I have a (low-voltage supplied) device A which I want to switch on and off with a timer, and a (high-voltage supplied) appliance B which must switch off immediately whenever A comes on, and (ideally) back on immediately whenever A shuts off. It's not that they'd be jointly overloading a circuit to be on simultaneously, there's just a functional dependence between their actions (A is bypassing a necessary input away from B).

Obviously I could connect A and B to separate timers, set to opposite schedules -- but not only would I prefer to only have to adjust one timer (I need to frequently change the schedule for A), I also need to keep the devices closely synced, not letting them potentially drift apart on different timers. It's important that B never runs while A is on, even for a minute.

Would I need to employ some kind of multi-way relay here? I'm trying to avoid a lot of serious wiring if possible, but maybe I can't. Unfortunately(?), it's the more dangerous higher-voltage circuit for B which is the more important one to interrupt.

EDIT: Since someone asked, B is actually a pair of appliances -- a dehumidifier and a window air conditioner -- which I've been treating as one load for this purpose (but perhaps should not). The B units contain odors and allergens that have proven difficult to fully clean out, so I'm piping their air outputs into one branch of an intake manifold for an air filtration unit ( which runs continuously on another circuit). At certain times though, I want the air filter to draw more air from the other intake branch -- so to switch over the primary air intake to the other branch, I'm using a 24V motorized normally-open duct damper, which is A, to shut the branch that was accepting the B outputs.

I want the B appliances to shut off when the A damper turns on to close their filter branch, because if they kept running while their filter branch were closed, they would end up pushing their unfiltered outputs into the living space, which is undesirable.

The duct damper A runs on a 24V circuit which is wired off a transformer that plugs into line voltage. A could potentially be controlled either upstream or downstream of this transformer.

The question of B's power draw seems not straightforward to me. Nominally the dehumidifier may draw up to 8 amps and the air conditioner up to 5 amps, but those are the running amps, not starting amps. I don't know if I should expect, say, a 20 amp relay to allow both of these appliances to start simultaneously without damaging the relay (or indeed, if a 20 amp relay would allow either of these inductive loads separately at startup -- I don't know the surge currents for either one). Would this issue be reduced by just running the appliances off a surge-suppressing power strip?

  • 2
    A diagram would help. But it sounds likd you need a standard rwo wsy relay that has normally open and normally closed contacts.only one gadget would receive power. Commented Apr 22, 2023 at 6:43
  • I believe you're right. I think can see how to wire it to make it work if the 'timer on/off' signal were flipping a standard mechanical light switch on/off (though I'm working through some confusion about the difference between that and a three way switch), and a two-way relay would be a version of that light switch that could be operated by the timer signal. Commented Apr 22, 2023 at 7:12
  • How much power does B draw? Commented Apr 22, 2023 at 13:18
  • What is supplying the (low) voltage to A—a battery?
    – Huesmann
    Commented Apr 22, 2023 at 13:40
  • Using a relay like a RIB2401D might be part of your answer. Two form C contacts, multi-volt coil. Commented Apr 22, 2023 at 16:56

1 Answer 1


So, relay modules that take low voltage coil/control inputs and can switch line voltage are a thing you can buy.

Here's one example that requires no line voltage wiring by you, and is a listed product. No affiliation, example, not endorsement.

120VAC DC controlled relay module from adafruit.com

Typically on the line voltage switching side they will have a common contact, and a normally open and a normally closed contact (normally meaning when the relay is not actuated.) Some only have two contacts, those are usually common and the normally open contact, and not ideal for your application.

So, your timer (is that line or low voltage, by the way? Could give more options for relay modules) turns on the low voltage to A and the low voltage to A is ALSO connected to the "coil" (control) input on a line voltage-switching relay, which has the power to B run through the common and normally closed contacts, which means B switches off as soon as there's power to A.

  • Thanks, I didn't know there were products like that. Timer is flexible, since I haven't set up the timer system yet, currently switching manually. Commented Apr 23, 2023 at 19:29

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