Powered Version of RJ11
In the powered version, Pins 2 and 5 (black and yellow) may carry low voltage AC or DC power. While the phone line itself (tip and ring) supplies enough power for most telephone terminals, old telephone instruments with incandescent dial lights in them (such as the classic Western Electric Princess and Trimline telephone models) needed different voltages than the phone line supplies. Typically, the power on Pins 2 and 5 came from a transformer plugged into a power outlet near one jack, wired to supply power to just that telephone (or to all of the jacks in the house, depending on local telephone company practices). It is now usually recommended that only the one local jack used by such older telephones be wired for power, to avoid any potential interference with other types of service that might be using pins 2 and 5 (black and yellow pair) in jacks in other parts of the house. The early Trimline and Princess phone dial light incandescent lamps were rated at 6.3 Volts, 0.25 Amp and the transformer output is approximately 6-8 Volts AC (while later Trimline versions had LED light sources, powered directly from the phone line, and the last Western Electric-made Princess version had no dial light).
Read more about this topic: Registered Jacks, RJ11, RJ14, RJ25 Wiring Details
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