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I wondering if this is a safe and working schematic. At AC voltage equal to or larger than 40 volts then switch goes to normally open and the battery (common) connects to load 2. At AC voltage source less than 40 volts the switch remains on normally closed and the battery is connected to load 1.

Given that there is an AC source and a battery source in this circuit but they don't interact this circuit is still safe?

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  • \$\begingroup\$ Got a datasheet for your relay? \$\endgroup\$
    – The Photon
    May 2 '18 at 4:55
  • \$\begingroup\$ no i don't. but i do understand the limits of voltage and current for a relay. \$\endgroup\$ May 2 '18 at 5:02
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  1. You don't need a DPDT relay. SPDT is enough. All the places where you drew a ground symbol in your schematic are considered to be connected to each other, so the second pole in your relay has no effect.

  2. Relays generally don't have very precise switching thresholds, so you're not likely to find one that will be guaranteed disconnected at (say) 35 V and guaranteed connected at 40 V.

    I've never designed with an AC-coil relay before, but to give an example the first one I found online is available with coil ratings of either 24 or 48 V AC. The 24 V version is specified to be connected when the coil voltage is at least 21.6 V (for 60 Hz systems), and to be disconnected when the coil voltage is below 7.2 V. At any voltage in between there's no guarantee what the switch will do.

    To get a precision switching threshold, you'll probably want some kind of comparator circuit to test the input signal amplitude, and then a buffer amplifier to drive a DC-coil relay.

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