Solid-state lighting is quickly become the most popular lighting option for a wide range of applications. As incandescent lamps have been rendered largely obsolete, given the U.S. government’s mandate to save energy, they are increasingly being replaced by Light Emitting Diode (LED) bulbs due to their long life (typically 25,000 hours) and the ease of adapting them to many different socket and shape requirements. As prices for LED bulbs continue to drop and become increasingly competitive with compact fluorescent (CFL) bulbs, some predict LED lighting’s share of the residential lighting market will reach nearly 50 percent by 2016 and more than 70 percent in 2020. However, from the designer’s perspective, it’s important to understand that LED lighting and dimmer controls present different challenges than earlier technologies.
Triacs (short for triode for alternating current) make up the heart of AC light dimming controls. These components can conduct current in either direction when turned on. In the past, triacs used in dimmers were normally characterized and specified for incandescent lamp loads, which have high current ratings for both steady-state conditions and initial high in-rush currents, as well as very high end-of-life surge current when a filament ruptures.
In contrast, LEDs have much lower steady-state current than incandescent lamps, and their initial turn-on current can be much higher for a few microseconds at the beginning of each half-cycle of AC line voltage. Typically, the current spike for an AC replacement lamp is 6–8A peak; the steady-state follow current is less than 100mA.
LED bulbs for home lighting might draw 7.5W (A19 bulb-450 lumens) or higher, with a chandelier typically having from four to ten bulbs. However, a string of 50 Christmas lights could draw as little as 4.8W. An LED flood lamp for a recessed ceiling fixture designed to replace a typical filament unit that produces 750 lumens consumes only 13W (BR30) in contrast with the old filament unit, which normally draws 65W.
The latest generation of triacs make designing an AC circuit for controlling LED light output very simple because relatively few components are required: a firing/triggering capacitor, a potentiometer, and a voltage breakover triggering device. Using two inverse parallel sensitive gate silicon-controlled rectifiers (SCRs) as the voltage breakover triggering device allows the controlling circuit to produce a wide range of light level outputs. This also allows achieving a low hysteresis control because two SCRs form a full breakback trigger. Figure 1 illustrates a circuit diagram for a control suitable for a recessed flood lamp (such as a BR30 LED lamp).
This circuit allows the bulb to turn on at nearly the full 180° on each AC half-cycle; the RC timed turn-on also may be delayed to a small conduction angle on each half-cycle for very low light output. A triac with low holding and latching current characteristics, allows the triac to remain on at very low current levels. Two inverse parallel sensitive gate SCRs (S4X8ES1) with their gates tied together produce a very low voltage triggering device with full breakback voltage, producing very low hysteresis. That allows the potentiometer to be set for a low conduction angle with turn-on being immediate when the line switch is turned off and on. The circuit diagram in Figure 2 improves upon older phase control/dimmer circuits that provided poor hysteresis by adding steering diodes around the C1 firing capacitor.
For applications in which a wide control range and low hysteresis are less critical, a quadrac device (a special type of thyristor that combines a diac and a triac) offers a simple variable light control alternative. Figure 3 illustrates how to reduce the component count further by combining the diac triggering device and an alternistor triac in a single TO-220 isolated mounting tab package. This control circuit allows a little lower full turn-on voltage due to higher VBO switching of the diac trigger device but offers a dimming function that operates from 175° to <90° of each AC half-cycle.