How to Choose a Dimmable LED ?

Dimming for LED lights?

Steve Law

22/12/2014

(source: http://stevelawblog.wordpress.com/2014/12/22/dimming-for-led-lights/)

The rising cost of electricity is certainly driving consumers, retailers and business owners to seek for energy efficient lighting technologies, and dimmable LED is consider as one of the most efficient choices. However, as people may know, there are a huge amount of different dimmable LEDs in the Australian market, and the dimming methodology for all these products are varied. When facing this massive amount of options, consumers are frequently encounter such question: Which dimmable LED should I choose?

How LED lights be dimmed?

Before answering this question, it is necessary to clarify that most of LED lamps are powered by electronic drivers, as they are more suitable for DC device. In general, there are mainly four protocols that are using in Australia:

1.     Phase-cut dimming

This method dims the output illuminance through cutting the phase of the AC current, which will decrease the power input to the light. There are mainly four types of phase-cut dimming methods:

a.    Leading Edge dimming:

This can alternatively be called as “triac dimming” or “incandescent dimming”. It turns off the sinusoidal signal of the electrical current at the front of the AC input (see figure 1). It fits the traditional incandescent lamps or magnetic low voltage transformer. Nevertheless, it can cause buzzing noise when dimmed, and when it is used for an LED lamp, it can produce a high inrush current to its electronic component, and eventually the entire luminaire will be damaged. [1]

Figure 1 leading edge dimming curve

Source: http://www.leapfroglighting.com/trailing-edge-or-leading-edge-led-dimming/

b.    Trailing Edge

This is also called “electronic dimming”, which can be considered as a reverse form of leading edge dimming, as it turns of the end of the AC input waveform (see figure 2). It can be used for electronic driver and low voltage transformers. In comparison to leading edge dimming, it will not exert high inrush current, while not causing buzzing sound when dimmed.[2]

Figure 2 Trailing edge dimming curve

Source: http://www.leapfroglighting.com/trailing-edge-or-leading-edge-led-dimming/

Summary for phase-cut dimming

After comparing these two Phase-cut dimming methodologies, it is clear that for most LED lamps, trailing edge dimming is a better option comparing to leading edge dimming. However, because LED retrofits are mostly for the existing incandescent or halogen fixtures, which makes many manufacturers designed their LED lamps to be suitable for leading edge dimmers. So when you choosing a dimmable LED, it is better to clarify the capable diming method for the product before getting a dimmer for it, or you can use a “Universal” dimmer, as it is suitable for both leading edge and trailing edge dimming.

2.     0-10V analogue:

As one of the earliest and simplest lighting control, 0-10V analogue dimming is used for early fluorescent dimming and still being used today. Nowadays, some of the LED fixture is also designed for this dimming methodology due to its simplicity. This 0-10V dimmer is connected to the circuit in parallel, and dims LED simply by adjusting the input to the lamp via scaling the DC voltage at the dimmer side. Theoretically, the LED will have 100% output when the dimmer is scaled at 10V; while the dimmer is at 0V, the LED will have 0% output, or be turned “OFF”. However, in many cases, the light can only be dimmed down to 10% due to the limitation of the power supply, and a switch or relay is needed to completely turn the light off. [3] Moreover, for a 0-10V dimming system, each control channel requires one wire, and the excessive amount of cabling can cause a considerable voltage drop that lowered the energy efficiency. [4]

3.     Pulse Wave Modulation (PWM) Dimming

In a technical perspective, PWM dimming can be categorised as a type of Digital dimming. Instead of adjusting the DC current, PWM dimming achieve the same effect by varying the duty cycle of the constant current. It switches the input current of the LED lamp at a high frequency between 0A and the rated current, and this modification of on-off rate of LED will result as the change of output percentage. This suitable for LEDs that need to maintain colour consistency when dimmed down to lower than 40%, and lighting applications with mixed colour control. Nevertheless, in order to be capable with a PWM dimmer, the power supply should be able to run the LED at a high enough frequency, and this will greatly increase the technical requirement of an LED driver[5].

4.     Digital Dimmings

(1)  DSI and DALI Control

The reason that I put these two dimming protocol together is that they are both developed under the working principle of 0-10V analogue lighting control.

DSI( “Digital Series Interface”) dimming technology is uniquely owned by Tridonic®, and it has been used for the control of LED interface or electronic Fluorescent ballast. This is also the precursor to DALI dimming system. [6]

On the other hand, although DALI is developed based on DSI protocol, it is a non-propertied technology, so it is now the most common type of commercial lighting control system, which is available from many different suppliers. Each DALI device has a “non-violate” memory, which contains its own settings. This allows DALI to operate without a central control point. [7]

(2)  Switch dimming

This is a stand-along digital protocol. It is a cheap solution that is commonly used for residential and hospitality systems. No control system is required for this type of dimmer.

(3)  DMX Dimmer

DMX (or “DMX512”) is the digital control system that is designed for RGB and RGBW colour change lighting system.Different from other digital systems, it allows complicated programming to communicate DMX devices through a ‘Data Bus’. It sends signal in an 8/16bit code package, which can instruct fixtures on everything from dimming to movement positions. All this feature makes it an ideal option for stage lighting and effect industry[8].

C-Bus……?

Although there are lots of people asking questions about “C-bus dimming”, however it is not a dimming protocol. This is actually a building control system that is owned by Clipsal® Australia. Due to its complexity, I will talk about it in another separate article.

Summary

To sum up, the conventional dimming method can be summarised as table 4 below. Hope this can give you a bit of idea when you choosing a dimmable LED.

Category Dimming

Protocol

Control system needed? (Y/N) Capability
Analogue Leading Edge N Incandescent Lamp; Fluorescent lamp with Iron-core ballast; LED with Leading edge dimming driver
Traling Edge N Halogen lamp;Fluorescent lamp with electronic Ballast;  Dimmable LED with electronic driver
0-10V dimming N halogen lamp;Fluorescent lamp with 0-10V Ballast;  Dimmable LED with 0-10V driver
Digital PWM Dimming N LED driver with high enough output frequency
Switch dimming N most of the single fixture dimming
DSI Y LED or other device with dimmable Drivers with DSI or 0-10V functions
DALI Y Most of major 0-10V ballast. LED or other device with dimmable Drivers with  DALI ,DSI or 0-10V functions
DMX Y RGB and RGBW LED control; Stage and effect lighting

Table 4 summary for dimming methods

p.s.: One of the discussion of the C-bus community forum has been cited for the information above:

http://www.cbusforums.com/forums/showthread.php?t=2626

Reference:


[1] http://www.leapfroglighting.com/trailing-edge-or-leading-edge-led-dimming/

[2] http://www.leapfroglighting.com/trailing-edge-or-leading-edge-led-dimming/

[3] http://www.lightology.com/index.php?module=tools_faq_0_10v_control

[4] http://www.epanorama.net/documents/lights/0to10v.html

[5] http://www.lutron.com/TechnicalDocumentLibrary/048360a_PWM_vs_CCR_LED_App_Note.pdf

[6] http://lightproject.com.au/the-basics-dimming

[7] http://lightproject.com.au/the-basics-dimming

[8] http://lightproject.com.au/the-basics-dimming


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