Triacs are at the heart of dimming controls for LED lighting. Triacs used in dimmers have normally been characterised 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.
LEDs have much lower steady-state current than incandescents, and their initial turn-on current can be much higher for a few microseconds of each half-cycle of AC line voltage. Therefore, a spike of current can be seen at the beginning of each AC half-cycle. Typically, the current spike for an AC replacement lamp is 6 to 8A peak; the steady-state follow current is less than 100mA. An LED flood lamp for a recessed ceiling fixture designed to replace a typical filament unit that produces 750 lumens consumes only 13W in contrast with the old filament unit, which normally draws 65W.
Designing an AC circuit for controlling LED light output is very simple when using the newest triac designs, such as the Littelfuse Q6008LH1LED or Q6012LH1LED Series, because the only components required are a firing/triggering capacitor, a potentiometer, and a voltage breakover triggering device. Two inverse parallel sensitive gate silicon-controlled rectifiers (SCRs), such as the Littelfuse S4X8ES1, can be used as the voltage breakover triggering device, allowing the controlling circuit to produce a wide range of light level outputs. Also, using these components as the triggering device allows achieving a low hysteresis control because two SCRs form a full breakback trigger.
If the application doesn’t demand a wide control range and low hysteresis, a simple variable light control may be designed using quadrac devices, such as the Littelfuse Q6008LTH1LED or Q6012LTH1LED Series (Figure 1). (A quadrac device is a special type of thyristor that combines a diac and a triac in a single package.) The circuit shown in Figure 2 minimises the component count 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 light dimming function that operates from 175° to <90° of each AC half-cycle.
Figure 2. In this circuit, the potentiometer is 250kO with built-in fixed end resistance of 3kO minimum. The quadrac device included offers low gate and holding current characteristics. RL is a minimum LED load of 10W. VC is the same as the triggering voltage of the built-in diac die.
To learn more about the latest LED lighting control options, download the free Controlling LED Lighting Using Triacs and Quadracs application note from Littelfuse.