TRIAC dimming tends to flicker when adjusted to low brightness (such as below 20%) on LED lamps. The main reason involves the mismatch between the working principle of TRIAC and the electrical characteristics of LED. The following is a detailed analysis:
1. Holding Current of TRIAC
TRIAC triggering principle:
TRIAC dimming adjusts the brightness by cutting the AC waveform (phase cutting). Each time the AC passes zero, the TRIAC needs to be triggered to turn on at a specific phase angle (such as 30°, 90°) and turn off when the current is lower than the holding current (usually 10-50mA).
LED has low power and low current:
Incandescent lamps have high power (such as 40W) and large current (about 170mA), so TRIACs are easy to maintain conduction; while LED lamps have low power (such as 5W) and the working current may be only 20-30mA.
When the brightness is adjusted to low, the conduction time is very short, and the current may be lower than the holding current, causing the thyristor to shut down prematurely, causing the lamp to restart continuously → flickering visible to the naked eye.
2. Compatibility issues of LED driver power
Traditional thyristor dimmers are designed for resistive loads (incandescent lamps), while LEDs are capacitive/inductive loads and require a dedicated driver power supply to convert the voltage.
Problems at low brightness:
The driver power supply may not be able to obtain enough energy in the extremely short conduction time, resulting in unstable output voltage → LED flickering or extinguishing.
Some low-end driver power supplies are not filtered enough and cannot smoothly process the "fragmented" current after phase cutting, exacerbating flickering.
3. Poor matching between dimmer and lamp
Leading Edge vs Trailing Edge dimmer:
Leading Edge dimmer (traditional TRIAC) cuts the first half of the waveform and is more prone to flickering at low brightness; Trailing Edge dimmer (such as MOSFET) cuts the second half, which is more suitable for LEDs, but the cost is high.
Dimmer minimum load limit: Some thyristor dimmers require a minimum load (e.g. 20W). If the total LED power is insufficient (e.g. only 10W), they will not work stably at low brightness.
4. Solution
(1) Choose LED lamps with good compatibility
Look for the "TRIAC dimmable" label and check the brand test report (e.g. support 10%-100% dimming).
Preferably use LED lamps with constant current drive and marked "low brightness without flicker" (e.g. Philips Hue, Osram).
(2) Change the dimmer type
Use a trailing edge dimmer (e.g. Lutron CL series) or digital dimming (e.g. DALI, 0-10V).
Use smart dimming solutions (e.g. Zigbee/Wi-Fi dimming) to completely avoid thyristor problems.
(3) Add a dummy load (temporary solution only)
Connect a resistive load in parallel (such as a 3W cement resistor) to increase the total current to maintain the conduction of the thyristor (but reduce energy efficiency, not recommended for long-term use).
Technical comparison: Why does PWM dimming not have this problem?
PWM dimming adjusts the duty cycle of brightness by quickly switching the LED (such as above 1000Hz) instead of cutting the AC waveform.
The current always passes at full amplitude, and there is no problem of insufficient current maintenance, so 1%-100% flicker-free dimming can be achieved.
The low-brightness flicker of thyristor dimming is essentially the result of the mismatch between "incandescent lamp era technology" and "LED characteristics". If stable dimming is required, it is recommended to gradually eliminate the thyristor solution and turn to PWM or intelligent dimming ( DALI )system.
