How to Select Power over Ethernet (PoE) Injectors
Image Credit: Baumer, Ltd., GlobTek Inc., SL Power Electronics
Power over Ethernet (PoE) injectors supply or inject direct current (DC) power through network cables to power network devices. They are also called midspans or midspan hubs. PoE injectors enable Ethernet, a popular local area network (LAN) protocol, to carry DC power along with data packets. This eliminates the need for separate power cables, reducing wire costs and simplifying troubleshooting.
Like other PoE products, PoE injectors comply with IEEE 802.3af, a power distribution standard from the Institute of Electrical and Electronics Engineers (IEEE). The 802.af standard specifies two modes of operation: Type A, where power and data are transmitted over the same data pairs as Ethernet; and T Type B, where power is transmitted over unused pairs.
How PoE injectors Work
PoE injectors are part of larger PoE systems that include network devices such as routers and switches. Using an uninterruptible power supply (UPS), power is injected at the network's head end. If necessary, midpoint spans powered by additional UPS systems can supply more power. According to the IEEE 802.3af specification, power must be injected every 328 ft., which is the maximum distance for an Ethernet connection.
Powered Devices and Power Sourcing Equipment
PoE networks consist of powered devices (PD) and power sourcing equipment (PSE). PDs do not require separate power cables for local alternating current (AC). Instead, they use a standard RJ-45 jack and can be positioned away from wall outlets. PSE outputs are limited to 15.4 W per RJ-45 port. Because longer Ethernet links exhibit a significant drop, voltages as high as 57 VDC may be used with PoE injectors.
Electrical and Interface Specifications
Electrical specifications for PoE injectors include inlet type, input voltage, data line pin-out, category 3 (CAT-5) cable pin-out, supply voltage, maximum current, nominal available output power, and nominal output voltage. Input currents may be specified at 110 VDC and 220 VDC, or over 100 - 250 VAC. Operating temperature and humidity are important environmental parameters to consider when selecting power over Ethernet (PoE) injectors for demanding applications.
Interface specifications describe information about the input and output sockets. For example, the input for an Ethernet 10/100 Base-T connection may use an RJ-45 female socket. The data and power output may also use a RJ-45 socket, but with DC voltage on wire pairs 7-8 and 4-5.