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From: John Popelish
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X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: Lumped Impedance
Date: Wed, 27 Nov 2002 13:30:18 GMT
NNTP-Posting-Date: Wed, 27 Nov 2002 08:30:18 EST
Paul Burridge wrote:
> Anyone care to provide an accurate explanation of what this is? I'v
> seen the term bandied about a lot but have no idea what it is.
Impedance is a two dimensional extension of the one dimensional
concept of resistance in that it relates the magnitude of current to
the magnitude of voltage. But, whereas resistance always has a
current that is instantaneously proportional to voltage (there is no
phase shift between them, regardless of frequency) this is not the
case when energy storage as well as dissipation is involved. Energy
can be stored in capacitance (electric fields) inductance (magnetic
fields) or waves (electromagnetic energy in route through some
medium). The full description of a system that involves any and all
of these is a general description that captures the current voltage
relationship at any frequency, but is pretty cumbersome to deal with.
But as long as you deal with only one frequency at a time, you can
approximate the full description by a combination of a single
resistance, capacitance and inductance in some configuration. Lumping
all the energy dissipation and storage mechanisms this way, into 3
equivalent components produces the "lumped impedance" approximation.
But don't expect that approximation to hold for any other frequency.
You have to recalculate the values of lumped resistance, lumped
capacitance and lumped inductance if you change frequencies, unless
the circuit is actually made up of ideal resistance capacitance and
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