Launch relay when power off

Discussion in 'The Projects Forum' started by parmaja, Apr 9, 2012.

  1. parmaja

    Thread Starter Active Member

    Jul 2, 2008
    37
    0
    Hi, team

    Description:

    This circuit launches the relay when the power comes off using a capacitor charged when the power on, so it is not needed any extra power for the relay.
    Increase the size of this C1 to increase the delay time, the power is 6v.

    Benefit:
    I have a server computer connected to a UPS that give me 10 minutes backup time when the main power goes off, at this time I must shut down the computer.
    The problem my UPS doesn't have any feature to send that signal to my computer, and any new UPS will be expensive.
    Also my computer has no any COM port to accept the old type of UPS.

    The circuit powered by a transformer connected directly to main power not to the UPS.

    I will take two wires from the Power Button (on the mother board) and link it to this relay. When the main power off, my computer still on, and gets a signal to shut down from that relay as like if we press the Power button, we need to config our system (Windows 2003) to take this action.
    Also, I configured my bios to turn on when the AC comes back, so when the main power come, my computer will turn.

    Until now I have tested it in the breadboard, and I need some one approve it before put it in my blog, or check it if we can improve it.

    This is my first circuit at my age, I am just a programer.
    Thanks in advance

    Zaher Dirkey
     
    Last edited: Apr 9, 2012
  2. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    What is the purpose of C2? It would do more on the base of the transistor (after the diode) instead of across the power supply (before the diode). Is that supply noisy DC, or AC?

    D1 and D2 are redundant. A single diode at the power supply would accomplish the same thing.

    There would be other ways to accomplish this, but if you're happy with your relay, go for it.
     
  3. parmaja

    Thread Starter Active Member

    Jul 2, 2008
    37
    0
    I am not sure about C2, it reduce the noise of DC that come from,
    I will move it after the diode.
    Ah, good :D

    I've searched for this circuit for days can not find one do the same.

    No not happy with Relay, but it is more safe when connect to computer wires.

    Thanks.
     
  4. chowdhas

    New Member

    Apr 6, 2012
    10
    0
    Hi Parmaja,
    I am also a novice so can't help with your circuit directly. But it sounds quite like one I needed help with - though I needed to switch a relay when the power went on as well as off. If you look at my thread "Differentiator ciruit help" http://forum.allaboutcircuits.com/showthread.php?t=68481 - you might find some useful ideas from the experts who responded.
     
  5. parmaja

    Thread Starter Active Member

    Jul 2, 2008
    37
    0
    Not worked, because D1 prevent the capacitor C1 from apply postive value on transisotor base, is that right?
     
    Last edited: Apr 10, 2012
  6. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    I don't understand your current arrangement. Please post another schematic.
     
  7. parmaja

    Thread Starter Active Member

    Jul 2, 2008
    37
    0
    1 - Thanks for the replay, I edited my previous post, (I have mistakes when quote)

    Same original schematic, if I replace D1 and D1 with one D2, and connect R1 to after D2, is that you meant?

    It is not working, because C1 applies a positive value on the Base of the transistor?.

    2 - I hear a noise from my relay, some time after a random period it is clicked, while the power on.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    Yes, but I misunderstood your circuit because I assumed (never assume!) it was a DC circuit. It must be AC? You need both diodes. Sorry about that.

    My comment on C2 is still valid - it's not doing anything except to put extra capacitive load on your power supply.
     
  9. parmaja

    Thread Starter Active Member

    Jul 2, 2008
    37
    0
    Thanks wayneh

    It is DC circuit.
    Now i have another problem when testing,

    2 - I hear a noise from my relay, some time after a random period it is clicked, while the power on.

    Is that becuase the relay is sensitive, or that BD140 is bad transistor for this kind of circuit?
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    Well now I'm confused again. With true DC, Q1 should be off, since the voltage at the base is only 0.125V less than the collector, as set by the R1 and R2. With the transistor off, the relay should not energize. The transistor won't turn on until power is lost, and then it will rapidly discharge C1 through the relay. Is this the intended behavior?

    With rippling DC, the transistor lets through enough current to chatter the relay. Putting C2 back in on the base-side of D1 – either end of R1 – may help quiet it down.
     
  11. parmaja

    Thread Starter Active Member

    Jul 2, 2008
    37
    0
    Yes, the relay work (chatter) only when power loss, I need it to press the Power Button in my server.

    I will test it and post the new schematic.

    Thanks a lot :D
     
  12. parmaja

    Thread Starter Active Member

    Jul 2, 2008
    37
    0
    Hi, i put this circuit in my blog, thanks again :D .
     
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