Very Simple Triac Dimming Circuit of LED Ceiling Floodlight

Triacs (short for AC triode) constitute the core of AC dimming control. These elements can conduct current in either direction after triggering (opening). In the past, triacs used in dimmers were usually aimed at the characteristics of incandescent lamp loads and were also dedicated to incandescent lamp loads. Incandescent lamp loads have high current ratings, high initial inrush current, and high inrush current at the end of filament breaking life under both steady-state conditions.

In contrast, LEDs have much lower steady-state current than incandescent lamps, and they can have higher initial conduction current in a few microseconds at the beginning of each half cycle of AC line voltage. Generally, the current peak of AC standby lamp is 6-8a (peak); Steady state continuous current is less than 100 mA.

The power consumption of LED lamps for home lighting is 7.5 watts (type A19 bulbs, 450 lumens) or higher, while a chandelier usually has four to ten bulbs. However, the power consumption of a string of 50 Christmas lights can be as low as 4.8 watts. The LED floodlights used in ceiling lamps designed to replace typical filament lighting can produce 750 lumens and consume only 13 watts (BR 30). In contrast, old filament lighting usually consumes 65 watts.

The latest generation of triacs makes the design of AC circuit for controlling LED light output very simple because it requires relatively few components: an ignition / trigger capacitor, a potentiometer, and a voltage breakdown trigger device. Two reverse parallel sensitive gate thyristors (thyristors) are used as voltage breakdown trigger devices, so that the control current can produce a wide range of optical level output. This also allows lower hysteresis control because the two thyristors form a comprehensive counterattack through trigger. Figure 1 is the control circuit diagram applicable to ceiling lamp (such as br30led lamp).

Figure 1: ceiling floodlight dimmer circuit using two reverse parallel sensitive pole thyristors

This circuit enables the bulb to turn on 180 degrees almost every half AC cycle; The remote control timing on can also be delayed to a smaller conduction angle every half cycle to achieve lower optical output. Triacs with low holding and locking current characteristics, such as Lite's q60xxlh1led series, enable triacs to maintain a very low current level. Two reverse parallel sensitive gate thyristors (s4x8es1), connected together, can produce a very low voltage and trigger the device with full counter breakdown voltage, resulting in a very low hysteresis. Thus, the potentiometer can be set for the low conduction angle that is immediately opened when the line switch is closed and opened. The circuit diagram in Fig. 2 shows the improvement of the old phase control / dimmer circuit that provides poor hysteresis by adding a steering diode around the C1 ignition capacitor.

Figure 2: dimmer circuit design to improve hysteresis.

For applications where wide control range and low hysteresis are not so critical, quadrac devices (a special thyristor combining DIAC (AC switching elements at both ends) and triac in a single package) can provide a simple variable light control method. Fig. 3 illustrates how to combine DIAC trigger device and AC triac in a single TO-220 independent installation package to further reduce the number of elements. Due to the high switching voltage VBO of DIAC trigger device, this control circuit has low full on voltage, but it can provide dimming function in the range of 175 degrees to less than 90 degrees every half AC cycle.

Figure 3: in this quadrac based dimming circuit, the potentiometer is 250 kohm and has a built-in fixed end resistance of at least 3 kohm. Quadrac devices are qxlth1 LED products with more sensitive triac silicon chips (low gate current and holding current characteristics). RL is the minimum LED load (10 watts). VC is equivalent to the trigger voltage of the built-in DIAC silicon wafer.

For more details on LED lighting dimming design, you can download free application instructions from Lite.

About the author: Koichiro Yoshimoto is the thyristor business development manager of the semiconductor division of Lite company. His responsibilities include finding and developing strategic growth markets and introducing new products to the power control products (with emphasis on thyristor products) market. He holds a bachelor's degree in industrial chemical engineering from the University of Tokyo, Japan. He joined Lite ten years ago and worked in the electronics industry for 20 years. Can pass kyoshimoto@littelfuse.com Get in touch with him.