How the PoE HAT Powers a Raspberry Pi

The Power over Ethernet (PoE) Hardware Attached on Top (HAT) allows a Raspberry Pi to receive both high-speed network connectivity and electrical power through a single standard Ethernet cable. By utilizing the unused wires or phantom power techniques of an Ethernet cable, the PoE HAT eliminates the need for a separate USB power supply. This article explains the underlying IEEE standards, the hardware components involved, and the practical benefits of using a PoE HAT for streamlined, single-cable Raspberry Pi deployments.

The Technology Behind Power over Ethernet

Power over Ethernet operates on standardized protocols established by the Institute of Electrical and Electronics Engineers (IEEE). The most common standards used by Raspberry Pi PoE HATs are IEEE 802.3af (PoE) and IEEE 802.3at (PoE+).

In a standard Cat5e or Cat6 Ethernet cable, there are four twisted pairs of copper wires. While 10/100 Mbps Ethernet only requires two pairs for data transmission, Gigabit Ethernet utilizes all four pairs. PoE technology works by injecting a direct current (DC) voltage—usually between 44V and 57V—into these wire pairs. Because data and power operate at vastly different frequencies, they can coexist on the same copper strands without interfering with each other, a method often referred to as phantom power.

How the Hardware Connection Works

For a Raspberry Pi to utilize this technology, it requires a compatible network switch or an injector, a compatible Pi board, and the PoE HAT itself.

• The PoE Switch or Injector: This acts as the power sourcing equipment (PSE). It detects whether the connected device supports PoE and, if compatible, safely injects the required voltage into the Ethernet cable.
• The Raspberry Pi Board: Starting with the Raspberry Pi 3 Model B+ and continuing through the Raspberry Pi 4 and Raspberry Pi 5, the boards feature four dedicated PoE pins located near the Ethernet jack. These pins tap directly into the Ethernet port’s internal transformers to extract the raw DC voltage sent by the switch.
• The PoE HAT: The Raspberry Pi’s processor and onboard components cannot handle 48V directly; they require a stable 5V (or 5.1V) supply. The PoE HAT functions as a step-down converter. When plugged into the Pi’s GPIO pins and the dedicated PoE pins, the HAT receives the high-voltage DC current and uses an onboard isolation transformer and a buck converter to efficiently step it down to the safe 5V required by the Pi.

Why Use a PoE HAT?

Deploying a Raspberry Pi with a PoE HAT offers several distinct advantages for both hobbyists and enterprise environments:

• Single-Cable Deployment: Eliminating the bulky USB power adapter allows the Raspberry Pi to be placed up to 100 meters away from a power outlet, as long as it is connected to a PoE-enabled switch.
• Centralized Power Management: Since the power originates from a central PoE switch, users can remotely reboot the Raspberry Pi by cycling the power on that specific switch port. Additionally, connecting the PoE switch to an Uninterruptible Power Supply (UPS) ensures all connected Raspberry Pis remain online during a power outage.
• Cleaner Installations: For cluster configurations (such as Pi Bramble clusters) or tight enclosures, removing the need for individual power bricks drastically reduces cable clutter and simplifies thermal management.