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Behind my doorbell panel, a 24VAC transformer is currently wired into mains (USA) using wire nuts. I am interested in tapping into mains to power an IoT device that would integrate with the doorbell (similar to this implementation). I wish to continue allowing the existing transformer to draw from mains while also powering a USB power adapter, all while staying compliant with electrical code. The resulting implementation would continue to fit within the doorbell enclosure.

Some options I was considering:

  • Split the existing connections (e.g. using Wago 221-413 connectors) and expose a non-grounded connector for use with a USB power adapter
  • Replace the direct connection to the transformer with a grounded connector (e.g. this), rewire the transformer to a plug, and drive both that transformer and the USB power supply with a splitter

Though less preferable, I was also considering tapping into the 24VAC output from the transformer to drive 5VDC, or having my device be battery-powered.

Any advice on how I should best proceed would be greatly appreciated.

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    Can you post a photo of the box that the current doorbell transformer is attached to? – ThreePhaseEel Dec 24 '19 at 15:05
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Wow, now that I see your picture, this thing is a hatchet job.

No, no. That transformer cannot just be stuffed loose in a plastic (!!) Box. There is no separation between mains and low voltage. What a fiasco.

I would extract the doorbell wiring from the box and bring it out to the wall through a hole outside the box. Now, with only mains wiring in the box, I would install 2 standard receptacles (path of least challenge) with 2-gang cover plate, and fit up a plug-in "wall wart" transformer for the doorbell. Use another socket for any bog standard USB supply from the local gas station.


It's all about separation

There should be a metal curtain between your mains and low-voltage wiring. This is where we get into mains wiring methods. This separation is easy with doorbell transformers; they make transformers that mount on a 1/2" knockout or a junction box cover plate. The mains wiring is contained; the 24V comes out to open screw terminals.

This concept of "metal curtain" must be respected. The single best way to achieve that is to install a common receptacle and plug a quality USB power supply (Apple iPad is my pick) in the receptacle.

  • Install any common 1-gang deep "handy-box" steel junction box right next to the doorbell box. That holds the receptacle. Assuming the power supply cable going into the doorbell box is full-current-rated, reroute it into the handy-box instead, remembering minimum slack (6" of wires free inside the box).
  • Connect from the handy-box into the old doorbell box. I'm a conduit person so I would go into a standard knockout via a steel conduit nipple to the handy-box, and use THHN wire between; this also carries ground to the metal doorbell box. But you could use a few inches of appropriate Romex (most places sell it by the foot, remember 6" tail on each end). Or alternately, just install a power cord (with strain relief) on the doorbell box.

  • Wire it up in the usual, Code-legal way, so both the receptacle and doorbell box get power.

    • In all things mains, hook up safety ground first always, and never disconnect it, not even for troubleshooting (it's never the problem).
  • Plug the USB power supply into one socket, and (if needed) the doorbell box's new line cord into the other.

  • The "hatchet job" comment leads me to believe maybe I should focus on getting the mess cleaned up before adding more to it, thanks! – Justin Bell Jan 5 at 20:06
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Use a bridge rectifier and a LM2956 module to make 5VDC from the 24VAC supply.

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Hard to say for sure without seeing a picture of your existing installation. But typical, and ideal here, is a box with transformer mounted on the outside and standard receptacles on the front for your USB adapter to plug in. Another question Can a doorbell transformer sit loose inside main panel? addresses the issue of mounting on outside vs. inside a breaker panel. This is much the same, just mount in on the outside of a regular junction box with receptacles in the same box - i.e., transformer on the side, receptacles in front.

Having trouble finding an exact match at the moment. This question What is this thing on my junction box? addresses a transformer on the outside of a box (which is what you should have) and discusses hooking up a separate box to the same power. But you can hook it all up with **one* box, as long as it is large enough (which should not be a problem as the transformer is outside so it only adds a pair of wires compared to the receptacles by themselves.)

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There are a few alternatives you could consider as well. For instance: if the IoT widget has a low power draw, as the ESP module would, power it from the low voltage side of the transformer. You'll need a little power supply board that can take about 16 VAC in and put out whatever DC voltage your circuit requires. (HVAC circuits use 24 VAC, but doorbells in USA usually run on a 16 VAC transformer.)

Another option is to detect the doorbell ring rather than relaying it. The ELK 930 doorbell and telephone ring detector kit does this. It installs in series with the existing doorbell circuit. Current flowing in the circuit when the bell is rung causes an opto-isolator to trigger; the isolated output is often connected to a security alarm system or a home automation system.. or a home-grown IoT device. The only problem with the ELK model in particular is that they just charge too much. It has been priced about US$60 for more than a decade..

  • I did in fact confirm it is 24VAC (~22.1). – Justin Bell Dec 26 '19 at 15:50

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