+ Post New Thread
Results 1 to 5 of 5
  1. #1
    Full Member level 6
    Points: 2,916, Level: 12

    Join Date
    Aug 2012
    Location
    .
    Posts
    376
    Helped
    7 / 7
    Points
    2,916
    Level
    12

    MAINS transfer relat circuit: does this look safe and reliable?

    This is a AC transfer relay circuit that transfers from 120V MAINS to battery backed 120V inverter when the MAINS fail.

    Load is 0.25A
    Relay is EE2-24NU surface mount rated 2A, 240Vac (24Vdc coil).
    TVS is to protect the capacitor.

    Why not just use an ac relay? 3x cost, larger and no SMT.


    Do you think it will be safe and reliable?

    •   AltAdvertisment

        
       

  2. #2
    Super Moderator
    Points: 78,697, Level: 68
    Achievements:
    7 years registered
    Awards:
    2nd Helpful Member
    betwixt's Avatar
    Join Date
    Jul 2009
    Location
    Aberdyfi, West Wales, UK
    Posts
    12,887
    Helped
    4303 / 4303
    Points
    78,697
    Level
    68

    Re: MAINS transfer relat circuit: does this look safe and reliable?

    If the relay contacts are sufficiently rated there is no reason why an AC relay can't be used. I am a little suspicious of the relay in the schematic, a nominal rating of 24V id OK, a minimum operating voltage of 18V is reasonable but a drop out voltage of only 2.4V sounds a bit low to me.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



    •   AltAdvertisment

        
       

  3. #3
    Super Moderator
    Points: 51,502, Level: 55

    Join Date
    Apr 2011
    Location
    Minneapolis, Minnesota, USA
    Posts
    12,641
    Helped
    2514 / 2514
    Points
    51,502
    Level
    55

    Re: MAINS transfer relat circuit: does this look safe and reliable?

    I like to experiment with simulations, so here's your schematic. 5k carries sufficient current to provide 24v 8mA. Notice the resistor carries 4W peaks.

    Click image for larger version. 

Name:	mains 120vac resistive drop diode smoothing cap 24vDC load 8mA.png 
Views:	0 
Size:	32.5 KB 
ID:	152562

    Is there a reason you ruled out a capacitive drop, to reduce mains voltage? It's feasible for low power requirements.



    •   AltAdvertisment

        
       

  4. #4
    Super Moderator
    Points: 254,978, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    44,430
    Helped
    13528 / 13528
    Points
    254,978
    Level
    100

    Re: MAINS transfer relat circuit: does this look safe and reliable?

    EE2-24NU has only 60W/125VA switching power, unlikely sufficient for your application.

    Generally, using at DT contact for mains switchover is never safe. The "open" arc won't be extinguished before the new contact closure, causing a short between both power sources. If switching under load, you need separate contactors for both sources with several 10 ms deadtime.



  5. #5
    Super Moderator
    Points: 73,726, Level: 66
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    14,964
    Helped
    3406 / 3406
    Points
    73,726
    Level
    66

    Re: MAINS transfer relat circuit: does this look safe and reliable?

    Hi,

    Maybe a safety relay like SF2-24V is more suitable.

    But I doubt that it can fulfill the need for "several 10ms deadtime".

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



--[[ ]]--