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Delay on a mains-powered relay?

Discussion in 'Electronic Design' started by DaveC, Jan 5, 2005.

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  1. DaveC

    DaveC Guest

    I've got several remote alarm sensors triggered by 120vac relays. When power
    fails, the alarm calls the right folks.

    Problem is that even the small power glitches get these guys out of bed. Not
    a good thing.

    How can I delay the de-energizing of these relays? 5 minutes without power is
    OK. Will a series diode and shunt cap do the trick? If so, what size cap? If
    not, what're my options?

    The relay is similar to this one:

    http://tinyurl.com/6pppw

    Thanks,
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

    This is an invalid return address
    Please reply in the news group
     
  2. Ken Taylor

    Ken Taylor Guest

    Hi. Hmm, sorry but you should Goo.... Altavista this. Use the term 'delay
    relay'. Here's one:
    http://www.automatictiming.com/pages_div/timedelayrelays.html

    The more run-of-the-mill parts places don't sell them, but relay specialists
    do (not sure what the situation is in your area).
    Farnell also have these (they're possibly easier to use):
    http://au.farnell.com/jsp/endecaSea...=null&comSearch=true&manufacturerDisplay=true

    (They operate USA too, I just didn't have the link handy)

    Cheers.

    Ken
     
  3. majortom

    majortom Guest

    What would be the purpose of setting off ur alarm every time power
    fails??
    Not being nosey, it's just that the only alarm panels (fire/security)
    I've seen had some form of
    battery backup, whether it be Nicad, Lead Acid, etc. to keep the system
    running
    normally under power failure.
     
  4. Guest

    Not being nosey, it's just that the only alarm panels
    (fire/security)
    system

    Yes, but they still dial home to report AC loss.
     
  5. majortom

    majortom Guest

    k, then yeah the brit guy above pointing toward the time delay relays
    might be a way to go...
     
  6. legg

    legg Guest

    Assuming power is available to make the call, your control circuit
    obviously doesn't depend on 120VAC being present, to function. It is
    also unlikely that your remote sensors run on 120VAC, as this could
    represent an unneccessary safety hazard.

    Therefore the delay is best performed in the device's control section,
    using the power provided to perform the control function.

    5 minutes is an incredibly long delay to ask of a de-energized relay.
    It also could represent a signifigant security breach, if ignored. Why
    power loss should affect remote sensors is a question you should raise
    with the vendor.

    You should get the liable vendor or service rep to set the product up
    to meet realistic expectations that don't compromise security or
    convenient function.
    ...............................................
    Imagine if the delay was required of the hardware relay alone -
    assuming a 100mW coil , that drops out at 45% of the label voltage;

    5 minutes hold-up requires 30j minimum of stored energy.
    (.1 x 60 x 5)

    If the initial stored voltage was 165V, and drop-out was 75V, the
    storage cap required would be 700uF

    If the initial stored voltage was 12v and dropout was 5V4, the storage
    cap required would be 3 Farads.
    ....................................................

    This example used an extremely sensitive device - your referenced part
    consumes 80 times the power in the example and would require 80 times
    the stored energy, 80 times the capacitance.

    The obvious source of stored energy for any controlled delay is the
    controller's supply.

    RL
     
  7. NSM

    NSM Guest

    | I've got several remote alarm sensors triggered by 120vac relays. When
    power
    | fails, the alarm calls the right folks.
    |
    | Problem is that even the small power glitches get these guys out of bed.
    Not
    | a good thing.
    |
    | How can I delay the de-energizing of these relays? 5 minutes without power
    is
    | OK. Will a series diode and shunt cap do the trick? If so, what size cap?
    If
    | not, what're my options?

    The normal method is a relay with a hydraulic delay. This means you don't
    need a no fail power supply. However these days, if you have backup power,
    there are solid state equivalents.

    For your method you would need a large cap run off say, 12 VDC - maybe 1000
    uF and up.

    N
     
  8. DaveC

    DaveC Guest

    So the answer is... no.

    If power fails at the remote site, so does the control circuitry and its
    power. So a traditional delay relay won't work.

    The relay, powered by mains, simply opens the wireless sensor's sense circuit
    when power goes away. I just want it to do so after waiting a few minutes to
    see if power comes back on.

    The alarm isn't for security purposes, just for letting those responsible
    know that certain things have occurred, one of which is power fail. The
    sensor is a wireless device run on a lithium battery, and is a packaged deal
    so has no spare power available. The relay's not near the alarm CPU, so that
    power isn't available, either.

    Suggestions for a simple mains-powered circuit that will open a pair of
    contacts about 5 minutes after de-energizing? If it's to be battery-powered,
    it needs to be super-low drain. I rather keep battery replacements down to
    every 2 years, at most.

    An idea that just struck me: the presence of mains power could keep this
    circuit de-energized. When power fails, the battery is connected to the
    circuit which starts the countdown. When zero is reached (ie, 5 minutes have
    passed), a pair of contacts would open. And the energizing circuit
    (triggering these contacts) would only have to be a one-shot; once the alarm
    is triggered, the circuit could de-energize. That way the battery is utilized
    only briefly, during power-fail situations. How might I construct such a
    circuit?

    Thanks,
    --
    Please, no "Go Google this" replies. I wouldn't
    ask a question here if I hadn't done that already.

    DaveC

    This is an invalid return address
    Please reply in the news group
     
  9. Ross Herbert

    Ross Herbert Guest


    The full specifications for this relay are shown here
    http://my.execpc.com/~indelect/potter/pbku.html

    Your problem is that you are using the relay coil to directly monitor
    the mains voltage and since the relay has a release time of around
    10ms it doesn;t take much of a glitch to cause the relay to release. I
    don't think there is much you can do to delay the release of the relay
    itself unless you go through a major re-design process. The simplest
    method of achieving a delay is by connecting the relay coil across the
    output of a full-wave bridge rectifier and then connecting a large
    electrolytic cap (say 4,700uF) across the coil. However, the low
    resistance of the 120Vac coil is a problem and you would need to
    replace the 120Vac coil with a 110Vdc coil of 10,000 or 6720 ohms
    resistance (see spec data) in order to achieve a significant delay.
    You might be able to get around 10 - 20 seconds delay (at a guess)
    using the above method depending on the contact spring tension and the
    coil used (10,000 or 6,720 ohms) but you may need to experiment with
    even larger capacitor values.

    In order to achieve delays in the minutes range you would need to have
    some ancillary solid state delay circuit (with its own fail safe power
    supply) which becomes initiated whenever a mains failure occurs and
    only after the time-out period of this circuit is the alarm sent to
    the monitoring folks. If a mains failure is a simple glitch or one
    lasting several minutes (shorter than the time delay period) then the
    circuit would be reset to await the next failure. As you can see this
    adds complexity and the need for a separate fail safe dc supply for
    the electronics and this may not be a viable option.
     
  10. John G

    John G Guest

    I never cease to be amazed at the questions asked in this NG from people
    who do not have a clue from grade school experiments about electronics
    but want to design or modify industrial situations where some manager
    should be putting up the money to employ a competent engineer skilled in
    the particular area.

    Relying on poorly informed advice is asking for trouble and a sure
    course to the courts for a damages claim.
     
  11. Impractically big. If you get one big enough, Feerguy will want to hear
    from you. (;-)
    Back-up battery and inverter. A standard UPS would probably do. Do you
    know how much power the system consumes?
     
  12. Guest

    Can you use a solid state relay or a DIP relay, instead of the
    presumably larger relay you're using now? If so, the coil current
    will be much less, and you might be able to get away with something
    like a wall transformer, a big capacitor, and the relay. The time
    delay won't be very exact, but it may work for your application.

    Matt Roberds
     
  13. Use a Thermal Relay - maybe called "Delay Relay". Best approach is to find
    out what the format/socket is, then one of the suppliers of process control
    gear will have something that plug into it.
     
  14. Use something like this maybe. View in fixed width font.
    ____
    +---------+ +------|AC +DC|---+-----+ +---->Out
    | | | | | _|_ |
    )||( | Bridge | +|C | | o |
    120Vac )||(18Vac |Rectifier| === | RL|----|N/O
    )||( | | | |_ _| o |
    | | | | | | |
    +---------+ +------|AC____-DC|---+-----+ +---->Out

    A 120:18V transformer on the mains, bridge rectifier,
    capacitor smoothing, and 24V comms relay.

    Relays have an On/Off hysteresis. Once it has been
    energised, a 24V comms relay will not then release
    until the voltage has reduced to below about 8V.

    24V sinking to 8V is not far off T = R*C.

    A standard 24Vdc high sensitivity comms relay has a coil
    resistance of about 2900 ohms. If C= 47000uF then the
    the relay will be held closed for about 136 secs after a
    mains failure.

    A 47000uF (at 35vdc working) is not cheap, and the thing
    is a little clunky, but easy to put together and not much
    to go wrong.
     
  15. Fred Bloggs

    Fred Bloggs Guest

    The wireless "sensor's sense" ciruit is most likely something with a
    weak pull-up of say 100uA, so that shunting the relay input terminals
    with C=100uA/(3V/300secs)=10,000uF 10WVDC should get you close:
    View in a fixed-width font such as Courier.

    +--------[100]--+------->
    | |
    o | +
    / === to sensor
    o | 10,000u
    | |
    +---------------+------->
     
  16. No Spam

    No Spam Guest

    Someone suggested hydraulic. I was thinking pneumatic.
    Checkout:
    http://ecatalog.squared.com/catalog/173/html/sections/21/17321027.html

    Not cheap but do have a 5 min +/- 12% version.
     
  17. I don't know what is powering the relays, but can you use a UPS to keep it
    powered, but then time out?

    WT
     
  18. No.These relays are designed to work in ac.There's no way in storing ac in a
    capacitor and diode circuit.Only expensive inverter designs.You need some
    engineer to construct a dedicated circuit, maybe with battery, for your
    purposes.
     
  19. CJT

    CJT Guest

    Engineering in the U.S. has never had the political clout needed for
    effective enforcement of registration laws. A PE license carries with
    it almost no recognition in a world where just about anyone can get
    away with calling themselves an engineer (except where public funds are
    involved). Doctors and lawyers, and even beauticians, have done a
    better job getting their expertise recognized IMHO.
     
  20. CJT

    CJT Guest

    It's an AC relay. Just how do you intend to attach that "big capacitor?"
     
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