Relay and Mains question

Discussion in 'General Electronics Chat' started by vectravanman, Jan 11, 2016.

  1. vectravanman

    Thread Starter New Member

    Mar 2, 2012
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    HI all

    I am looking to design a circuit which when mains power drops (power cut for example) the load will also turn off. The load is a test rack which when power drops it will run on a UPS, but i want to do a circuit using a relay, probably even a Crydom SSR which will switch off the load when there is a power cut, bypassing the UPS. So i am not switching mains, i am using it as the input, and will be looking to switch low (10V) positive DC voltage off when there is a power cut. IS there a cost effective way of doing this without buying lots of components and building a PCB.

    Thanks for any assistance.
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Not trying to be snarky or anything, but if you want the rack to go down when power goes down, can't you just remove the UPS?

    ak
     
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  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What do you want to happen when mains power comes back? Automatic turn-on of the load, or stay off until manually turned on again?
     
  4. vectravanman

    Thread Starter New Member

    Mar 2, 2012
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    Sadly not an option, the equipment is designed to stay on and because the UPS doesn't last very long if there is a power cut when it loses battery life, the equipment applies a load to what its testing which recently damaged some of the test equipment. So i need a 'hard off current limiting circuit' which will cut current to the UUT when mains power goes off.
     
  5. vectravanman

    Thread Starter New Member

    Mar 2, 2012
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    Would ideally want the load to turn back on but it is not a necessity.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Just use an AC relay with coil rated for the mains voltage
    Connect the coil to the mains and the relay NO contact in series with the load.
     
  7. vectravanman

    Thread Starter New Member

    Mar 2, 2012
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    I was thinking about just doing that, just needs to be damn safe and not too pricy. Will require a few relays.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    How many loads?
    You can buy relays with multiple poles.
     
  9. vectravanman

    Thread Starter New Member

    Mar 2, 2012
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    Yeah i know, no more than 6 loads just now but increasing in the future.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    You can buy 6PDT relays which would switch 6 separate loads.
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    IF we're talking about removing power from multiple AC powered devices,
    AND IF all are powered from the same mains feed,
    THEN why do we need more than 1 or 2 poles?

    ak
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Because that's not what we are talking about. ;)
    He needs to remove the DC loads that the test equipment is driving when the AC fails.
     
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  13. vectravanman

    Thread Starter New Member

    Mar 2, 2012
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    Yeah crutschow you are correct, thanks for the assistance. Will post an update and circuit when i'm finished.
     
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