Trailing-edge dimmers offer smoother, quieter control for modern lighting systems—but their inner workings often remain overlooked. This post sheds light on the circuitry behind the silence. Sometimes, the most elegant engineering hides in the fade, where silence is not a flaw but a feature.
Let’s get started.
Dimmers serve as an effective interface for controlling energy-efficient lighting systems. And dimming methodologies are broadly categorized into forward-phase dimming (leading-edge), reverse-phase dimming (trailing-edge), and four-wire dimming, commonly referred to as 0–10 V analog dimming.
This post specifically examines reverse-phase dimming, also known as trailing-edge dimming, which is particularly well-suited for electronic low-voltage (ELV) transformers and modern LED drivers. Its smoother voltage waveform and inherently lower electromagnetic interference (EMI) make it ideal for applications requiring silent operation and compatibility with capacitive loads.
Leading and trailing edge dimming
In a leading-edge dimmer—also known as a triac dimmer or incandescent dimmer—the electrical current (sinusoidal signal) is interrupted at the beginning of the AC input waveform, immediately after the zero crossing. This dimming method is traditionally used with incandescent lamps or magnetic low-voltage transformers.
On the other hand, a trailing-edge dimmer interrupts the current at the end of the AC input waveform, just before the zero crossing (Figure 1). This technique is better suited for electronic drivers or low-voltage transformers with capacitive loads.
Figure 1 In trailing-edge dimming waveform, conduction begins mid-cycle, and current is interrupted before zero crossing to suit capacitive loads. Source: Author
In a nutshell, a trailing-edge dimmer is an electrical device used to adjust the brightness of lights in a room or space. It operates by reducing the voltage supplied to the light source, resulting in a softer, dimmer glow.
Unlike leading-edge dimmers—which cut the voltage at the beginning of each AC waveform—trailing-edge dimmers reduce the voltage at the end of the waveform. This “trailing edge” approach enables smoother, more precise dimming, especially at lower brightness levels.
Trailing-edge dimmers are particularly well-suited for LED lighting. They tend to be more efficient, generate less heat, and offer better compatibility with modern electronic drivers. The result is a quieter, flicker-free dimming experience that feels more natural to the eye.
Figure 2 The popular DimEzy brand for trailing-edge rotary dimmers embodies compact engineering optimized for retrofit installations. Source: LiquidLEDs
It’s important to note that most mains-powered LED bulbs are not dimmable. Even among those labeled as dimmable, compatibility with dimmer types can vary. Many require dedicated trailing-edge dimmers to function correctly; using the wrong dimmer may lead to flickering, limited dimming range, or even premature failure. Always check the bulb’s specifications and pair it with a suitable dimmer for reliable, smooth performance.
Moreover, since LED bulbs and dimmers are mains-operated, even minor mishandling can lead to electric shock or fire hazards. Always choose compatible components and follow safety guidelines.
Trailing-edge dimmer design: The starting point
Building a trailing edge dimmer is not trivial; but it’s far from overcomplicated. Below is a conceptual block diagram for those poised at the starting line.
Figure 3 A conceptual block diagram highlights the key functional units coordinating trailing-edge dimming. Source: Author
From the block diagram above, several distinct functional stages interact with each other to perform the overall dimming functionality. In a trailing-edge dimmer circuit, the power supply delivers a stable low-voltage DC source to power control and switching stages. The zero-crossing (ZC) detector pinpoints the exact moment the AC waveform crosses zero volts, providing a timing reference for phase control.
Based on this, the timing control block calculates a delay to determine when to switch off the load during each half-cycle, shaping the trailing edge of the waveform. This delayed signal is then fed to the gate driver, which conditions it to reliably switch the power MOSFETs, the primary switching elements that interrupt current partway through each cycle, enabling smooth dimming with minimal noise and flicker.
So, for your trailing-edge dimmer, the selection of components involves careful consideration of their roles in the dimming process.
- Power supply (DC): This supply will power the control circuitry, including the digital logic and gate drivers. Its voltage and current rating must be sufficient to reliably operate these components, especially under varying load conditions.
- Zero-crossing (ZC) detector: This detector is fundamental for timing the dimming cycle. It senses when the AC waveform crosses zero, providing a synchronization point. The ZC detector should be fast and accurate to ensure precise dimming.
- Timing control: This element, often integrated with digital logic, dictates the duration for which the power MOSFET remains on during each AC half-cycle. For trailing-edge dimming, the gate pulse is enabled at the ZC signal and disabled after a specific ON-time pulse width.
- Digital logic: This is the brain of the dimmer, interpreting user input—for instance, from a potentiometer or button—and controlling the timing logic. It might involve simple logic gates or a microcontroller. One document mentions a triple 3-input NOR gate for control, indicating the use of basic digital logic.
- Gate drivers: Gate drivers are essential for efficiently switching power. They provide the necessary current and voltage levels to turn the MOSFETs on and off quickly, minimizing switching losses and heat generation. Proper selection ensures a clean gate drive signal.
- Power MOSFETs: The power MOSFET acts as the main switching element, controlling the power delivered to the load. It must be chosen based on the load’s voltage and current requirements, with low on-state resistance (Rdson) for efficiency and adequate heat dissipation capabilities. For AC dimming, devices capable of handling the AC voltage and current, such as specific MOSFETs or IGBTS designed for phase control, are necessary.
Recall that a trailing-edge dimmer operates using transistor switches that begin conducting at the start of each half sine wave. These switches remain active for a defined conduction angle, after which they turn off, effectively truncating the AC waveform delivered to the load.
This approach results in smoother current transitions. The electronic load benefits from the gentle rise of the sine wave, and once the switch turns off, any residual energy stored in inductive or capacitive components naturally dissipates to zero. This behavior contributes to quieter operation and improved compatibility with sensitive electronic loads.
Up next is the practical schematic of a trailing edge phase control rotary wall dimmer designed without a microcontroller and originally introduced by STMicroelectronics over a decade ago.
Although this elegant concept now calls for a few updates—mainly due to the unavailability of certain key components (fortunately, drop-in replacements exist)—it remains an invaluable design reference, at least to me. I could not have expressed it better myself, so here is the link to its full documentation.
Figure 4 Rotary wall dimmer circuit employs reverse-phase control to regulate mixed lighting loads. Source: STMicroelectronics
Happy dimming
In summary, there is not much more to add regarding trailing-edge dimmers for now. However, it’s worth noting that these dimmers can also be built using a microcontroller, which is especially useful for smart lighting systems. Compared to specialized dimmer ICs, microcontrollers provide more freedom to create custom dimming profiles, incorporate user interfaces, and connect with smart home technologies like Wi-Fi or Bluetooth.
That is all for now. But don’t let the dimming stop here.
Dive deeper into the fascinating world of trailing-edge dimmers. Experiment with different component combinations, explore their impact on dimming performance, and share your discoveries with us.
What will you create next? Let’s know your thoughts or any challenges you encounter as you build your own dimming solutions. Your insights could light the way for others.
Happy dimming!
T. K. Hareendran is a self-taught electronics enthusiast with a strong passion for innovative circuit design and hands-on technology. He develops both experimental and practical electronic projects, documenting and sharing his work to support fellow tinkerers and learners. Beyond the workbench, he dedicates time to technical writing and hardware evaluations to contribute meaningfully to the maker community.
Related Content
- Dimmer With A MOSFET
- Secrets of Analog Dimming
- A matter of light — PWM dimming
- How to design a dimming fluorescent electronic ballast
- DC/DC Converter Considerations for Smart Lighting Designs
The post Behind the curve: A practical look at trailing-edge dimmers appeared first on EDN.
