Electronic Design

Power Over Ethernet

Sponsored by: NATIONAL SEMICONDUCTOR

What is the basis for Power over Ethernet (PoE)?

The IEEE 802.3af standard for PoE enables all data-terminal equipment (DTE) to receive power over the same cabling used for data. It specifies the protocol for delivery of 48 V dc over unshielded twisted-pair cables, such as Category 5. This eliminates the need for local power sources.

What does IEEE 802.3af cover?

The IEEE 802.3af standard presents the requirements for providing and receiving power over existing cabling. It involves power-sourcing equipment (PSE), which provides the power on the cable, and the powered device (PD), which receives the power. This standard defines the interface between the PSE and PD as it relates to the associated detection and classification protocol. The figure shows a simplified typical PoE system.

How does the PoE System know when power is required?

Look for a valid PD. The PSE looks for a valid PD by sending out 2.8 to 10 V across the power lines. A valid PD detects this voltage and places a resistance of 23.75 to 26.25 kž across the power lines. Upon detection of the resulting current, the PSE concludes that a valid PD is requesting power.

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