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Solid State Lighting for Incandescent Replacement

3 Existing Control Types
Existing dimming controls can be divided into two broad categories: incandescent phase-cut dimmers and fluorescent controls. The division is more conceptual than practical, as some fluorescent ballasts respond to a phase-cut signal, and there exist some LED power supplies that respond to control types originally intended for fluorescent ballasts. This paper addresses applications using phase-cut dimmers typically used with incandescent lamps.
3.1 Incandescent Phase-Cut Dimmers The phase-cut dimmer was developed for incandescent lamps. One of the advantages of phase-cut dimmers is that a standard switched lighting circuit may be easily retrofitted with existing wiring by merely changing the conventional two-way switch to a dimmer control. In a standard installation, current that is conveyed through wires to the lamp powers the light fixture. In a simple switched lighting circuit, a switch in the “on” position will allow the current to pass unmodified through the switch to the lighting fixture. The switch in the “on” position allows the fixture to use electricity all the time. Figure 1 shows this concept.
8 Global standards under development have generally adopted this definition for LED light sources. They will become formalized with the publication of completed consensus industry standards.
© Copyright 2010 by the National Electrical Manufacturers Association. 7
LSD 49-2010

A phase-cut dimmer is a unique kind of switch that uses a special circuit to “turn on and off” the current supply to the fixture at regular intervals, typically 120 times per second. This is fast enough that the human eye does not perceive any interruption, in the same way that celluloid movies look continuous even though they are actually a series of static images shown in very quick succession.9
The amount of time the current is “on” determines how bright or dim the lamp will be. For each “on/off” cycle (1/120th of a second), the more time current is “on,” the brighter the lamps will be and the more energy used. This is shown in Figure 2.
In the United States, power is delivered from the electric utility as a sinusoidal voltage, with a root mean square (rms)10 value of 120 volts and repeating 60 cycles per second (see Figure 3). A phase-cut dimmer contains a thyristor semiconductor device (triac, field effect transistor, or silicon controlled rectifier) that acts like a switch. When the thyristor turns “on,” current flows through the circuit; when the switch turns “off,” no current flows through the circuit. By turning the switch “on” and “off” twice every line cycle, the rms voltage applied to the lamp is reduced from 120 volts to a lower value, so the rms current that flows is also reduced. As the rms current flowing through the lamp is reduced, it dissipates less power, and so produces less light.