Leading Edge and Trailing Edge Dimmers Explained
Both dimming methods regulate the brightness of a luminaire by cutting off part of the AC sine wave. The more that is cut, the less energy reaches the lamp – and the lower the brightness.
Leading Edge Dimmer (R/L)
The leading edge dimmer cuts the beginning of each half-wave. The voltage therefore starts with a delay and then rises steeply. This technique is the older of the two variants and was the standard for incandescent and halogen lamps.
Suitable for:
- Incandescent lamps
- Halogen lamps with conventional (wound) transformers
- Selected LED lamps that are explicitly marked as compatible
The steep voltage rise can cause flicker, buzzing or a limited dimming range with sensitive LEDs.
Trailing Edge Dimmer (R/C)
The trailing edge dimmer cuts the end of each half-wave. The voltage starts immediately and is gently switched off at the end. This method operates much more gently and is the preferred dimmer type for modern LED luminaires.
Suitable for:
- LED lamps (dimmable)
- Halogen lamps with electronic transformers
- Electronic LED drivers
The gentle switch-off reduces flicker and noise, and generally enables a smoother dimming curve down to very low brightness levels.
How Do I Choose the Right Dimmer?
The choice of the right dimmer depends on the lamp type:
- LED lamps: Use a trailing edge dimmer (R/C). The lamp must be marked as "dimmable".
- Incandescent and halogen lamps: Use a leading edge dimmer (R/L) or universal dimmer.
- Low-voltage halogen with electronic transformer: Use a trailing edge dimmer (R/C).
- Low-voltage halogen with wound transformer: Use a leading edge dimmer (R/L).
Universal dimmers automatically detect the connected load and select the appropriate method. They offer the greatest flexibility but are more expensive to purchase.
Observe Minimum and Maximum Load
Every dimmer has a minimum and a maximum load. With LED lamps that have low power consumption, the minimum load of the dimmer may not be reached – the result is flicker, unstable dimming behaviour or a lamp that does not turn on at all. Make sure that the total wattage of the connected lamps is within the permissible load range of the dimmer.
Frequently Asked Questions (FAQ)
Can I continue using my old dimmer for LEDs?
In many cases, no. Older leading edge dimmers are designed for the high load of incandescent lamps and often do not work reliably with LEDs. An LED-compatible trailing edge dimmer or universal dimmer is the safer choice.
Why does my LED buzz when dimming?
Buzzing is often caused by an incompatibility between the dimmer and the LED driver. Switching to a trailing edge dimmer or a dimmer recommended by the luminaire manufacturer usually resolves the issue.
What do the abbreviations R, L and C mean?
R stands for resistive load (incandescent lamp), L for inductive load (wound transformer), C for capacitive load (electronic transformer/LED driver). An R/L dimmer is a leading edge dimmer, an R/C dimmer is a trailing edge dimmer.
In Summary:
Choosing the right dimmer is crucial for flicker-free, smooth dimming results. For modern LED lighting, a trailing edge dimmer (R/C) or universal dimmer is generally recommended. At Leuchtenland.com you will find the recommended dimmer type specified for every dimmable luminaire. For more on the topic of dimming, see our detailed lexicon article Dimmable and Dimmers Explained.