Signs of the Times - April 2014 - (Page 36)

LED Upd a te By M. Nisa Khan, Ph.D. LED Driver Utilization Basics Basic challenges and solutions L ED lamp manufacturers employ specific electric drivers to operate their proprietary lamps and, typically, signmakers reference the manufacturers' lists to learn driver-requirement details on how best to light their LED-illuminated signs. Less technically interested end users, however, simply connect an LEDlamp or luminaire unit, just as they would other lamp types. Unlike incandescent lamps, LED and gas-discharge lamps require added electrical parameter controls to stabilize their operation and increase longevity. Incandescent lamps are more simple - the mainsource current passes through the light-emitting element, i.e., a tungsten wire coil, which is the only resistive load. The coil's resistive heating generates the incandescent bulbs' glowing light. For gas-discharge lamps, e.g., tubular, fluorescent lamps, the load current must be limited externally; otherwise, its negativedifferential resistance will cause the lamp's inside current to rise indefinitely, which can prematurely burn it out. Because of this characteristic, installed electrical or electronic ballasts must limit the current, to prevent catastrophic damage to the lamp. Fig. 1: LED I-V Curve 36 SIGNS OF THE TIMES April 2014 Why use an LED driver An LED is a semiconductor diode and, therefore, can only be turned on when the electrical drive across it is positive. Further, electrical inputs to LED light sources must be controlled for optimum performance and prolonging lamp lifespans. The electrical drive is denoted as forward voltage (VF) with its analog, forward current (IF). Under positive bias VF, current IF passes through the LED and generates light through a process known as electroluminescence. An LED's inherent properties are described by their current-voltage (I-V) and light current (L-I) characteristic dependences. See Figs. 1 and 2. Fig. 2 shows both the LED and laser zones specified by spontaneous and stimulated emissions, respectively. Current control preferred Essentially, an LED is monochromatic; that is, it emits light of a single color, although not fully saturated color, like a laser can produce. The LED semiconductor material's structural properties determine the emitted-light color. One such property is the amount of energy that separates the "bands," meaning the areas where the electrons can Fig. 2: LED L-I Curve Dr. M. Nisa Khan is president of IEM LED Lighting Technologies. She consults in the solidstate lighting industry and educates consumers about LED lighting. She has a bachelor's degree in physics and mathematics, and master's and Ph.D. degrees in electrical engineering. Email her at nisa.khan@iem-led.com flow (the conduction band) and where they can fall into (the valence band). Although an LED turns on at a fraction of a volt, it is optimally "on" when the VF across it, multiplied by the electronic charge, equals the separation energy. This condition allows electroluminescence interaction to occur efficiently, which then produces light with a dominant wavelength, and this value is determined by the inherent relationship between energy and wavelength. More technically, this special value, VF-OPT, is an inherent property for an LED of a specific color; V F-OPT ~ 3.6 V for blue InGaN LEDs; VF-OPT ~ 2.0V for red LEDs. Most LED manufacturers refer to these as VF values. Any small fluctuation in applied VF will generate large IF variations because the I-V curve of a diode is exponential. Large IF variations, in turn, cause significant fluctuations in the output light because L and I are directly proportional, as seen in the L-I curve (Fig. 2). To achieve a steady light output, an LED driver must continuously provide a constant current to the LED or any ensemble of LEDs. LED drivers are voltage regulators Essentially, all LED drivers that generate a constant-current output are voltage regulators (VR) that can also sense the LED current. Such a VR (Fig. 3) is constructed with a feedback circuit that comprises a sensor resistor (RS) across which a voltage drop (VCS) can be measured and used as a reference for adjusting the circuit parameters. This ensures that a constant IF flows through any ensemble of LEDs.

Table of Contents for the Digital Edition of Signs of the Times - April 2014

Signs of the Times - April 2014
Columns & Departments
Features
ST Update
Technology Update
ISA Sign Expo 2014 – Making the most of it
Vinyl Apps
Strictly Electric
LED Update
Software Update
Technology Review: Roland’s SOLJET Pro 4 XF-640 printer
Technology Review: EFI’s VUTEk GS2000/3250 digital-hybrid printers
Sign Museum News
New Products
The 2014 Intl. Sign Contest
2nd Annual Readers’ Choice Award
For the Record
ISA Expo Adds SEGD Component
Surprising Evidence about Neon’s History
Editorially speaking
Advertising index

Signs of the Times - April 2014