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.
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.
- 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)
- A PD is a device powered by a PSE.
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.
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.
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 4 shows the basic circuit I am working with here.
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.
- The load power, PLoad, is limited to 25.5 W.
- The minimum source voltage, VMin, is specified as 50 V.
This exercise was a useful refresher exercise as to how PoE delivers power over category 5e cables. My application requires 15.3 W, so the 25.5 W capability of PoE+ will provide enough power for my application plus some reserve power in case we want to add more features later.