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Why I hate L

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  • mariss92705
    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
      inductor dissipates it in about 0.011 seconds. This is about 100,000
      times worse! Why? Read on.

      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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