LED Power Supplies
By now, if you work with lighted signs, you probably use LEDs, which offer a number of advantages over traditional incandescent and fluorescent lighting. They provide a long operating life, low power consumption, reduced heat emission and now come in a wide variety of colors, intensities and form factors. From channel letters to lighted cabinets to full-motion billboards, LEDs are definitely lighting the way.
When designing a lighted sign, we tend to think in terms of the lights, color and intensity, but without a proper power supply you wind up with a sign that is only good for daytime viewing. So let’s get into the basics of powering LED lighting and how to make the proper decision when choosing a power supply.
AC/DC, W + V
The usual lamps and fixtures we use in our homes are normally powered by 110-120 VAC (volts alternating current) power. The LED modules we use for sign applications are usually DC (direct current), so we can’t directly tap into standard power. In addition, the lights are typically driven at a much lower voltage of 12-24 V. The power supply, in essence, transforms the higher-voltage AC to the lower-voltage DC needed for the sign. When building a lighted sign, you will find a wide variety of modules and an equally wide variety of power supplies offered by the manufacturers. So, how do you select the correct power supply?
One of the first questions to ask is, “What is the voltage requirement of the modules?” Most of the modules you find will be 12 V, though a number are 24 V. The higher voltage can allow for longer runs and brighter modules. The point is that you want to ensure that you purchase a power supply that matches your module needs. From there you will notice that the next specification will be the maximum wattage the supply can handle. Here you will need to do a little simple math, finding the wattage of the individual modules. Then you need to determine how many modules you will use in the sign. Multiply that number by the wattage. For example, you may need 100 modules that each consumes 0.72W. You would come up with 72W total consumption. Now you need to add a 20% overhead, so that the power supply is not overtaxed. Multiply 72 x 1.2 and you come up with 86.4W. This means a 60W power supply will not be enough; you will need at least a 100W unit.
So that’s it, right? Not yet. Is this an indoor or outdoor application? If outdoors, check the power supply’s IP (ingress protection) rating. You’ve probably seen these numbers in relationship to phones, GPS units, cameras and many other items. You will typically see a number like IP67. The first number, 6, has to do with dust-sealing. The “6” indicates that the item is sealed from dust. The “7” has to do with the water-resistance. In this case, it means the power supply can be immersed in 3 ft. of water for 30 minutes. Higher numbers are better, with IP67 being a fairly typical rating. You will also have to ensure that the power supply has UL/CSA ratings as well. As a final note, if your customer wants to dim the illumination and you have no other controller in the circuit, you’ll need to acquire a power supply that is rated for dimming.
Choosing the proper power supply will help avoid dim lighting or improper lighting during a rainstorm. Finally, check product specification sheets for mean time between failure (MBTF), a common indicator of quality. LED drivers, unlike some sign industry products, are fairly simple. Don’t skimp on the wattage, check your specs and you’ll have the correct supply for the job.