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Al Marble of Advance and Sameer Sodhi, Marketing Manager–LED Systems for OSRAM SYLVANIA, Inc. offer the following design and application tips for architects and designers seeking to specify an LED system:
• Do not overload the driver. LED drivers are rated for a maximum load that must be paid proper attention. “One of the most common mistakes is to connect too many LED strings in series,” says Sodhi. “Putting too many strings in series may result in too low a voltage being available to the last string(s) in the chain.”
• Be aware that there are heat issues with LEDs even during normal operation. “LEDs are occasionally and incorrectly believed to generate little or no heat,” says Marble, pointing out that there can be substantial heat generated in higher-wattage LED fixtures. “Hopefully, the integrator/fixture manufacturer designed appropriate heat sinks for the system. Still, allowing ample heat dissipation in the installation is good practice, such as mounting to metal or allowing some ventilation if possible.”
• Pay special attention to the environmental rating of the driver: Most drivers are “dry location only” in type and must be installed in a weatherproof electrical enclosure if used outdoors. Damp location drivers should be used in signs or raceways where some moisture is expected, and wet location drivers are typically supplied in a pre-assembled, sealed enclosure for mounting outdoors. “Make sure that the driver is rated for use in its environment,” says Marble. “And make sure that the driver has been evaluated and rated for use within the particular LED system.”
• Check the voltage rating of the LED load being used against the rated output voltage of the driver, and use only a driver that is correctly matched to the load. When the wrong driver is used, the LEDs will either not light or may operate at higher currents than intended. For example, using a 12V driver on a 10V LED load could result in significantly shorter life of the module.
• Check the quality of the DC outputvoltage of the driver. “To maximize the light output from the LEDs without overstressing them requires a constant DC current to be maintained through them,” says Sodhi.
• Remote mounting of the driver can result in voltage drops and power losses on the DC wiring that must be properly accounted for.
• For optimal performance, choose drivers that have short-circuit protection, that are designed specifically for the given application, and that can handle temperature extremes. “Off-the-shelf DC power supplies are typically designed for room temperature applications such as IT or telecom,” says Marble. “Such power supplies may operate erratically or not at all under the rigors of a lighting application.”
• Be aware of ambient temperatures at the application. While LEDs have the ability to start at temperatures as low as -40 ºC, operating them at cold ambient temperatures can cause operating problems. LEDs draw higher power at cold ambient temperatures, the opposite of what happens with fluorescent lamps, and this can lead to system malfunction. “For outdoor applications where the power supply is mounted remotely, the maximum LED load on the driver should be de-rated by 10-20 percent to avoid system conflicts during cold temperatures,” says Sodhi.
• UL Class 2 ratings, required for LEDs in sign application, can also benefit general lighting applications. “UL Class 2 mandates that the driver has voltage, current and power below certain levels on the secondary,” says Marble. UL Class 2 rated LED drivers provide electrical isolation from the AC line voltage, which allows for safe handling of the LEDs being operated at low-level DC voltages.
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