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Inductive clamp energy

Discussion in 'Electronic Design' started by The Phantom, Jun 21, 2007.

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  1. The Phantom

    The Phantom Guest

    There have been several threads about clamping relay coils, etc. There are
    some subtleties connected with this whole topic that I haven't seen
    mentioned. I've posted a paper from 1968 over in ABSE that explains it

    This is an interesting topic with practical applications, and I hope the
    paper stimulates some discussion.
  2. That was posted over 8 hours ago. Nothing in abse yet.
  3. Winfield

    Winfield Guest

    I was able to grab it about 7pm EDT last night, 9 hrs ago.
    But I haven't read or even more than glanced at it yet (it
    looked complicated), so you're at no disadvantage, so far!
  4. The Phantom

    The Phantom Guest

    Well, it does take a little while to digest it all.

    Briefly, what they're showing is something that, to borrow a phrase John
    Popelish used nearly 3 years ago, "...we knew but didn't realize." Any of
    us could figure it out, if only we realized we should.

    As shown in this app note:

    the suppressor (clamp) can be connected across the switch *or* across the
    relay coil (inductor).

    If the clamp is across the inductor, then a simple zener can't be used,
    because the zener will conduct in the forward direction when the switch is
    on; another diode must be placed in series with the zener. A bidirectional
    suppressor (two zeners in one package, or a MOV) solves that problem, but
    may be more expensive or hard to get.

    If the clamp is across the switch, then a simple zener can be used, but as
    shown in the paper I posted on ABSE, the dissipation in the clamp is not
    just due to the energy stored in the inductance.

    The thing to realize is that when the clamp is across the switch and the
    switch turns off, the current in the inductor is decaying and the power
    source, inductor and clamp are all carrying the same current. The
    direction of current in the DC power source is such as to represent energy
    flow *out* of the source. Since we believe in conservation of energy, we
    see that the energy that must be absorbed by the dissipative elements
    (mainly the resistance of the wire of the inductor and the clamp) will be
    *not only* the energy stored in the inductor at turn-off. The energy
    supplied by the power source during time while the inductor current is
    decaying must be absorbed, as well as the inductor's stored energy.

    The authors of the paper I posted over on ABSE show that if the clamp
    voltage (when the clamp is across the switch) is only slightly higher than
    the power source voltage, the time for the inductor current to decay is
    increased, and the energy the clamp must absorb can be substantially more
    than L*I^2/2.

    The situation is different if the clamp is across the inductor, and this
    factor should be taken into account when deciding where to place the clamp.

    The increase in energy to be absorbed when the clamp, or snubber, is across
    the switch is a phenomenon that also should be taken into account in
    switching power supplies.
  5. The Phantom wrote:
    The possible up side of having the snubber across the switch
    (that might pay for the higher power dissipation during
    inductive discharge, in some cases) is that the supply
    current is continuous during the process, so the supply
    voltage is not bumped up as much as when the snubber is
    across the coil and the supply current switches of as fast
    as the switch current drops.

    Generally, I would rather use bypass capacitance to smooth
    the bump than dump extra energy into the clamp.
  6. The Phantom wrote:

    When I read this, I assumed you were remembering something
    someone else had said, but had my name attached to the
    memory. I was wrong. I found it with Google and enjoyed
    reading the thread all over again. It is a shame my brain
    is full and I have to lose a memory to make room for a new one.
  7. Jon

    Jon Guest

    I don't get out much...What is ABSE?
  8. If your newsgroup browser supports it, you can use this link to get
    directly to the indicated article:

    Otherwise, look for alt.binaries.schematics.electronic and search for
    the subject looking like: Energy in clamps--from SED - Inductive
    Energy Calculations...

  9. Rich Grise

    Rich Grise Guest

    He's a googlie - they don't get binaries.

    What Jon needs to do is contact his ISP, and get the IP/URL of his
    newsserver, and find a real newsreader (butthook distress sucks), and then
    he can post/read anywhere he wants to.

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