@iscuva.ISCS.COM (Jim Cathey) writes:

>Switching power supplies might very well object to square(ish) waves as input.
>The problem is inrush current through the diodes and into the capacitors.
>This can cause premature failure on both components. 

What problem?   :-)

First of all, if you really mean inrush current as in what you get when you
first turn the switch on: Since switching supplies don't try to synchronize
the application of line power to the capacitors with a zero-crossing (at least,
the cheap ones don't), you could just as easily be turning on the switch at
the peak of the sinewave as near the zero - more likely in fact, considering
the shape of the sine curve, especially after it's been chopped off by the
load of all those *other* switching supplies out there. For the same peak
input voltage, which is what switchers care about, the peak inrush current
is the same for sine or square.

Secondly, rms running current will be lower for a square wave, since the
conduction angle is much greater than for a sine wave, even the chopped-off
version. The rectifiers will run cooler, last longer. The capacitors will,
too - lower rms current -> lower core temperature, and the single most telling
variable affecting electrolytic capacitor life (in normal use :-)) is
temperature.

						Jeff Winslow