I can find lots of powerstrips ( https://www.amazon.com/gp/product/B00ACZPZIU ) that are autoswitching to save electricity by urning off outlets when a device on a control outlet is not drawing current. What I want to do is the opposite - to power on a separate outlet when the device on the control outlet is NOT drawing current (ie normally-closed relay).

If there are no smart power outlets / powerstrips on the market that do this (for a reasonable price), I am thinking about buying a relay and wiring it myself.

It would modify the functionality to turn it into a fail-over power device by inverting the relay functionality that is built in from functioning as “normally open” to “normally closed” using an ac to ac relay (https://www.amazon.com/gp/product/B08TR7WJX2). As a result, when the device on the control outlet (a fan) fails, the outlet would detect the drop in current and turn ON the other outlets, powering another standby fan.

Can anyone either suggest where I might find such a device, and if not, can you weigh in on this design?

Thanks.

 

Welcome to AAC.

It seems that what you want to do is trivial. If you use the sort of power strip you describe you can power a relay from one of the switched outlets with the secondary fan’s power connected through the NC contacts. When the power strip turns off, the relay will de-energize, closing the NC contacts.

If this is what you want to do, then the only challenge is finding a really of sufficient ampacity, with a form factor that allows you to build something conveniently sized and electrically safe.

Assuming it is rated sufficiently for the fan motor, something like this relay from Amazon might be a good choice. It has recessed quick connect terminals and comes with pre-terminated jumpers using insulated terminals. You would need to mount it in a box, with a power cord on one side and an outlet on the other.

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I would consider using an outdoor style, plastic junction box as the housing. For example, this two gang box might be suitable. Suitable covers that include one outlet and one blank don’t see readily available, so this blank cover and one (or more) of these outlets that panel mount into a square hole would be my choice. On the other hand, if you don’t fancy cutting a square hole (really not too hard but if you are not familiar it can be daunting) something like this (much more costly) outlet designed for mounting in a round hole is an alternative.

l to r: 2 gang box, blank plate, square mount receptacle, round mount receptacle
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The wiring is trivial, but if you have any questions, please don’t hesitate to ask. I will just say, as you probably already know, wire the L (Live) side through the relay and pass the N (Neutral) side through.

 

Thanks Ya'akov.

Can you confirm that A or B in the attached diagram is what you were thinking?

 

Actually, based on your last comment, I believe this is the diagram that represents what you were saying, right?

 

Sorry to spam this post, but I think this diagram may represent a cleaner implementation.

 

What would be a bit of a challenge is sensing when one mains powered device is using power, without interfering with it's operation or attaching to it any way.
The simple way to do that is to have a pair of diodes in reverse-parallel in series with the line lead of the power connection for the monitored device. When the device draws current there will be a 0.7 volt square wave voltage developed across the diode pair that can be used to operate a relay to provide whatever function is required. This is a quite old concept that still works very well. The caution is thet the sampled voltage is at mains potential and so is a bit hazardous.