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4 Top Benefits of Power over Ethernet (PoE)in Embedded Design 2018-05-10

By: Chris Binnie

PoE might be considered in any design where data is going to be passed over an Ethernet cable (eg. Cat5). The power is either passed over the spare pairs of the cable or via the centre tap of the data transformer, so PoE can be used no matter what the speed of data. There are many articles written about the specifications (IEEE802.3af and IEEE802.3at) and implementation of PoE so I am not going to cover them here.

Definitions of PoE Acronyms

First I need to clear up a couple of commonly used acronyms in the PoE world. PSE (Power Sourcing Equipment) is the product that puts the power onto the Ethernet cable - typically the router or Ethernet switch. PD (Powered Device) is the collective term for the product that sits at the other end of the Ethernet cable.

When PoE May Not be Beneficial

Let me start by suggesting some areas where PoE is not likely to be suitable for your application. In order to comply with the specs, the maximum power that will be available at the end of the Ethernet cable (maximum length 100m or 330ft) is 12W and 24W respectively. Therefore, if the PD is going to exceed this level, PoE is not going to be suitable for your design if you want to comply with the IEEE specs. There are ways around this power restriction, but let's stick with the IEEE limitations for now!

1. PoE’s Ease of Installation is a Big Benefit

The main benefit of PoE is to make the PD easy to install. The person installing the product simply finds the right location for the device, clicks the Ethernet cable into the RJ45 socket, and that's it. Power and data will travel down the same Ethernet cable, so no need to find the mains outlet. As an example, think of a security camera - these are usually located high up, even outdoors, where an AC outlet is unlikely to be.

2. The Value of PoE Can Far Outweigh its Cost

On the face of it, adding PoE could add $5 to $10 (for 12W) or $10 to $15 (for 24W) to your bill of materials (BoM). So taking our security camera as an example, that could mean that the selling price of the Ethernet camera rises from say $200 to $250. Not great right? ....... WRONG! This would be the best $50 spent by the installer. Sticking with our security camera example, if the installer wants to put the camera where there is no AC outlet they either have to trail AC cables to the nearest outlet or install an outlet where the camera is going to be located. It would have to meet safety regulations, building codes etc, or even require a specialist electrician, far outweighing the incremental cost of the camera.

3. Safety – as Classified by UL

PoE typically uses 48V DC, which has been classified as a safe voltage by UL Standards, the organization that benchmarks power supplies. PoE also has built-in safety features, so if there is any interruption in the connection, the PSE automatically stops sending power, and then starts a handshaking type process before sending power again. This means if there is damage to the cable, for example fraying or even a rodent attack, the power is stopped. With mains power, a rodent could easily cause sparking, resulting in fire.

4. Reliability Even in the Face of a Power Loss

How many times have you lost power? Unless you have a backup generator, any mains powered equipment would be shut down as well. Since the data network is usually supported with a UPS (Uninterruptible Power Supply), any loss of mains power would mean that the UPS can maintain the integrity of the PoE network. Using our security camera example, this would mean that the camera is still powered by the network UPS.

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