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From: jgk@osc.COM (Joe Keane)
Newsgroups: sci.electronics
Subject: Re: Watt Hour Meters
Summary: If you want you can build a really good power supply.
Keywords: power supply
Message-ID: <1897@osc.COM>
Date: 20 Jan 90 01:07:20 GMT
References: <MARK.90Jan5105134@retix.retix.retix.com> <5170068@hplsla.HP.COM>
Reply-To: jgk@osc.COM (Joe Keane)
Organization: Object Sciences Corp., Menlo Park, CA
Lines: 17

I write:
>A _good_ switching power supply looks like a resistive load, the resistance
>slowly changing in response to changes in output current and input voltage.

In article <5170068@hplsla.HP.COM> tomb@hplsla.HP.COM (Tom Bruhns) writes:
>What sort of resistance do you suppose a switching supply looks like?  

The term `switching power supply' often means just a normal power supply
(transformer -> rectifier -> big capacitor) with a switching voltage
regulator.  Mr. Bruhns is right about these: the rectifier only draws current
on a small part of the cycle, putting nasty harmonic currents on the line.

Anyway, it's good for a power supply to look like a resistor from the line
side, and for a large one it becomes an important issue.  There are a number
of techniques for achieving this; more phases and resonant shunts are classic,
but you can also use a switcher.  It's interesting, but you could write a
whole book on it...