unique relay operation needed

Discussion in 'The Projects Forum' started by jakestevens, Jul 13, 2011.

  1. jakestevens

    Thread Starter New Member

    Jul 13, 2011
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    I am looking for a relay that when power is initially applied to the coil the contacts don't change state until power is removed. Then after a set time the contacts should go back to normal state. Any ideas?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, it's pretty hard to tell what you want the relay to do from your description.

    What is the voltage and current rating required of the contacts?
    Does the coil side need to be at the same voltage as the contacts, or is it different?
    What state are the contacts supposed to be in when power is first applied?
    What is "normal"? Normally closed (NC), or normally open (NO)?

    Also, I can't really picture what you want the relay to do. You say that you don't want the relay to change state when power is 1st applied to the coil (which is what the coil is for; change the state of the contacts) - and then when the power is removed, the contacts are supposed to move to a different state?

    [eta]
    Why don't you start out by describing what it is that you want to control with the relay, and what the relay will use to energize/de-energize its' coil with.
     
    Last edited: Jul 13, 2011
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    You want a relay to close for a pre-determined time when you REMOVE power?

    Huh?

    Got battery?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Thanks, ErnieM - now I "sort of get" what our OP wants to do. I missed the actuation on closing part.

    What our OP was trying to describe would make sense if they wanted to do something like discharge caps in a power supply when the power is turned off, without having to use a bleeder resistor or the like.

    But, we still need a better description of what they are actually trying to do.
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    A ratchet type relay would do what he wants as far as doing something when power is removed after being applied, but I can't figure out how he would restore to original state after a given time. Sounds like he needs a couple of inter-connected individual timers.
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    If I wanted to do it with readymade relays, I'd use one normal relay and one "true off delay" relay.

    The NC contact of the normal relay is in SERIES with the NO contact of the off-delay relay.

    - Apply voltage to both at the same time.
    - The NC opens , the NO closes
    - turn off the power supply
    - the NC of the normal relay closes and the NO of the delay relay remains closed for the adjusted time.

    The actuation time of the off-delay relay has to be slower than that of the normal relay, if not you can put an On-delay relay in series with the Off-delay relay.

    If seen Off-delay relays for up to 300 seconds..

    Of course this solution is expensive. If you build your own circuit it'll get much cheaper.
     
  7. strantor

    AAC Fanatic!

    Oct 3, 2010
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    if you want a time delay after power is removed, then you should be looking for electropneumatic off-delay timer relays. They will switch immediately when power is applied (which if I understand correctly is what you don't want), but you could get around this easily by adding another relay in series (contacts in series, coils in parallel).
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    darn, beat me typing
     
  9. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    LOL, I hadn't read your post while I was writing...
     
  10. strantor

    AAC Fanatic!

    Oct 3, 2010
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    likewise. I think we hit submit at the same time
     
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