LED Dimming Key Principles and Selection Guide

Imagine setting the mood for a romantic dinner with soft lighting or creating an immersive home theater experience by dimming the lights. Dimming technology makes these scenarios possible. But how do LED lights achieve dimming? What types of dimmers should you choose? This article explores the principles and methods of LED dimming while providing practical guidance for selecting the right dimming solutions.

Dimming refers to reducing the light output of a fixture, typically measured in lumens (lm), also known as "luminous flux." When a light is dimmed, its lumen output decreases accordingly. In simple terms, dimming is the process of controlling a light's brightness.

The answer is yes—theoretically, all LED lights can be dimmed. However, the practicality depends on the type of driver controlling the LED. To determine whether a specific LED fixture is dimmable, it's essential to understand the relationship between the LED and its driver.

All LED lights require a driver, an electronic device that converts mains AC power (typically 230V, 50Hz) into low-voltage DC power (usually 12V or 24V) to operate the LED. This is because LEDs cannot use mains power directly and instead require low-voltage DC current.

In commercial lighting fixtures—such as LED panel lights, downlights, and spotlights—the driver is often separate from the fixture itself. This allows installers to access the driver and, if compatible, connect dimming control wires alongside the power supply.

For LED bulbs, the driver is integrated and inaccessible. Since the LED and driver form a sealed unit, no dimming control wires can be connected.

To answer the question, "Can this LED light be dimmed, and how?" you must first identify whether the driver is separate and accessible or integrated with the LED.

If the driver is separate or accessible within the fixture, the best dimming method is to use a dimmable driver. This approach is most common in commercial and industrial lighting installations.

In commercial fixtures, the LED driver is typically standalone, allowing installers to replace it with a dimmable version if needed. A dimmable driver serves two functions: acting as both a driver (converting AC to DC) and a dimmer (adjusting the power delivered to the LED).

Dimmable drivers reduce power to the LED using two primary methods:

• Pulse Width Modulation (PWM): The driver rapidly switches the LED on and off. The longer the "on" pulses and shorter the "off" pulses, the brighter the LED appears, and vice versa. If this occurs at a frequency above 200Hz, the human eye perceives a steady brightness level.
• Amplitude Modulation (AM): The driver adjusts the output current to the LED. While this eliminates flickering, some LEDs may exhibit slight color shifts at low current levels.

Some drivers combine PWM and AM for optimal performance.

Several methods can be used to control dimmable drivers, each requiring a continuous power supply and a control signal. The most common methods include:

• DALI (Digital Addressable Lighting Interface): The most widely used communication protocol in commercial lighting control, DALI signals are transmitted via two additional wires alongside the power supply.
• 1-10V Analog Control: An analog communication protocol that also uses two additional wires.
• Switch Dimming, Touch Dimming, or Push Dimming: These methods use an additional wire to supply power to a dedicated terminal on the driver. A momentary (non-latching) switch controls the dimming function—quick presses turn the light on/off, while holding the switch brightens or dims the light.
• Corridor Function: A variation of switch dimming that uses a latching switch, such as a motion sensor or timer, instead of a momentary switch.
• DMX (Digital Multiplex): Originally developed for theatrical lighting, DMX is now used for color-changing applications like facade lighting.
• Wireless Control: Technologies like Zigbee and Bluetooth enable wireless communication with LED drivers.

For most LED bulbs and many residential fixtures, the driver is sealed inside, making it impossible to send a dimming control signal. In these cases, dimming must be achieved by installing an external dimmer on the power supply line.

This approach requires the driver to be designed for compatibility with external dimmers. Such drivers are labeled as "dimmable," but even dimmable LEDs may not work with all dimmer types. Always check the manufacturer's recommendations for compatible dimmers.

The most common LED dimmers are phase-cut dimmers, which work by switching the AC power on and off twice per cycle, reducing the energy delivered to the LED. There are two types:

• Leading-Edge Phase-Cut Dimmers (Triac/Forward Phase): These dimmers cut off the current at the zero-crossing point and turn it back on later in the cycle. The longer the "off" period, the dimmer the light.
• Trailing-Edge Phase-Cut Dimmers (Reverse Phase): These dimmers turn the current on at the zero-crossing point and cut it off later in the cycle, typically using an IGBT (Insulated Gate Bipolar Transistor).

To select the best dimmer for a specific LED bulb or fixture, consult the manufacturer's recommendations. If no list is provided, follow these steps:

• Calculate the total wattage of all bulbs/fixtures on the circuit and choose a dimmer with a compatible load range.
• Opt for a trailing-edge dimmer or one advertised as "LED-compatible," as these are more likely to work with dimmable LEDs.