Power over Ethernet (PoE) is not rocket science, but it’s not plug-and-play magic either. This short primer walks through the basics with a few practical nudges for those curious to try it out.
It’s a technology that delivers electrical power alongside data over standard twisted-pair Ethernet cables. It enables a single RJ45 cable to supply both network connectivity and power to powered devices (PDs) such as wireless access points, IP cameras, and VoIP phones, eliminating the need for separate power cables and simplifying installation.
PoE essentials: From devices to injectors
Any network device powered via PoE is known as a powered device or PD, with common examples including wireless access points, IP security cameras, and VoIP phones. These devices receive both data and electrical power through Ethernet cables from power sourcing equipment (PSE), which is classified as either “endspan” or “midspan.”
An endspan—also called an endpoint—is typically a PoE-enabled network switch that directly supplies power and data to connected PDs, eliminating the need for a separate power source. In contrast, when using a non-PoE network switch, an intermediary device is required to inject power into the connection. This midspan device, often referred to as a PoE injector, sits between the switch and the PD, enabling PoE functionality without replacing existing network infrastructure. A PoE injector sends data and power together through one Ethernet cable, simplifying network setups.
Figure 1 A PoE injector is shown with auto negotiation that manages power delivery safely and efficiently. Source: http://poe-world.com
The above figure shows a PoE injector with auto negotiation, a safety and compatibility feature that ensures power is delivered only when the connected device can accept it. Before supplying power, the injector initiates a handshake with the PD to detect its PoE capability and determine the appropriate power level. This prevents accidental damage to non-PoE devices and allows precise power delivery—whether it’s 15.4 W for Type 1, 25.5 W for Type 2, or up to 90 W for newer Type 4 devices.
Note at this point that the original IEEE 802.3af-2003 PoE standard provides up to 15.4 watts of DC power per port. This was later enhanced by the IEEE 802.3at-2009 standard—commonly referred to as PoE+ or PoE Plus—which supports up to 25.5 watts for Type 2 devices, making it suitable for powering VoIP phones, wireless access points, and security cameras.
To meet growing demands for higher power delivery, the IEEE introduced a new standard in 2018: IEEE 802.3bt. This advancement significantly increased capacity, enabling up to 60 watts (Type 3) and circa 100 watts (Type 4) of power at the source by utilizing all four pairs of wires in Ethernet cabling compared to earlier standards that used only two pairs.
As indicated previously, VoIP phones were among the earliest applications of PoE. Wireless access points (WAPs) and IP cameras are also ideal use cases, as all these devices require both data connectivity and power.
Figure 2 This PoE system is powering a fixed wireless access (FWA) device.
As a sidenote, an injector delivers power over the network cable, while a splitter extracts both data and power—providing an Ethernet output and a DC plug.
A practical intro to PoE for engineers and DIYers
So, PoE simplifies device deployment by delivering both power and data over a single cable. For engineers and DIYers looking to streamline installations or reduce cable clutter, PoE offers a clean, scalable solution.
This brief session outlines foundational use cases and practical considerations for first-time PoE users. No deep dives: just clear, actionable insights to help you get started with smarter, more efficient connectivity.
Up next is the tried-and-true schematic of a passive PoE injector I put together some time ago for an older IP security camera (24 VDC/12 W).
Figure 3 Schematic demonstrates how a passive PoE injector powers an IP camera. Source: Author
In this setup, the LAN port links the camera to the network, and the PoE port delivers power while completing the data path. As a cautionary note, use a passive PoE injector only when you are certain of the device’s power requirements. If you are unsure, take time to review the device specifications. Then, either configure a passive injector to match your setup or choose an active PoE solution with integrated negotiation and protection.
Fundamentally, most passive PoE installations operate across a range of voltages, with 24 V often serving as practical middle ground. Even lower voltages, such as 12 V, can be viable depending on cable length and power requirements. However, passive PoE should never be applied to devices not explicitly designed to accept it; doing so risks damaging the Ethernet port’s magnetics.
Unlike active PoE standards, passive PoE delivers power continuously without any form of negotiation. In its earliest and simplest form, it leveraged unused pairs in Fast Ethernet to transmit DC voltage—typically using pins 4–5 for positive and 7–8 for negative, echoing the layout of 802.3af Mode B. As Gigabit Ethernet became common, passive PoE evolved to use transformers that enabled both power and data to coexist on the same pins, though implementations vary.
Seen from another angle, PoE technology typically utilizes the two unused twisted pairs in standard Ethernet cables—but this applies only to 10BASE-T and 100BASE-TX networks, which use two pairs for data transmission.
In contrast, 1000BASE-T (Gigabit Ethernet) employs all four twisted pairs for data, so PoE is delivered differently—by superimposing power onto the data lines using a method known as phantom power. This technique allows power to be transmitted without interfering with data, leveraging the center tap of Ethernet transformers to extract the common-mode voltage.
PoE primer: Surface touched, more to come
Though we have only skimmed the surface, it’s time for a brief wrap-up.
Fortunately, even beginners exploring PoE projects can get started quickly, thanks to off-the-shelf controller chips and evaluation boards designed for immediate use. For instance, the EV8020-QV-00A evaluation board—shown below—demonstrates the capabilities of the MP8020, an IEEE 802.3af/at/bt-compliant PoE-powered device.
Figure 4 MPS showcases the EV8020-QV-00A evaluation board, configured to evaluate the MP8020’s IEEE 802.3af/at/bt-compliant PoE PD functionality. Source: MPS
Here are my quick picks for reliable, currently supported PoE PD interface ICs—the brains behind PoE:
- TI TPS23730 – IEEE 802.3bt Type 3 PD with integrated DC-DC controller
- TI TPS23731 – No-opto flyback controller; compact and efficient
- TI TPS23734 – Type 3 PD with robust thermal performance and DC-DC control
- onsemi NCP1081 – Integrated PoE-PD and DC-DC converter controller; 802.3at compliant
- onsemi NCP1083 – Similar to NCP1081, with auxiliary supply support for added flexibility
- TI TPS2372 – IEEE 802.3bt Type 4 high-power PD interface with automatic MPS (maintain power signature) and autoclass
Similarly, leading semiconductor manufacturers offer a broad spectrum of PSE controller ICs for PoE applications—ranging from basic single-port controllers to sophisticated multi-port managers that support the latest IEEE standards.
As a notable example, TI’s TPS23861 is a feature-rich, 4-channel IEEE 802.3at PSE controller that supports auto mode, external FET architecture, and four-point detection for enhanced reliability, with optional I²C control and efficient thermal design for compact, cost-effective PoE systems.
In short, fantastic ICs make today’s PoE designs smarter and more efficient, especially in dynamic or power-sensitive environments. Whether you are refining an existing layout or venturing into high-power applications, now is the time to explore, prototype, and push your PoE designs further. I will be here.
T. K. Hareendran is a self-taught electronics enthusiast with a strong passion for innovative circuit design and hands-on technology. He develops both experimental and practical electronic projects, documenting and sharing his work to support fellow tinkerers and learners. Beyond the workbench, he dedicates time to technical writing and hardware evaluations to contribute meaningfully to the maker community.
Related Content