Additionally, incandescent dimmers can also be categorized into two main types: “leading edge” (also called forward phase-cut) and “trailing edge” (also called reverse phase-cut). The former uses a TRIAC as the semiconductor switch, and the vast majority of incandescent dimmers in residential buildings are of this type because of the lower cost of the design. The TRIAC is turned on when it receives an electrical pulse at its “gate” (one of the terminals of the device) and stays on until the electric current falls below the TRIAC’s “holding current” very near the end of the half cycle of the ac wave form, which in the United States operates at 60 Hz and ideally (and very closely in practice, too) has the form of a sine-wave. The earlier the TRIAC is turned on in the half cycle, the brighter the lamp operates. The resulting voltage waveform at the lamp is illustrated in Figure 4.9. The design of the dimmer converts the user action—such as moving a slider up and down—to the proper timing of this TRIAC gate pulse. The operation of the TRIAC is illustrated in Figure 4.9. Electronic switches that employ a TRIAC simply turn it on at the beginning of the half cycle to operate the lamp at full on.
The TRIAC works very well with incandescent lighting because the TRIAC’s holding current (below which the TRIAC will not remain on) is much smaller than the current in even the lowest wattage incandescent lamps, such as a 25 W lamp. However, with CFLs, and even more so with SSL devices that require very low power, the current required to operate the lamps may be smaller than the holding current, and this can lead to observations of flickering or other improper operation. In addition, other problems have been observed with keeping the TRIAC reliably in conduction with certain LED lamps. In some cases, the problems manifest themselves when the total load (number of lamps) is actually increased, meaning that the minimum load requirement in an incandescent dimmer is not the only condition that needs to be satisfied. Leading edge dimmers that are used in commercial grade lighting controls sometimes provide a continuous gate signal rather than a pulse, and this action is very successful in keeping the TRIAC on even with smaller loads. Additionally, trailing edge dimmers, which use transistor switches that require a continuous signal to keep them on and are not characterized by a holding current, may also avoid this problem. Trailing edge dimmers were originally designed for low-voltage (incandescent) lighting using an electronic transformer to step the 120 Volts ac (Vac) line voltage to the 12 Vac required by the lamps. These transformers were developed to make them lighter, smaller, and also often less expensive than core and coil wound transformers, and they utilize capacitors on the “front end” for energy storage inside the device. When a capacitor suddenly experiences a high voltage, a large inrush of current occurs, and many such electronic transformers are not compatible with leading-edge dimmer designs. Trailing edge designs reverse the process of switching by turning the transistors on in the beginning of the half cycle and turning them off at some point before the end of the half cycle. Front end capacitors do not cause problems for this mode of operation, so these types of dimmers are more compatible with SSL drivers such as those used in medium screw-base incandescent replacement lamps.
The lighting controls industry is developing new dimmer designs specifically for LED lamps that are used as replacements for incandescent lamps. It is reasonable to expect that the lamps will operate well with the new designs, but the industry has estimated that there are more than 150 million leading-edge dimmers installed in U.S. homes, and it is probably impractical to expect to replace them all as LED lamps become more popular. NEMA (2010b) has developed a new standard, NEMA SSL 6-2011, to address the retrofit issue,
FIGURE 4.9 Waveforms illustrating leading-edge dimming control.