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Thread: Switching AC / DC Relays and

  1. #1

    Default Switching AC / DC Relays and

    Still some comments:

    Controlling AC type relays or contactors is tricky and cannot be done
    easily with electronics (particularly not with transistors).
    In case an electronic control circuit is necessary, a triac or similar
    device could be used.
    These should preferably be controlled in "zero current" mode, that is, the
    control circuit must wait until the current is zero and not the voltage of
    the AC supply. (90 degrees lagging current)

    There are (or were), special ICs for this application.

    If no zero current mode is possible nor used, then a snubber circuit should
    be connected in parallel with the coil, but still it is tricky business.
    (the Triac typicaly will not switch off reliably or at all, and also could
    be damaged). A snubber must contain a resistor AND a capacitor in series
    (rather difficult to design, except by trial and error).

    I have never seeen shunt (parallel) resistor with AC contactor coils to
    improve the switching, though who knows, I may be getting too old.

    Another comment, in case of DC powered coils and wheel diodes:

    Circuit theory easily shows that the maximum current that an inductance
    will try to pass through an external circuit is equal (and NOT higher) to
    the current that was circulating on it immediately before the power is
    switched off.

    I don't like analogies, but as an exception: you don't expect your car to
    jump into hyperspace like the Enterprise when you clutch suddenly; it will
    try to keep its previous speed for a while and (more or less) quickly slow
    down.

    Inductances are similar. So no risk of overloading the wheel diode if it is
    rated at least for the normal operating current of the relay coil.

    Note that we a re lucky enough or Nature is so wise, as to have the voltage
    on the inductance reverse its polarity (Lenz law) upon switch off, so that
    we can connect our diodes without polarity problems.

    But the real problem is that if you do not provide an alternative path to
    the current the inductors try to maintain after being switched off, the
    indutance will try to overcome the resistance by proportionally increasing
    the voltage on its terminals (the infamous spike that burns any transistor).

    ANY resistance on the circuitt (such as the "shunt" resistance we are
    talking about) can only increase the voltage over the original value
    applied to the inductance.

    That is, simply put, why diodes are the best solution in DC circuits (they
    have practically zero resistance in the conducting state).

    Regards,

    Jose Luis
    At 09:25 05/10/2001 -0400, you wrote:
    >Mark,
    >I wasn't quite "with it" yesterday, and didn't think your question through
    >completely.
    >A "clamping" or "wheel" diode across a relay's coil works great *IF* the
    >voltage applied to the coil is DC. However, this won't do for AC, as the
    >clamping/wheel diode would present a dead short to ground for 1/2 off the
    >cycle (when the AC voltage went negative).
    >
    >For this reason, in AC circuits only, you must use a shunt resistor across
    >the relay's coil (or other inductive load).
    >
    >-----Original Message-----
    >From: Mark Adams [mailto:madams@a...]
    >We were looking at some BOSCH relays and they listed an option that
    >included a 'shunt resistor' and the ones we currently purchase with an
    >included diode. Now, the question is why is the shunt resistor there?
    ><snip>
    >
    >
    >
    >To UNSUBSCRIBE, just send mail to:
    > basicstamps-unsubscribe@yahoogroups.com
    >from the same email address that you subscribed. Text in the Subject and
    >Body of the message will be ignored.
    >
    >
    >Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/


    [Non-text portions of this message have been removed]
    Last edited by ForumTools; 09-30-2010 at 04:57 AM. Reason: Forum Migration

  2. #2

    Default

    Switching highly inductive AC relay coils (large contactors) is very
    easy to do. Most Solid State Relay modules do employ zero switching.
    An appropriatly sized SSR can easily handle the demands of a
    contactor coil. Also, a transistor can drive a small DC relay, which
    can then switch the contactor. Finally, the difference between an AC
    coil and a DC coil is that the AC coil has a "chatter" ring on the
    coil. A coil is after all a coil. Almost any electromechanical relay
    with an AC coil can be operated by DC. A relay with a DC coil wont
    work well with AC due to the absence of the chatter ring on the coil.
    Over the years I have driven many AC coil relays with DC with no ill
    effects.

    Precautions should be taken to assure that the power supplying
    sensitive electronics is well filtered and regulated. I almost always
    use .1uF caps across Vdd and Vss pins

    My two cents

    Regards
    rich


    --- In basicstamps@y..., Jose Luis Juarez <jl.ingenieria@i...> wrote:
    > Still some comments:
    >
    > Controlling AC type relays or contactors is tricky and cannot be
    done
    > easily with electronics (particularly not with transistors).
    > In case an electronic control circuit is necessary, a triac or
    similar
    > device could be used.
    > These should preferably be controlled in "zero current" mode, that
    is, the
    > control circuit must wait until the current is zero and not the
    voltage of
    > the AC supply. (90 degrees lagging current)
    >
    > There are (or were), special ICs for this application.
    >
    > If no zero current mode is possible nor used, then a snubber
    circuit should
    > be connected in parallel with the coil, but still it is tricky
    business.
    > (the Triac typicaly will not switch off reliably or at all, and
    also could
    > be damaged). A snubber must contain a resistor AND a capacitor in
    series
    > (rather difficult to design, except by trial and error).
    >
    > I have never seeen shunt (parallel) resistor with AC contactor
    coils to
    > improve the switching, though who knows, I may be getting too old.
    >
    > Another comment, in case of DC powered coils and wheel diodes:
    >
    > Circuit theory easily shows that the maximum current that an
    inductance
    > will try to pass through an external circuit is equal (and NOT
    higher) to
    > the current that was circulating on it immediately before the power
    is
    > switched off.
    >
    > I don't like analogies, but as an exception: you don't expect your
    car to
    > jump into hyperspace like the Enterprise when you clutch suddenly;
    it will
    > try to keep its previous speed for a while and (more or less)
    quickly slow
    > down.
    >
    > Inductances are similar. So no risk of overloading the wheel diode
    if it is
    > rated at least for the normal operating current of the relay coil.
    >
    > Note that we a re lucky enough or Nature is so wise, as to have the
    voltage
    > on the inductance reverse its polarity (Lenz law) upon switch off,
    so that
    > we can connect our diodes without polarity problems.
    >
    > But the real problem is that if you do not provide an alternative
    path to
    > the current the inductors try to maintain after being switched off,
    the
    > indutance will try to overcome the resistance by proportionally
    increasing
    > the voltage on its terminals (the infamous spike that burns any
    transistor).
    >
    > ANY resistance on the circuitt (such as the "shunt" resistance we
    are
    > talking about) can only increase the voltage over the original
    value
    > applied to the inductance.
    >
    > That is, simply put, why diodes are the best solution in DC
    circuits (they
    > have practically zero resistance in the conducting state).
    >
    > Regards,
    >
    > Jose Luis
    > At 09:25 05/10/2001 -0400, you wrote:
    > >Mark,
    > >I wasn't quite "with it" yesterday, and didn't think your question
    through
    > >completely.
    > >A "clamping" or "wheel" diode across a relay's coil works great
    *IF* the
    > >voltage applied to the coil is DC. However, this won't do for AC,
    as the
    > >clamping/wheel diode would present a dead short to ground for 1/2
    off the
    > >cycle (when the AC voltage went negative).
    > >
    > >For this reason, in AC circuits only, you must use a shunt
    resistor across
    > >the relay's coil (or other inductive load).
    > >
    > >-----Original Message-----
    > >From: Mark Adams [mailto:madams@a...]
    > >We were looking at some BOSCH relays and they listed an option that
    > >included a 'shunt resistor' and the ones we currently purchase
    with an
    > >included diode. Now, the question is why is the shunt resistor
    there?
    > ><snip>
    > >
    > >
    > >
    > >To UNSUBSCRIBE, just send mail to:
    > > basicstamps-unsubscribe@y...
    > >from the same email address that you subscribed. Text in the
    Subject and
    > >Body of the message will be ignored.
    > >
    > >
    > >Your use of Yahoo! Groups is subject to
    http://docs.yahoo.com/info/terms/
    >
    >
    > [Non-text portions of this message have been removed]
    Last edited by ForumTools; 09-30-2010 at 04:57 AM. Reason: Forum Migration

  3. #3

    Default

    Another possibility for switching an AC load is to put it in series
    with the input (AC) terminals of an appropriately rated full-wave
    bridge. Your switching transistor goes inside the bridge and sees
    only DC. Comments about transients still apply. If the control
    circuit doesn't require much current, you can often develop a
    half-wave supply for it, with a separate rectifier off the same AC
    supply referenced to the (-) side of the bridge.

    This is works best for fairly small loads with control circuits that
    don't have any opportunity for user contact, like fully-enclosed
    appliance timers etc. since it is NOT isolated from the power line.

    I'd draw it but I'm ASCII-art-impaired :-)

    Reg Neale
    Last edited by ForumTools; 09-30-2010 at 04:57 AM. Reason: Forum Migration

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