Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!decwrl!ogicse!ucsd!chiton!cdl
From: cdl@chiton.ucsd.edu (Carl Lowenstein)
Newsgroups: sci.electronics
Subject: Re: Bullshit, or NOT? AC line problems.
Message-ID: <695@chiton.ucsd.edu>
Date: 13 Feb 91 23:12:51 GMT
References: <1991Feb5.005045.388@miavx2.ham.muohio.edu> <2470012@hp-vcd.HP.COM> <14050@encore.Encore.COM>
Reply-To: clowenstein@ucsd.edu (Carl Lowenstein)
Organization: MPL of SIO at UCSD
Lines: 28

In article <14050@encore.Encore.COM> jcallen@encore.Com (Jerry Callen) writes:
|In article <2470012@hp-vcd.HP.COM> johne@hp-vcd.HP.COM (John Eaton) writes:
|>Todays switching supplies are superior in size,cost and efficiency
|>but are lousy as loads. They act as negative resistance by drawing
|>more current as the voltage decreases and tend to only draw current
|>during the peak times of the AC cycle.
|
|Wait a sec - isn't it true that ALL power supplies, once past the initial
|power-on state, only draw current during the peaks in the AC cycle?
|The filter caps will only draw current when the voltage out of the 
|bridge is greater than the voltage on the cap, right?

ALL covers quite a bit of territory.  Back in the olden days, we learned
how to build power supplies with "choke input filters" that drew current
from the AC source during the whole cycle.  (except for a small time
region as the voltage crossed 0 and the rectifiers switched).

The modern switching supply tends not to have any 60-cycle iron-core
components, thus saving weight and volume.  But the external efficiency
is lower, because of the peak current drain through the power grid.
And, as pointed out above, the switching supply does act as a negative
resistance load, and can adversely affect the stability of the power grid.


-- 
        carl lowenstein         marine physical lab     u.c. san diego
        {decvax|ucbvax} !ucsd!mpl!cdl                 cdl@mpl.ucsd.edu
                                                  clowenstein@ucsd.edu