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snubber in contactor coil

Discussion in 'Electronic Design' started by a-bike, Oct 14, 2006.

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  1. a-bike

    a-bike Guest

    I have a problem of noise caused by the conmutation of a contactor that is
    hunging the dsp in the control board of my system. Some one told me the
    posibility of connecting a snubber or varistor in parallel with the
    contactor coil in order to reduce that noise. Does anybody knows about the
    method used to dimensioning the snubber?
     
  2. A common AC (you didn't say whether the contactor coil is AC
    or DC operated, so I guessed) coil snubber consists of a
    resistor in series with a capacitor, connected across the
    coil. When the driving contacts opens, the coil current
    detours to the capacitor, which limits the rate of change of
    voltage as it absorbs the energy that was stored in the
    coil. The lower the value of the resistor, the better this
    works. Except that when the driving contact closes at peak
    voltage, there is a large inrush current as the cap is
    popped up to that peak voltage, while the contact is
    bouncing. Bad for both RFI and contact life. So a
    compromise must be struck on the resistor value. Since the
    coil draws essentially no current during the bounce time,
    because of its inductance, I usually size the resistor to
    limit the peak current to no more than the contact current
    rating. The capacitor value and voltage rating must be
    large enough to contain the peak stored coil energy without
    exceeding its voltage rating. But way too much capacitance
    causes the series resistor to get hot while the coil is
    energized.

    In some cases, I have used a smaller capacitor than could
    safely store the peak coil energy, and limited its peak
    voltage by paralleling it with an MOV.

    So to design an effective snubber that doesn't get too hot,
    or damage the contacts, you need to start with information
    about the contact rating (that is driving the coil) the peak
    driving voltage, the coil's peak current and its inductance
    (to come up with the (I^2*L)/2 energy that gets transfered
    to (V^2/C)/2 in the capacitor.

    Or you but an off the shelf RC snubber rated for the coil
    voltage and hope it works and survives.
     
  3. John Popelish wrote:
    (snip)
    Sorry. The energy stored in the capacitor is (V^2*C)/2
     
  4. Fred Bloggs

    Fred Bloggs Guest

    It's hard to tell whether you're talking about snubbing the coil or the
    contacts or both? And the OP is classically conclusory: has no idea in
    hell what he's doing but still *knows* a snubber is the answer. You are
    a glutton for punishment it seems.
     
  5. 0.1 uF cap in series with 100 ohm resistor.
     
  6. Chris

    Chris Guest

    Either that or John's just a natural teacher.

    Cheers
    Chris
     
  7. Step 1) Look up contactor in catalogue.
    Step 2) Order the matching supressor/snubber/whatever it is called in catalogue.

    Then fun starts:

    3a) Find out that the leads, the mechanical fixing e.t.c. do not match
    contactor.
    3b) Find out that contacts need snubber after having got one for coil.
    3c) Find out that coil needs snubber after having got one for contacts.
    3d) Find out that this is after all not the real problem having got all snubbers
    available.
     
  8. Tom Bruhns

    Tom Bruhns Guest

    A snubber cross the coil will keep the flyback energy from the
    inductance from exceeding the breakdown voltage of the driving device,
    if properly designed, but the coil side should not be causing the sort
    of noise that would hang the DSP. I'd guess more likely the very fast
    risetime transients from the contacts opening and/or closing are the
    culprit. You can also apply a snubber to them. The snubber design
    depends on the load they are driving: the voltage, the current, and
    whether the load is resistive or inductive (or possibly capacitive).
    Beware that optimal design of a snubber for contacts is different than
    optimal snubber design for a solid-state switch driving a relay or
    contactor coil. You should be able to find web references for both; I
    have in the past, and some are quite good.

    Cheers,
    Tom
     
  9. Mike Monett

    Mike Monett Guest

    Hi Tom,

    Back in the old days, a laser company moved in next door and all our
    sensitive equipment went belly up. It turns out they used 10KW heaters to
    soften the glass, and the temperature control was a simple thermostat.

    Every time the contacts opened the arc resonated with stray inductance and
    capacitance in the wiring, and generated a burst of 40MHz noise, measured
    with my HP spectrum analyzer. The RFI went everywhere and was impossible to
    filter or shield against.

    Hertz would have been proud:)

    I talked to the owner and he allowed me to add simple snubbers across all
    the contacts. I believe it was something like 47 ohms in series with 0.1uF,
    but this was a very long time ago. Anyway, it was pure guesswork.

    Since there was no way of figuring out the wire inductance in the ratsnest,
    there was no calculation involved, and the first try solved the problem.

    I guess the trick was to prevent the arc from starting in the first place.

    Regards,

    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    http://silversol.freewebpage.org/index.htm
    SPICE Analysis of Crystal Oscillators:
    http://silversol.freewebpage.org/spice/xtal/clapp.htm
    Noise-Rejecting Wideband Sampler:
    http://www3.sympatico.ca/add.automation/sampler/intro.htm
     
  10. Tom Bruhns

    Tom Bruhns Guest

    Yep, exactly: for opening contacts, keep dv/dt low enough that the
    contacts spread fast enough that no significant arc develops. The arc
    itself should not be the problem; it's the very fast dv/dt when the arc
    finally quits. And to do the job, you need capacitance, with just
    enough series resistance to keep the current pulse in the capacitor
    when the contacts close under control. But the snubber across a
    contactor coil driven by a solid-state switch such as a triac will
    typically use a much larger resistance, too large to work well for a
    contact snubber.

    Arcs/sparks have a bad reputation for causing EMI, but it's not the arc
    itself, it's that the arc can act like a very fast switch. Spark
    transmitters used the spark to connect a capacitor in parallel with a
    coil, with the capacitor pre-charged to typically a rather high
    voltage. Though the spectrum was fairly broad, most of the energy was
    concentrated at the resonance of the capacitor and inductor. Spark
    transmitters generating up to a significant fraction of a MEGAwatt
    output were built and put in service to communicate roughly half-way
    around the world at what we'd now call VLF frequencies: in the tens of
    kHz.

    Cheers,
    Tom
     
  11. Rich Grise

    Rich Grise Guest

    I heard or read somewhere once something to the effect that the resistance
    should equal the inductive reactance, anthough they didn't say at what
    frequency, and as you've said, there was no way to measure the inductance,
    so a WAG, if it worked, was obviously the right answer. ;-) Also, the guy
    that made that reactance remark just said to use a large enough capacitor
    that its reactance is negligible; I'd think "critically damped" would be
    the value that would resonate at the inductance you used for the resistor
    calc, at the frequency of interest.

    At least that's a starting point - I tried to learn about snubbers once,
    but it turned out to be as much black magic as science, much like ferro-
    resonant transformers. :)

    Cheers!
    Rich
     
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