THAT Home Automation Topology

The Open Home Automation and Control System

«  THAT Power Supply  »


One of the fundamental components that every THAT module will need is a Power-over-ethernet (PoE) power supply. Although not required, it is likely that most THAT modules will make use of identical or very similar power supplies. One such design is presented here. The power supply module has two input sources (PoE and an external AC/DC input) and two outputs (5 and 3.3 Volts DC).

To handle the PoE functionality, as per the IEEE 802.3af standard, we have selected a PoE module from the manufacturer Silver. The model we are using is the Silver Ag9400-S PoE power supply module. This modules in hardware handles the "handshake" procedure for identifying and obtaining power from a PoE powering device. It outputs regulated 5 Volts DC from a nominal 48 Volt PoE source.

To generate regulated 5 Volts DC from the auxilary AC/DC input, we have selected the LM2575 IC. This device utilizes a couple external components to create a high-efficiency buck converter, which is one type of switch-mode power supply (SMPS). From either 5 Volt DC source listed above, we use an MC33269 IC, which is a linear voltage regulator, to provide 3.3 Volts DC.


The latest schematic for the Power Supply is shown below. Some notes about this design can be found following the schematic.

Power Supply Schematic, version 0.3


  • R2 is used to increase the output voltage of the PoE module slightly. This is necessary since the diode D8 will drop about 0.3 Volts. The addition of R2 boosts the output of the PoE module to approximately 5.3 Volts to compensate for the drop, resulting in a 5 Volt DC output.


The parts list that corresponds to schematic version 0.3 is shown below. All parts are available from except for the PoE module by Silver.

Power Supply Parts List, version 0.3

Printed Circuit Boards:

A Printed Circuit Board design for the Power Supply Board has been completed. The PCB is a standard, 2-layer board with through-hole components. It measures 83.5 mm x 52.2 mm. The PCB design layout is shown below, followed by a photo of the PCB populated with components.

Power Supply PCB layout, version 0.3 Power Supply board, version 0.3


The Power Supply board contains two female pin header connectors, in addition to a 5.5mm OD / 2.5mm ID barrel jack. The barrel jack is AC/DC input from an unregulated auxiliary power supply (such as a wall-wart transformer).

One 3-pin header is for the Power Over Ethernet (PoE), and it connects to the PoE output header on the current THAT Ethernet Interface board. The other 3-pin header is the power output header, which carries 5 Volts, 3.3 Volts, and Ground. It is designed to connect to the mainboard of a THAT module.