## Why I hate L

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• Hi, This is meant tongue in cheek, so please take it that way. As everyone knows, R, L, and C is the holy trinity when it comes to electronic circuits. R
Message 1 of 1 , Aug 24, 2002
Hi,

This is meant tongue in cheek, so please take it that way.

As everyone knows, R, L, and C is the holy trinity when it comes to
electronic circuits. R dissipates energy while L and C stores and
transfers it. L and C are to a great degree mirror images of each
other; C stores energy in the form of an electric field while L
stores it in a magnetic field. There is a symmetry or equality
between L and C in ciruit theory.

In the real world though, I avoid using L like the plague when I
design circuits. To me, L is an inferior and flawed component
compared to C. Here are my reasons:

1. Energy density:

A 100uF / 100V capacitor I use is 0.4" in diameter and is 1" long. It
will store a maximum of 1 joule (100uF times 100V^2 = 1J) of energy.
Its volume is 0.1256 cubic inches (pi * 0.2^2 * 1), so you can say
its energy density is about 8 joules per cubic inch (about .5 joule
per ml for you metric types).

A 150uH / 5A inductor I also use is 0.7" in dia. and is 1.25" long.
It will store 0.00375 joules (150uH * 5A^2). Its volume is 0.488 cu.
in. so its energy density about 0.008 joules per cu. in. This a
thousand times worse than the capacitor!

2. Energy storage:

Ideally C is supposed to store its charge when open circuited and L
is supposed to do the same when shorted for all eternety. Sadly this
is not so.

My 100uF cap will dissipate its charge in 1,200 seconds, my 150uH

3. Parasitics:

In theory, R is only R, C is only C, and L is only L. In the real
world each component has R, L and C. What makes a component a
capacitor is that it is mostly C with only a little R and L.

My 100uF cap has 30nH of series L, 0.12 Ohms of series R and 4Meg of
parallel R. My 150uH inductor has 62pF of parallel C and 0.04 Ohms of
series R.

The capacitor has a time constant (RC) of 400 seconds (100uF * 4Meg).
Three time constants is considered discharged because only 5% of the
original charge remains. The 150uH inductor has a time constant (L/R)
of only 0.00375 seconds. This is becase the parasitic R is much
greater in the inductor than in the capacitor. This makes the
inductor much more "impure" relative to capacitors and resistors.

4. Cost:

My 100uF cap costs \$0.18 my 150uH inductor costs \$1.42. It costs me
\$0.18 to store 1 joule of energy in the capacitor. It costs me
\$378.67 to store that same joule of energy in the inductor! That is
2,100 times more expensive!

Those are the reasons I hate L.

Mariss
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