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LED Power Supplies: Driving the Diode

​Ryan N. Fugler is a former editor of WRAPS magazine. 

Power is an interesting attribute, and many individuals are awed by it. But the vehicle that provides the most power isn’t always the best fit; however, a low-power solution may not get the job done effectively either. For example, you wouldn’t use a jet engine to power your lawnmower; on the other hand, you’ll need more than a double-A battery.

It’s not all that different with LED lighting.

“If you over-power your LEDs they’re not going to last as long,” explains Jim Redmer, director of marketing, Wensco Sign Supply. “Same thing if you under-power your LEDs—they won’t be as bright and they won’t last as long. If you do everything the right way, your LEDs should last a minimum of 50,000 hours.”

To provide some context, the LED power supply (or driver) “is an AC to DC transformer. It steps down the line voltage (120V-277VAC) to usable 12V or 24V (DC) for the LEDs to use,” explains Fritz Meyne, Jr., vice president sales, Bitro Group. “Additionally, modern power supplies automatically offer 50 to 60Hz.”

LEDs are distinctively different than traditional bulbs, and when they are used in signage projects there are certain elements to consider. One of the most obvious is the driver, so let’s take a more detailed look at these drivers and how they allow LEDs to function in sign displays.

Understanding the Driver

To start, “LEDs cannot be driven directly from the line,” says Herm Harrison, vice president of the Foster Transformer Company. He explains that LEDs’ forward voltage drop must be overcome before it conducts. “Once this threshold is crossed, the LED begins to conduct and emit light. As current begins to flow through the LED, it warms, and as it warms, the resistance drops causing more current to flow, which causes the resistance to drop further leading to what is termed thermal runaway. If the current is not regulated to a safe value, the LED will burn out and this is where the driver comes into play. An LED driver is designed to regulate both output voltage and current to safe levels.”

With this definition of how an LED driver performs, we can look at the benefits that come into play. At the top of the list is the fact that LEDs’ low voltage brings forth qualities that are not present in other lighted sign options. According to Bryan Vincent, a partner at Principal LED, “Low voltage reduces shock hazard during installation, reduces the amount of UL testing that is required (<15V DC is considered inherently safe by UL standards), and the driver can have built-in surge protection to protect the LEDs in the event of a lightning strike or power surge.”

These are widely accepted benefits of LED configurations, and Meyne expands on the pluses of the driver.

“The other function of an LED power supply is that it contains circuitry which limits the current output to a maximum of 5A; as well as having short-circuit protection/current limiting function,” he explains. “All of this is so that the power supply can be classified as a UL Class; and be safely usable for low-voltage LED installations. The Class 2 designation also allows sign installers and fabricators to work with the secondary side products (and not just electricians).”

Of course, with advantages also come potential setbacks. With all types of LED-based electric signs, there is a heat element involved. There are also ways for external environmental entities to impact performance.

“Overheating and water damage (are) two of the biggest causes of premature failure,” states Harrison.  “Following manufacturer recommendations is the best protection. Retrofitting LEDs to existing fixtures without adequate ventilation can lead to overheating and failure. Both the LEDs and drivers can be weather-sealed, but if there is insufficient space or ventilation to dissipate the heat buildup, problems will ensue. Manufacturers go to great lengths to identify proper installation procedures, which should be scrupulously followed to avoid problems.”

And one manufacturer, Principal LED, suggests that even the mounting of the sign can lead to problems if approached incorrectly.

“If the drivers and wiring are in the bottom of the sign and there is poor drainage, this can result in the power supplies shorting out,” says Vincent. “If the drivers are going to be placed inside the sign, I recommend mounting them on the vertical side of the sign with access through a hatch or door. An even better option is to mount the power supplies inside an enclosure box at the base of the pole.”

In the same vein, Meyne is surprised that LED signage is continuing to be improperly installed given the sensitivity that is associated with failure due to environmental elements.

“It is amazing as power supplies and ballasts have been used in our industry for so long, yet we do see them mounted in the bottom of a sign cabinet,” states Meyne, “and this is where we see the most failure from water damage.”

Environmental Considerations

This discussion about how to protect power supplies–and LEDs in general–from elements that could potentially damage the sign has much to do with where the sign is located and the quality of the components.

Wensco's Redmer points to the IP (International Protection or Ingress Protection) Ratings for LEDS. “There is an IP rating for LEDs, lamps, power supplies and all types of things,” he explains. “Most LEDs nowadays are IP66, IP67 or IP68. The first digit stands for solids protection, and the second number stands for liquids protection. The digits indicate conformity with the conditions of how far they can go.”

He adds that a lower IP Rating would mean susceptibility to dust particles, water leakage and more, but those with the higher IP Ratings can survive such elements more dependably.  “If you’re outdoors, you don’t really have to worry about the LEDs for the most part, especially if they have IP67 or IP68 ratings,” Redmer says. “And the same goes with the power supplies.”

Specific to the driver, there seems to be a consensus that a certain type should be used outdoors. Vincent suggests using a Class 2 driver in these situations.

“For electric signs, you should always use a Class 2 driver,” he shares. “Keep in mind that there are UL drivers in SAM (Sign Accessories Manual) that are non-Class 2 (isolated) drivers. These may be used in special circumstances, but they are not automatically qualified to be used with Class 2 low-voltage LEDs.  So be careful, there are a number of vendors that are selling low voltage drivers above 12V, 5A (60W) per channel and falsely claiming that they are Class 2. They should not be used, as this is a safely concern and fire hazard.”

From a general sense, LED signs perform well in a variety of environments. It’s simply the planning that must go into the installation and type of products used that can vary.

“LEDs can be used virtually anywhere fluorescent or incandescent lighting has been used. The important thing is to make sure the LEDs and drivers are rated for the environment where they will be installed,” explains Harrison. “LEDs and drivers designed for indoor use should never be installed where they will be subject to rain, snow, condensation, etc. Follow the manufacturer's recommendations and select components designed for the conditions where they will be used.”

Avoiding Trouble

Since even the most skilled electric sign installers can run into problems with LED drivers, it follows that everyone involved with these signs should scrupulously follow the appropriate steps.

First off, “Make sure the driver is rated for the input voltage,” says Vincent. “Many drivers are what we call universal (i.e. 110-277VAC input); however, some will accept 110V only or 110/220V only. This is a simple but unfortunately common mistake—the installer does not verify the output voltage of the site.  This can cause the power supply to fail.”

Meyne expands by adding, “If a product's operating voltage is unknown or in doubt, it is always safer to try hooking up to a 12V power supply first, before trying 24V. A 24V LED on 12V power supply will simply not light up (or be very dim). A 12V LED on 24V power supply will kill the LED, due to overcurrent.”

Next, when looking at purchasing the LED sign’s components, keep in mind that, “Often the driver and LED light are supplied as a matched package by the manufacturer. If purchased separately, a sign maker must first determine if a constant voltage or constant current driver is required,” says Harrison. “Strip lighting is often constant voltage and rated in watts per foot. The wattage of the driver is determined by multiplying the length of the strip by watts per foot. Fixed length strips may simply be rated in watts.”

Redmer, whose company supplies a range of LED products from multiple manufacturers, reminds sign shops that manufacturers are a great source for inquiries about LED projects. He says, “Manufacturers package LEDs by the bag and one power supply can power a bag. Nowadays, LEDs are being used for florescent replacement and are being put together in products like Principal LED’s Quick Stick, which is basically LEDs put on a bar. You have to know how many feet you can run on one power supply; you should know the specs for the LED that you’re putting in the sign and follow the manufacturer’s recommendation.”

Finally, there are testing procedures that will help to determine if the power supply is appropriate for the sign.

“Once the LEDs are wired in place and the correct input voltage is confirmed, the power supply should be wired to the LEDs and test lit,” says Vincent. “Check the output voltage with a meter to make sure the LEDs are receiving the correct input from the output of the driver.”

The main point with getting an adequate test result is to ensure that the LED is not over- or under-powered.

“Driving 12 VDC LEDs at slightly less than 12 VDC will result in a minimal loss in output, but driving them over 12 VDC can lead to overheating and loss of life,” says Harrison. “Over-voltage can come from high line voltage, or use of an excessively oversized driver.”

Life and Location

One certainty of an LED driver unit is that its lifespan is uncertain. The ambiguity of how long it may last is always in question. That does not mean you should expect a short life from the driver—these were built to last along with the LEDs.

“If the LED company is offering a five-year warranty on the LEDs,” Vincent notes, “you should demand a driver whose warranty is in lock-step with the LEDs as a system.”

He adds that many drivers can last somewhere in the 10-year range if operated properly, however there are many more factors that can contribute to a driver’s malfunction before the actual LED.

“The driver is 100 times more likely to fail than the LED,” Vincent continues, “so I recommend using LED drivers that have at least a five-year product and labor warranty that matches the LEDs being provided.”

Other factors may contribute to the life of an LED driver. Some are environmental, as noted previously, others are factors of the sign and components themselves. 

“The lifetime of a driver depends on a number of parameters,” says Vincent. “Quality of components such as the FETs, capacitors, and other active and passive components inside; thermal characteristics of the unit and its ability if the driver to dissipate heat; and the load (the lower load on the unit, the longer it will last).”

But when the necessary pre-planning, attention to installation, and testing have been done, an LED sign can be a cost-effective, long-lasting solution for users.