Living in a place where the electric service isn’t particularly reliable can be frustrating, whether that’s because of a lack of infrastructure, frequent storms, or rolling blackouts. An option for those living in these situations is a backup generator, often turned on and connected by an automatic transfer switch. These are necessary safety devices too; they keep power lines from being back-fed by the generators. But there are other reasons to use transfer switches as well as [Maarten] shows us with this automatic transfer switch meant to keep his computers and Internet powered up.
The device is fairly straightforward. A dual-pole, dual-throw relay is housed inside of an electrical junction box with two electrical plugs, each of which can be connected to a different circuit or power source in [Maarten]’s house. The relay coil is energized by the primary power supply, and when that power is lost the relay automatically changes over to the other power supply, which might be something like a battery backup system. [Maarten] was able to get a higher quality product by building it himself rather than spending a comparable amount of money on a cheap off-the-shelf product as well.
This might not seem like too big of a deal, but another person, [Stephen], came across this build and had a different use case for it. [Stephen] lives in a place currently experiencing rolling blackouts, and needed something to reliably power a piece of medical equipment through the night. Commerical offerings were too slow, and during the power transfer process the equipment would power off and need to be reset. [Maarten]’s design was fast enough that this didn’t happen, saving [Stephen] the hassle and expense of trying to find one that would work as well.
One of the perks of building your own tools and equipment like this is getting exactly the right parts for the job. In [Maarten]’s case it was a relay that had the right current ratings to switch his PC and Internet modem/router, and in [Stephen]’s case it was finding a relay that could perform the transfer fast enough, and both were able to do it for as inexpensively (or more) than anything they could find in a store. If you plan to perform the related task of connecting a generator to your home, you might not need a transfer switch at all, though. You can instead use an interlock plate which is even simpler than that.