Nov 302009

Jim Benya Looks At Basic LED Circuits in the Fall 2009 LED edition of Architectural Lighting. Benya points out LEDs are fussy about power and heat. LEDs operate on low voltage DC power and require a Transformer, Voltage Regulator, and Driver in order to function in our 120/277 volt world. The Transformer converts line power to 24 volt DC current. The Voltage Regulator keeps the Voltage constant and the Driver keeps the current constant. Any changes in the current or voltage can cause the LEDs to fail.

Once converted to low-voltage DC, power then passes through the LED to generate light. But as with a fluorescent lamp, there must first be a circuit that regulates the amount of energy—or the lamp will blow up. For a fluorescent lamp this circuit is called a ballast; for solid-state lighting it is called a driver. The typical contemporary white LED is designed to operate at either 0.35A, 0.70A, or 1A depending on watts, brand, and model. It is the driver’s job to regulate the DC power for the specific LED. An added complication is that seldom in lighting is one LED enough, and it is common to have several LEDs mounted and wired together. For example, a basic downlight might employ 20 or more 0.25W or 0.5W LEDs.

Drivers vary from simple resistors to integrated circuits that precisely control the diode itself. Add Dimming to fixture and the circuit becomes even more complex. Voltage can not be cut to the LED so the circuit has to vary the ratio of on/off cycles per second to achieve the effect of dimming. If you wish to control the color of your LED lighting system you add either RGB (Red, Green, Blue) LEDs to your dimming driver or RGBA (Red, Green, Blue, Amber) LEDs to your dimming driver and balance the color by varying the dimming to each LED.

Benya points out, the industry has no standards for Drivers, “We need standards in solid-state lighting that go well beyond the lumen measurement, efficacy, and life testing that have dominated solid-state lighting discussions to date. For instance, due to their poor 70 percent efficiency, drivers are often avoided when the energy efficiency of solid-state lighting is discussed.. As it is, LEDs have already developed a reputation for failing to meet rated performance and life.”

The good news is the industry went through similar troubles with the introduction of T8 fluorescent lamps and electronic ballasts. It is just a question of time before the issues of LEDs get sorted out.

Contant-Current Circuit

120v AC --> 24V DC Transformer --> Driver --> LED - LED - LED -->

Constant-Voltage Circuit

120v AC --> 24V DC Transformer --> Voltage Regulator --> Driver --> LED
                                                     --> Driver --> LED
                                                     --> Driver --> LED

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