PIXEL CONTROL SIGNALS
There are three ways to drive a string of smart pixels. These are listed below from best to worst. The ideal solution from my perspective would be a single string Ethernet receiver with a DC to DC converter for power over Ethernet. This solution would require one Ethernet cable to each string of lights.
a) BEST – Drive a string with a ground isolated control line. This can either be an Ethernet input or a isolated RS-485 receiver. This method provides complete ground isolation. As a third alternative, you could use an opto-isolator at the pixel string to provide ground isolation.
b) GOOD – Drive a string with RS-485 receivers. The nominal RS-485 receiver will continue to work with up to ±7Vdc ground offsets in the circuit, and some receivers support up to ±25Vdc offset voltages. This provides some measure of safety in driving the string of lights, since there can be some variation in the grounds. If you are using ground isolated power supplies, then all grounds are tied back to the controller through the control cables.
c) OK – Drive the string with 5Vdc CMOS logic. The above two methods have to convert Ethernet/RS485 to 5Vdc logic to drive pixels. Ideally this would be right at the start of the string or within a few inches. This method generally refers to driving smart pixels several feet away or more. This method should be used for large display items. The reason to limit the distance of the wiring is that 3.3Vdc/5.0Vdc logic can not tolerate an input from 0.5 to 0.7Vdc above or below the ground/voltage rails (WS2811 0.5V). There are internal diodes to power/ground on all inputs to protect the inputs. If diodes are turned on with a voltage spike with any amount of current and you will damage the input. The nominal set of 50 smart pixels pulls 0.06 amperes each for a total of 3 amperes. If you run power 5 meters/16.4 feet the voltage shift on the ground will be (0.5Vdc – 20AWG, 0.31Vdc – 18AWG, 0.19Vdc – 16AWG). Any connections in the wiring will also had voltage shift. So long runs of cabling will cause failures.
As with any control signal there are two recommendations:
a) Always use twisted pair when wiring control signals (signal/ground or differential pair) (this is improve signal quality).
b) Use a separate power ground, to keep power out of the control signal powering.
POWER SMART PIXELS
There are three generally ways of powering your smart pixels. The method will depend upon the element type. In generally, the best way to distribute power is with a high voltage, the same as power companies.
a) POWER OVER ETHERNET: For me this is the ideal method to drive most of the elements in your display. Lights 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. Using 5Vdc strings will allow more elements to be driven off a single power supply. 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).
Voltage Current 5V Current 12V
24V 0.694 1.667
36V 0.463 1.111
48V 0.347 0.833
56V 0.298 0.714
The nominal power for a 5V/12V string is about 16.5W/40W. So with a 200W 48Vdc power supply you can drive 12-5V elements or 5-12V elements. This is the major advantage of the 5Vdc strings.
The specification for power over Ethernet suggest using either 48V or 56V power. The specification limits the maximum current in the cable to 0.5 amperes. The RJ45 connectors are rated at 1.5 amperes per contact. For Christmas lighting our cable lengths are generally under 50 feet, so I would limit the current to about 1.0 ampere per element. NOTE: POE uses two connectors pins for running the power/ground, however if the resistance is higher in one of the connections, one wire will generally carry most of the current. So using higher current with only two pins is not a good practice. Fusing to limit current in connectors is recommended.
b) LOCAL POWER SUPPLY: Generally most of the elements in your yard should use a local power supply. A small IP67 5V/12V power supply is inexpensive ($5-$8). This allows the power to be applied at the start of the string, and avoid generating ground offsets in the control logic (must be ground isolated supply). It is also recommended in using a small supply for power injection along a long length of pixels. When using small power supplies, there is no need for long runs of large gauge wiring. Using inexpensive extension cords is a good way to distribute power. Fusing is not required.
c) HIGH WATTAGE POWER SUPPLY: This type of power supply should be used for the larger elements in your yard, like a mega tree or sign. With this type of power supply (more than 50-100W), you should use fusing. It is important to use large gauge wire (at least 16AWG) for the power and to keep the power cabling short to minimize ground offsets. I would not recommend injecting power into the same string twice off the same power supply. If using power injection in a large element use a second power supply to inject power into the string(s), with the ground floating from the other power supply. This will avoid ground loops in your system.
The star ground is a fallacy. For a star ground to work, it would require no resistance in the cabling. So there will always be variations in the grounds.