# A Quick Power Over Ethernet Review

Quote of the Day

Until lions have their historians, tales of the hunt shall always glorify the hunter.

— West African proverb. A variation on Winston Churchill's "History is written by the victors" quote.

## Introduction

Figure 1: Hookup for a 30W PoE Type 2 System.

I have been asked to write some requirements for an optical product that is powered using Power Over Ethernet (PoE). It has been a few years since I have worked on a PoE-based design, I thought it would be useful to review the standard and ensure that I still understand it. This is a good exercise in basic electrical design and will also illustrate how to design circuits using Mathcad utility functions that I have written over the years.

My objective in this post is to show how a useful amount of power (25.5 W at the load) can be transferred over an Ethernet cable. I will be avoiding discussions on the protocol details associated with PoE because that would result in an enormous post.

## Background

### Definitions

Power over Ethernet (PoE)
PoE is an IEEE standard for sending power and data over the same category 5e Ethernet cable, which contains four wire-pairs (i.e. 8 wires total). PoE is enormously popular because only one cable is required to network an Ethernet-fed device, which greatly reduces the cost and complexity of networking remote devices, like cameras. For the version of PoE discussed in this post, power is transmitted over two  wire-pairs by applying a DC voltage between each pair (see Figure 1). Superimposing DC on the wire-pairs does not interfere with data transmission because Ethernet uses differential signalling.
Type 2 PoE
Type 2 PoE is an IEEE standard (802.3at) for transferring as much as 25.5 W over an Ethernet cable. The standard is also known as  "PoE+".
Power Supplying Device (PSE)
A PSE is a device that provides power on an Ethernet cable.
Powered Device (PD)

### PoE Basics

Here are the key points about a type 2 PoE system discussed in this post:

• The source power is limited to 30 W.
• The wire temperature is assumed to be no more than 50°C.
• All design work will assume category 5e cable, which means 24 AWG wire.
• I will be using two of the four Ethernet pairs for power transmission.
• The category 5e cable length is limited to 100 m.
100 m is also the maximum reach for data transfers on Ethernet. This means that you can use PoE on any Ethernet network.

## Analysis

### Modeling Resistance of Annealed Copper Wire

Figure 2 shows my linear interpolation of some annealed copper wire resistance data that I found years ago. I believe it was from an old Bell Telephone, but I do not recall the original source – certainly something I googled. I have scanned the original table into an Excel workbook.

Figure 2: Linear Interpolation of Copper Resistance Data.

### One-Way Cable Resistance

Figure 3 shows how to compute the resistance of a 100 m long, 24 AWG, category 5e wire at 50 °C using the functions shown in Figure 2. My calculations show the maximum wire resistance is ~10 Ω, which does not include connector losses. The standard actually assumes 12.5 Ω, which will provide a reasonable amount of margin.

Figure 3: One Resistance of a Category 5e Cable Wire.

Figure 4 shows the basic circuit I am working with here.

Figure 4: Circuit Model for PoE (Source).

### PoE+ Analysis

My intent here is compute a few of the key product parameters of my PoE+ driven system, like maximum input power and internal heat generation (shown shaded green in Figure 5).  To perform my analysis, I need to state a few PoE+ characteristics:

• The source power, PSource, is limited to 30W.