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stand-off voltage of zener TVS device for a relay coil

Discussion in 'Electronic Design' started by Matt, Jun 12, 2007.

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  1. Matt

    Matt Guest

    I'm switching a 5VDC relay coil using a FET and a microcontroller.

    I'm comfortable using a bidirectional transient voltage suppressor
    (which functions like two zener diodes back-to-back) across the
    coil---except for a little issue with the stand-off voltage. I think
    that that is supposed to be the normal operating voltage of the device
    (in this case 5V). The stand-off leakage current seems to be the
    current when the device is run at the stand-off voltage (800uA for the
    5V device).
    That doc also mentions a "test current" of 10mA. I'm guessing that that
    is what defines the clamping voltage. Am I understanding these parts of
    the datasheet?

    Browsing at Digikey, I notice that some of the TVS devices have a "5%"
    designation, but that the listings for most of the TVS devices don't
    mention such a percentage. Would you suppose that that is a tolerance
    for the stand-off voltage or maybe for the clamping voltage? If so, I
    expect that a -5% error might cause some noticeable increase in the
    stand-off leakage current, which is already considerable at 800uA.

    My impulse is to simply use a TVS with a 6 (six) volt stand-off voltage.
    I expect that that would take care of any leakage, yet keep the
    clamping voltage well below any level that might damage the transistor.

    Do you see any downside in using a 6V TVS with a 5V coil?
  2. Eeyore

    Eeyore Guest

    It looks like it.

    If you want your relay to 'snap' open you'll have to use a higher voltage part
    than that though.

  3. tlbs101

    tlbs101 Guest

    You are almost always "safe" with just a diode-drop up to about 1 volt
    over the rated coil voltage, for doing no harm to the rest of the
    driver circuitry.

    I have a current (as in 'now') design using 28VDC relay coils. They
    are rated for a maximum voltage of 32 VDC, and the TVS snubber device
    is rated for 42 VDC. My relay drivers will withstand 50V at their

    Find out what the Vds-max. is for your FET. 80% of that max. is a
    good place to start in sizing the TVS.

    Good luck,
  4. Matt

    Matt Guest

    The Fairchild and Vishay datasheets I found are silent about the
    technology of the devices. The ST datasheet below is better in that it
    states that the TVS devices are made with transils. It also makes
    clearer the meanings of stand-off, breakdown, and clamp voltages. Those
    are the voltages at 800uA, 10mA, and peak-pulse currents respectively.
  5. Mook Johnson

    Mook Johnson Guest

    Why not just an simple diode across the coil. Most applications don't care
    about the slowing of the turnoff due to the freewheeling currents.
  6. Matt

    Matt Guest

    42V clamping voltage, not 42V stand-off or breakdown voltage, I hope.
    It is 100V.
    I guess you're saying it's good to have as high a clamping voltage as
    possible, as long as the driver circuitry is not endangered. You
    mention a place "to start"---what would make somebody change from that
    80% starting point?
  7. I found this on the Onsemi site,
    It may be of interest

  8. Matt

    Matt Guest

  9. Matt

    Matt Guest

    You have a rather narrow range (28V to 50V) to work with, so you might
    benefit from a lower clamping factor. I found the following at the link
    provided by martin.
  10. mpm

    mpm Guest

    I read this. Sounds like crap to me.
    They do not quantify "optimal" life.

    So if a relay is good for 100,000 closures, and spending all this
    money on suppression gets you up to 100,041 closures, I say: "Forget

    As for the 9.8mS claim using just a diode, this seems excessively long
    to me. And in any event, is an event less than one AC cycle in
    duration assuming 60-Hz. Next thing you know, they'll want us to use
    a zero-crossing detector.....

    I remain unconvinced that anything more than a simple reversed-bias
    diode across the relay coil is needed here.

    And remember, this "study" is coming from a company that makes relays
    (and therefore has a vested interest in lowering warranty returns,
    etc...), and also makes zeners, diodes and resistors. Coincidence??

  11. Matt

    Matt Guest

    I searched their website for the zener diodes and TVS devices
    recommended in those appnotes. I did not find any. I did find
    gas-discharge tubes, MOVs, capacitors, and resistors. They do not
    recommend capacitors and resistors for this application. They give
    honorable mention to MOVs, but they clearly prefer TVSs.
    Probably their P&B people want the customer to use the relay products in
    the best way.
  12. nospam

    nospam Guest

    It sounds quite reasonable to me.

    Assuming a relay doesn't fall to bits mechanically first what it is 'good'
    for depends on what the contacts are switching. The faster you open the
    contacts the faster you extinguish the arc drawn between them and the less
    contact material get vaporised.

    It is a shame they didn't go as far as running some trials, not hard to
    rattle a few loaded relays for a couple of months with different coil
    'suppression' circuits and then inspect the contact wear.

  13. Matt

    Matt Guest

    The other app note shows V and I versus time for a 1) 12V relay alone,
    2) relay with a diode, and 3) relay with a diode and a 24V zener.

    Is the extra cost that much of an issue? Digikey is selling TVS devices
    for $0.11 each in quantity 750. If that is $0.08 more than a diode, the
    difference is under 5% of the cost of the relay, probably closer to 2%.
  14. Matt

    Matt Guest

    Got it.
  15. Eeyore

    Eeyore Guest

    I still don't understand why you don't use a (reverse blocking) diode and a
    zener in series.

    There no danger of any forward conduction at all and I expect it'll be both
    cheaper and easier to obtain the parts.

  16. Matt

    Matt Guest

    I don't see much problem finding the parts. Yes, I guess the two-part
    design would be a little cheaper. I may be a little tight on board space.
  17. mpm

    mpm Guest

    The article smacks of a "white paper".
    They should have run the trials.

    I am really tempted to peek at their relay offering to see how many
    might come with internal reversed bias-ed diodes...
  18. mpm

    mpm Guest

    Hey Matt. I thought your TVS's cost more than that.
    In small quantities, it's probably a wash.

    Still, designing for cost efficiency is a REQUIRED skill in today's
    job market!
    When you're making a million widgets, every penny saved is worth

  19. mpm

    mpm Guest

    Actually, I suspect for AC loads it depends on WHEN the contacts are

    Which brings me to a story about a transfer switch (a big relay).
    I'll keep is short...

    Mnf = Zenith. (I mention this because it is a real POS! - Don't ever
    buy one.)
    400-Amp, 3-ph, 480/277 Volt.

    This baby had the most elaborate zero crossing detector you ever saw.
    It must have cost a couple hundred dollars - easy.

    This xfer switch was used to switch a FM transmitter from main to
    emergency generator power automatically. If power was lost, the
    transfer switch would start the generator and then switch to it. When
    main (utility) power returned, it would switch back.

    The phase monitor would try to throw the switch at the exact instant
    in time when the two AC waveforms (obviously not synched to each
    other), crossed the zero volt reference at the same time. (This would
    guarantee minimum voltage on the contacts, etc... and is a great

    The problem was, the switch timing was so bad (awful, actually!), that
    I don't know, anywhere from 10mS to maybe as high as 500mS might
    elapse before the contact would actually engage. (Poor mechanical
    switch actuation, not a command problem!). Like I said, real piece of

    And of course, you can guess what happens next: Generator output on
    top of cycle, utility on bottom, switch closes... main breaker pops.
    Damn!! Off the air. Again! (And isn't this exactly what an
    "emergency" generator is supposed to prevent??

    Problem was we had 8 of these babies in service at the facility.
    (Hey, they were cheap and I didn't spec 'em!!). Our "solution" was a
    forced timed shutdown after utility power restored, and we ended up
    throwing the switches in sequence, a couple at a time. Totally not
    the ideal fix.

  20. When you're making a hundred widgets, every hour spent on engineering
    is worth around a dollar apiece.

    Best regards,
    Spehro Pefhany
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