PoE Injector

Overview

A PoE injector, or midspan, adds power to an Ethernet run that comes from a switch with no power-over-Ethernet support. It sits between the switch and a remote device such as a wireless access point, IP camera, or VoIP phone, merging the data link with a DC supply onto one cable so the device needs no separate power outlet nearby.

The data link from the switch lands on the Data Input Port, an RJ45 jack whose magnetics pass the signal through untouched. Power comes from an external AC-DC Power Supply brick fed by an IEC Power Cord and entering through the DC Barrel Jack. Inside, the PSE Mainboard is the power-sourcing stage; it raises and isolates the rail, then hands the combined signal to the PoE Output Port, which carries both to the powered device. A LED Indicator Board shows power and delivery state, and a Surge Protection Board clamps lightning and ESD transients on the outdoor-facing pairs.

How it works

Power-over-Ethernet never blindly energizes the cable. When a device is plugged into the output, the PSE controller on the mainboard first applies a low probing voltage and looks for the standard 25 kΩ signature resistance inside the powered device. Only after that detection succeeds does it run a classification step, reading how much power the device requests, and then switch the full 50-plus-volt rail on through its pass MOSFET with a controlled inrush ramp. A current-sense shunt lets the controller hold the load within its class budget and trip on a short or overload.

The injection itself is electrically simple. This unit powers the data pairs (Mode B): the supply is applied to the center taps of the output magnetics, so DC rides as a common-mode offset on the same wires carrying Gigabit data, while the transformers keep the two from interfering. At the far end the powered device pulls the DC back off the center taps and recovers the data normally. Because the powering rail floats behind an isolation transformer, the data side stays referenced to the switch and a 1500 V barrier separates the two domains for safety and noise immunity.