What is this low voltage power supply leading into the wall?

Question

I found this in the back-corner of our second-floor closet:

Presumably this is not to code. Has anyone seen anything like this before, or have any ideas what it's for?  Unplugging it doesn't seem to have broken anything, and there aren't any small electronics on any of the nearby walls.

freshop · Accepted Answer

It looks like the power supply for an old alarm system.

batsplatsterson · Answer

I agree it's likely this is an alarm system power supply, I know Honeywell had some 9VAC panels and likely others.
Believe it or not it may be code compliant, the rules for class 2 wiring are pretty loose.  It's not really workmanship anyone would brag about but it's a pretty common hack.
If the cable is suitably rated CL2 or better it's compliant.  However it looks like a zipcord which is not likely rated for installation inside walls on class 2 circuits.  However, at 9VAC, it's not much of a hazard.

Tonny · Answer

Nobody mentioned door-bell.
Many electrical doorbells use a AC transformer with output anywhere in the 5 to 20V range.
9 and 12V AC are most common.

What is this low voltage power supply leading into the wall?

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