Grid Return Alarm

Discussion in 'The Projects Forum' started by RodneyB, Oct 19, 2014.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
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    I witnessed the worst accident ever yesterday that will haunt me tell the day I die and could have been prevented.

    Where I live there is long periods of no power. There is no warning when the power is going to go or come back and is often off for 16 hours at a stretch.

    Stoves and heaters are often left on and thus this accident. I would like some help to develop an alarm that when the mains returns after a power cut a buzzer sounds, It needs to have a push button pushed to reset it, thereby preventing accidents.

    I have unsuccessfully searched the net for ideas. So any help will be greatly appreciated
     
  2. ISB123

    Well-Known Member

    May 21, 2014
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    The power company could send automated phone calls 30 minutes before the power comes back on.Buzzer could trigger another explosion since it would require electrical power and be connected to the power line so basically everything else would work including stoves and heaters.The buzzer would have to run off the power grid and signal would have to be sent from the power company to start the buzzer.
     
  3. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    I live in Zimbabwe, There is no way the power company would phone. The warning buzzer is to indicate that power has been restored. It stops buzzing when you push the reset button. When the Buzzer sounds you no power has been restored when resetting the buzzer you can check to switch off any appliance that was left on when the power went
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    A simple self-latching relay. The relay needs to have Form C (No/Nc) contacts. Coil would have to be 240Vac.

    The Push button latches it to clear the alarm. The power failure causes it to unlatch (armature drops out), turns on lamp or buzzer.



    pfa.jpg
     
    Last edited: Oct 19, 2014
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  5. Johann

    Senior Member

    Nov 27, 2006
    190
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    I am away from home, but hope to post a simple circuit soon. (My "scanner" is a cheap cell phone camera).
     
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  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If it's a low-voltage buzzer you need to drive, then the attached circuit could be used, powered by a DC-output wall-wart such as a phone charger.
    MainsFailAlarm.gif
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The circuit in post #4 could include a 100a/200a contactor (U1) that would hold the power off until reset, if you want disconnection to remain until someone is present.
    In industrial equipment this is often done with a magnetic trip breaker, voltage has to be present to latch the breaker in.
    Max.
     
    Last edited: Oct 19, 2014
  8. Johann

    Senior Member

    Nov 27, 2006
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    As promised, herewith my attempt (primitive!), but all depends on availability of components. I do understand your position in Zim!
    The circuit components are not critical, so you may make use of a wall wart that gives you roughly 12 V DC and connect the rest as shown. Depending on what relay you can lay your hands on, will determine the size of the capacitor in series with it. The 1M ohm resistor bleeds away the charge when the power is off. When power comes on, charging current will flow into the capacitor and through the relay coil until the cap is full. Thereafter the relay will theoretically drop out again, but before that happens, its one N.O. contact will latch it on. The other N.O. contact will turn on the 12V piezzo sounder (or buzzer). This will continue to make a noise until the reset button is pressed. My apologies for the image from the cell phone!
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If it is ~16 hrs at a stretch I would think you would want disconnect in case someone is not present?
    Max.
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Sounds like a single phase DOL starter circuit to me..
     
  11. Johann

    Senior Member

    Nov 27, 2006
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    A single phase DOL starter will do the opposite: You have to start it and when mains fail it will stop and will not start automatically when mains returns.
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Thats exactlty what the op is trying to achieve, manual on not automatic.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That is my interpretation also.
    Max.
     
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  14. Johann

    Senior Member

    Nov 27, 2006
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    30
    My fault! Did not read the problem properly! Shows what travelling can do to your concentration! You're right, D.O.L. is perfect with contactor rating equal to rating on Main breaker.
     
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