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  1. #1
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    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?

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    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.
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    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.



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    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.



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    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
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