Types of Dimming for LED Lights|
Article - January 30, 2016 By LarsonElectronics.com
Types of Dimming for LED Lights
Dimming LEDs is a feature that is in demand by lighting specialists, professionals and mainstream consumers. There are several ways to dim LEDs; and choosing the correct method requires knowing the technical prerequisites of the desired dimming functions and luminary builds.
With this in mind, below expounds on different dimming options for LED lights.
Forward Phase Control (leading edge or triac dimming)
Forward phase control is considered to be the most widespread method for dimming traditional luminaries. During installation, this technique does not require re-wiring and can be used in setups that require retrofits. This allows the dimmer to work with existing switches and drivers, resulting in lower setup costs. According to the National Electrical Manufacturers Association (NEMA), over 150 million residential spaces rely on forward phase dimming devices. This method is commonly used for controlling incandescent lamps, such as A19 and PAR bulbs, and magnetic low-voltage (MLV) light sources.
Leading-edge dimmers work by controlling the energy going into the lamp load, as the input voltage increases after the zero crossing. During this process, a silicon component (SCR or triac) turns the AC waveform on, as it goes through the cycle. The point at which the waveform turns on varies, depending on the level of dimming set by the operator. One should carefully note that two-wire forward phase dimming controllers should not be used with Electronic Low Voltage (ELV) drivers, because it could lead to buzzing or flickering. Furthermore, triac dimmers are highly sensitive to electrical loads. Because of this, pairing this dimming method with LEDs may result in inconsistent performance, such as pop on and drop out. In order to meet the dimmer’s minimum load requirements, multiple lamps may be installed per controller.
Three-wire forward phase control supports a separate line-voltage wire that pushes the signal of the phase control separately from the power wirings. This dimming technique is ideal for fluorescent luminaries, and is less prone to electrical noise. Individuals may use this method (with compatible fixtures) to achieve dimming as low as one percent.
Reverse Phase Control (trailing edge dimming)
Reverse phase control dimming is usually incorporated with ELV dimmers. The process works by cutting the negative portion of the half-cycle. In reverse phase dimming, electronic components in the driver or ballast are powered before the dimming controls take effect. LEDs tend to work very well with trailing-edge dimming techniques, because most LED drivers rely on ELV transformers.
The benefits of reverse phase dimming include the following: quiet performance, longer lifespan (compared to forward phase dimming devices) and compatibility with low voltage luminaries. However, because trailing edge technology is relatively new, dimmers that incorporate the method tend to be more expensive. Re-wiring may also be needed during installation, because a neutral wire is required for the dimmer to function properly. Problems associated with this requirement could arise if the electrical back box does not have a neutral available. In Europe, trailing-edge dimming controls are favored due to their low electromagnetic interference (EMI) properties.
Pulse Width Modulation (PWM)
PWM involves rapidly turning an LED on and off to achieve dimming. For example, at 100 percent output, the LED remains constantly on. At 50 percent output, the light is only on half of the time, resembling a digital signal with a 50 percent duty cycle- or an ideal square wave (it is impossible to generate a perfect square wave in live applications). At 75 percent duty cycle, the digital signal is more active in the high state, while spending less time in the low state. It is important to consider that constant-voltage LED drivers almost always utilize PWM methods.
This technique is effective for maintaining the light’s characteristics, such as color temperature, during dimmed output. Because of this, PWM is suitable for strict LED dimming requirements below 40 percent. Additionally, because of the dimming control’s ability to maintain consistent color throughout the process, lighting specialists may use PWM for color mixing applications.
For example, when applied to RGB LEDs, full light output (by equally powering all three colors in the fixture) will result in white light. To get teal, blue and green may be set at full light output while turning off red. To generate an orange light, red must be set to full output, blue fully off and green at 50 percent duty cycle. Two-wire PWM dimming is mostly implemented in the car manufacturing sector to support interior lighting systems.
Constant Current Reduction (CCR)
CCR dimming is achieved by increasing or decreasing the current being applied to the LED. When the current is increased, light output is also increased and vice versa. This technique can be implemented to decrease risks related to flickering, and may help improve system efficacy. Applications that require deep dimming typically do not use CCR.
This method is effective for the following environments:
• Outdoor and damp locations
• Locations with strict EMI and dimming standards, such as medical facilities
• Locations with high activities and rotary machines
Complex lighting configurations where lengthy wires are used from the driver to the fixture, may also incorporate CCR dimming. By comparison, PWM dimming is not efficient for such configurations, because long wires may disrupt the precise dimming process (i.e., fast rise and fall times).