Pixel Extender & POE

Transmitter Board

Quad_TX_POE.jpg

These boards are both thru hole designs that can be used as pixel extenders, but also support power over Ethernet (POE). These boards only support single channel outputs, and therefore do not support WS2801 pixel types.

The transmitter is shown above with the power connector and fusing for use in power over Ethernet.

Bill of Materials: Quad_POE_BOM_revA

Receiver Board

RX_12V.jpg

The receiver board can either be wired for 5V or 12V pixels. The picture above shows it for 12V pixels, with the power supply installed.

Bill of Materials: RX_Board_POE_BOM_RevA

Introduction to POE

Power over Ethernet has been a painful journey for me and I am an electronics engineer. Standard POE wiring has transformers to isolate the sensitive electronics from the high voltage on the lines. With this version there is no isolation from the power. Also, generally you want the ground to be made before the high voltage. If the high voltage makes contact first it will destroy the buffers. This is why I recommend using the shielded cabling, so ground is made first by the case. In addition, I managed to buy some of the fake DC to DC boards and half of them went bad after the first year. Hopefully, they would fail in a open, but these failed in a short and applied the full 36Vdc to my strings. This is why there is an over-voltage diode on the input to my receivers. My original receivers were surface mount and it totally destroyed my tree topper. So even though I used 38 receivers last year, it has been a learning experience. POE is recommended for the more experienced individuals, but I have already experienced most of the problems and fixed them.

Please use Shielded Ethernet cable to avoid issues.

Power over Ethernet is the ideal way to drive pixel string. One wire to the string for both power and control. Pixel strips/strings are spread across your yard and on the house, and running a single Ethernet cable for both power and control is very attractive. I would generally limit this to 5Vdc smart strings of 50 pixels or less for the following reasons. The 5Vdc pixels pull much less power/current than 12Vdc strings. The lower current allows the cable to go further. And finally, you can drive more pixels off a single power supply at 5Vdc. The table below provides an example of how much current is required for different injected voltages for 5V/12V strings (assuming 90% efficiency on DC/DC converter).

Injected – 5V  – 12V
Voltage – Amps – Amps
———————–
24V – 0.694 – 1.667
36V –  0.463 – 1.111
48V – 0.347 – 0.833
56V – 0.297 – 0.714

The nominal power for a 5V/12V string is about 16.5W/40W. So with a 400W power supply you can drive 24-5V elements or 10-12V elements.

POE Specifications

The Ethernet standard for POE recommend using 48V or 56V. The higher the voltage, the more power you can send over the cable. This is why our AC power distribution uses high voltage lines, it minimizes the loss in the cable due to resistance. Less current, less power loss. In addition, specification limits the current to 0.5 amperes. This is probably due to multiple devices on a single cables and vary cable lengths.

The RJ-45 connectors used for Ethernet are rated at 1.5 amperes per pin. Power over Ethernet uses two pins each for power and ground. This implies that you can send more power. This is partially true, but one of the pins will have less resistance and end up carry more current. The maximum power is generally limited by heat due to resistance of the pins and wiring.

My Implementation

For my implementation I wanted to use daughter cards on my controllers that support power injection. Each board is basically a modification of my quad RS-422/RS-485 driver. Two modifications have been made to the board. First, four 2.0 ampere fuses were added. Fuses are not generally required, unless you are using a large power supply, over 100W. Second, the RS-422/RS-485 drivers used in the originally design are expensive, so I moved to a pin compatible part AM26C31 ($3.86 versus $1.16).

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