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How to Dim LED With Mainstream Dimmers?

Author: Apogeeweb
Date: 29 Jun 2021
 492
how to dim leds

Introduction

In the lighting industry, people often have a misunderstanding about dimming LED lights. And the reality is that the application of LED light source dimming technology in engineering is often unsatisfactory. Why is this the case? Is the LED light source dimming technology immature, or the technology is difficult to master? So this article analyzes the LED dimmer to help readers to fully understand and master it.

Dimming all kinds of LEDs?

Catalog

Introduction

Ⅰ Dimming LED Lights

1.1 What is Dimming?

1.2 LED Dimming Circuit Example

1.3 LED Dimming Using SCR

Ⅱ Dimming LED Using PWM

2.1 LED Dimming Current

2.2 LED PWM Dimmer

2.3 LED PWM Dimming Advantages

2.4 LED PWM Dimming Problems

2.5 DALI PWM Dimmer Introduction

Ⅲ Main LED Dimmers Comparisons


Ⅰ Dimming LED Lights

1.1 What is Dimming?

LED dimmer switch is an electrical device that changes the luminous flux of the light source in the lighting device and adjusts the illuminance level. The purpose of the dimmer is to adjust the different brightness of the light. By reducing or increasing the rms voltage, the light output of different intensities produced by the average power lamp is promoted. Although variable voltage devices can be used for various purposes, this regulation is aimed at controlling lighting. 
Regarding the LED dimmer switch, we must first understand the volt-ampere characteristics of the LED. That is, the characteristics of the current flowing through the LED PN junction with voltage. Generally, the reverse characteristic curve changes steeply. When the voltage exceeds a certain threshold, the current will rise exponentially, thereby breaking down the LED PN junction. The forward voltage of the LED is also determined by its forward current. It can be seen from the figure that the change of the forward current will cause the corresponding change of the forward voltage, to be precise, the decrease of the forward current will also cause the decrease of the forward voltage. Therefore, when the current is lowered, the voltage of the LED will also decrease, which will change the relationship between the power supply voltage and the load voltage.

diode volt ampere characteristics

Figure 1. Diode Volt-Ampere Characteristics Curve

Therefore, from the volt-ampere characteristics of the LED, we can know that the dimming of the light source cannot be achieved simply by reducing the input voltage or input current of the LED. In addition, the waveform of the sine wave of the LED is different from the waveform of the incandescent lamp, so it cannot simply change its conduction angle to achieve the purpose of dimming.
LED dimming methods can be divided into analog dimming and digital dimming. Analog dimming is to achieve dimming by changing the current in the LED loop. The power supply voltage remains unchanged, and the current in the loop is changed by changing the resistance value to achieve the effect of changing the brightness of the LED. Many analog dimming is an extension of this method. Its advantage is that the current can be continuous, but the range of adjustable current is often limited by hardware, and there are few adjustment gears. This method is not ideal for high-precision lighting equipment.
Digital dimming, also known as PWM (Pulse Width Modulation) dimming, uses PWM waves to turn on and off the LED to change the on-time of the forward current to achieve the effect of brightness adjustment. This method is based on the fact that the human eye is not sensitive enough to the brightness flicker. If the frequency of brightness and darkness exceeds 100Hz, the average brightness is seen by the human eye, not the LED flickering. PWM adjusts the brightness by adjusting the ratio of light and dark time. In a PWM cycle, because the perceived brightness of human eyes to flicker is a cumulative process, that is, the brighter time accounts for the greater the proportion of the entire cycle. The longer the time, the brighter the human eye feels.

LED Dimming Tech

1.2 LED Dimming Circuit Example

For example, in an LED lamp with an input of 24V, 8 1W high-power LEDs are connected in series. When the forward current is 350mA, the forward voltage of each LED is 3.3V, then 8 pieces in series is 26.4V, so a constant current source greater than 24V should be used. However, in order to dimming, the current is reduced to 100mA. At this time, the forward voltage is only 2.8V, and 8 pieces are connected in series to 22.4V. The load voltage becomes lower than the input voltage, so that a constant current source larger than 24V cannot work, and finally the LED will flicker.
In this case, you may choose a step-down (wide voltage) constant current source, such as a 10V-30V constant current source for dimming. However, if this type is adjusted to a low forward voltage, the LED load current will also become very low. So the step-down ratio is very large, beyond the normal working range of constant current source, which will make LED unable to work and cause flicker. In addition, LED works at low brightness for a long time, which will reduce its efficiency and increase the temperature rise. Because the efficiency of the step-down constant current source is related to the voltage ratio, the larger the voltage drop ratio, the lower the efficiency. And greater power loss on the chip will damage the life of the constant current source and the LED light source.

1.3 LED Dimming Using SCR

Ordinary incandescent lamps and halogen lamps usually use thyristors for dimming. Because they are pure resistance devices, and do not require the input voltage to be a sine wave. Their current waveform is always the same as the voltage waveform, no matter how the voltage waveform deviates from the sine wave, changing the effective value of the input voltage will dim the LED light .
However, the adjustment of LED light source by thyristor dimming will cause unexpected problems, that is, the LC filter at the input will cause the thyristor to oscillate. This oscillation is indifferent to the incandescent lamp, human eyes can't see it at all because of thermal inertia. However, this dimming method will cause the driving power of LED produce audio noise and flicker. It will also destroy the waveform of the sine wave, thereby reducing its power factor value (usually lower than 0.5), which greatly reduces the system efficiency of the LED. Moreover, the thyristor dimming waveform increases the harmonic coefficient, and the non-sinusoidal waveform will cause serious electromagnetic interference on the line to pollute the power grid.

Dimming LED Lights with PWM

Ⅱ Dimming LED Using PWM

2.1 LED Dimming Current

Here, you may ask: Lower voltage or current or thyristor dimming methods are not suitable for LED light source dimming, so what is the most suitable method?
Is it an analog (0-10V) dimming method? May be not. Analog dimming faces a severe challenge, which is the output current accuracy. Almost every LED driver needs some kind of series resistance to distinguish the current, and the tolerance, offset and delay in the analog (0-10V) dimming drive cause a relatively fixed error, which will reduce the accuracy of the output current, and the final output current cannot be specified, controlled or guaranteed. Therefore, to ensure the dimming effect of the LED light source, one of the important rules is to reduce the output current error and improve the current accuracy in a closed loop system.

2.2 LED PWM Dimmer

The PWM dimming method can solve the above problems very well. Because diode characteristics, LED can realize fast switching, and its allowable switching speed can be as high as microseconds or more. Therefore, as long as the power supply is changed to a pulse constant current source, the brightness can be changed by PWM. This PWM dimming. This method is like a sluice that opens and closes in microseconds or more. The switching frequency of it is so fast that humans can’t recognize the state of its opening with the naked eye. As a result, people can only identify the speed of its switching frequency by the amount of water downstream. In addition, because the sluice changes the duty cycle of the output water flow (effective water flow), it does not change the instantaneous water pressure and flow rate, so the opening and closing action of the sluice gate will not affect the hydropower generation. The amount of water flowing down and power generation are just changed. Therefore, the PWM dimming method does not change the instantaneous voltage and current of the input LED PN junction, but changes the duty cycle of the output current to change LED brightness.

2.3 LED PWM Dimming Advantages

1) There will not be any LED chromatogram shift, because the LED always works between the full amplitude current and 0.
2) It has a very high dimming accuracy, because the pulse waveform can be controlled to a high precision.
3) Even if the light is dimmed in a wide range, there will be no flicker. Because it will not change the working conditions of the constant current source (boost ratio or step-down ratio), problems such as overheating are less to occur.
4) It can be combined with digital (DALI/DSI/DMX 512) control technology for control, because the digital control signal can easily be transformed into a PWM signal.

2.4 LED PWM Dimming Problems

1) Because the LED is in a fast switching state, if the working frequency is very low, the human eye will feel flicker. In order to make full use of the residual visual phenomenon of the human eyes, its operating frequency should be higher than 100Hz, preferably 200Hz.
2) Eliminate the howling caused by dimming. Although the human eye can't detect it above 200Hz, it is within the range of human hearing until 20kHz. At this time, it is possible to hear the slightest voice. There are two ways to solve this problem: One is to increase the switching frequency above 20kHz, out of the range of human hearing, another is to find out the sound-producing device and deal with it.
At present, some manufacturers have solved the above problems well. A good LED light source dimming technology needs a good LED control signal technology to match and cooperate in order to become an effective, stable and reliable system. For example, the LED PWM dimming method has the advantage that the digital control signal can easily be converted into a PWM signal. At the same time, in the digital control signal of lighting, DALI (Digital Addressable Lighting Interface) has the unparalleled superiority of other lighting digital control methods, and it is also the mainstream of the current digital control application in the lighting industry. Therefore, the matching of PWM dimming mode and DALI takes into account their respective advantages, where PWM dimming technology solves the final dimming problem of LED light sources, and DALI solves the control, feedback and networking of each LED light.

LED PWM Dimming Circuit

Figure 2. LED PWM Dimming Circuit

2.5 DALI PWM Dimmer Introduction

The biggest feature of DALI technology is that each lamp has an independent address. Through the DALI system software, a single lamp or any lamp set can be accurately dimmed and switched, regardless of whether the lamps are on strong current loop or not. That is to say, the lighting control has nothing to do with the strong current circuit. The DALI system software can independently address single or multiple lamps on the same strong current circuit or different circuits, to achieve individual control and arbitrary grouping set. This concept brings great flexibility to lighting control, which can meet different LED lighting requirements. Even after installation, they can still modify the control requirements at will, without having to do anything to the wiring.
The following are the application advantages of PWM dimming method combined with DALI.
1) The design is simple and easy to implement.
In the design, as long as they are connected to each other through the digital signal interface, they are connected in parallel to the 2-core control line. All design process can be programmed by computer software during installation and debugging, which not only saves design costs, but also improve working efficiency.
2) Simple and economical installation
The DALI control line has no special requirements for the wire and no polarity requirements during installation. It only requires the main power line to be separated from the control line. The control line does not need to be shielded. When the current on the control line is 250mA and the line is 300 meters long, the drop does not exceed 2V. The control line and the power line can be parallel, no need to bury the line separately. The compact design of the control components does not require a special control cabinet, so installation is simple and economical.
3) Simple and convenient operation
The PWM LED driver with DALI control can automatically handle filament preheating, ignition, dimming, switching, fault detection and other functions. The user interface is very friendly. Users can operate and control without deep understanding, such as sending a change. According to the command of the scene, each relevant LED driver calculates the dimming rate according to the difference between the current brightness and the required installation brightness to achieve that all the LED light sources are synchronized to the required scene brightness.
4) Accurate and reliable control
DALI is a digital signal, which is different from an analog signal. The signal of 1010 can realize disturbance-free control, and will not distort the control signal due to long-distance voltage drop. Therefore, even if the DALI digital signal control line and the strong wire are in the same line and tube, it will not be disturbed. The DALI signal is two-way transmission, which not only transmits control commands forward, but also feeds back the information of the LED driver's status, fault information, switch, and actual brightness value to the system.
5) Wide range of applications
Nowadays, DALI interface is not only used for fluorescent lamp ballast dimming, various electronic transformers for halogen lamps, electronic ballasts for gas discharge lamps. DALI technology also employed in wide range of LED light control makes it more and more widely.

 

Ⅲ Main LED Dimmers Comparisons

1) SCR Dimming

Figure 3. SCR Dimming Circuit Diagram

✅Advantages: It has the advantages of high adjustment accuracy, small size, light weight, easy remote control, etc., which occupies a leading position in the market.
❎Disadvantages: The front-cut LED dimmer is prone to generate noise, so it is not recommended for high-demand occasions. The minimum load will vary depending on the LED dimmer and light source. It is necessary to consider derating to adapt to the spike caused by the driver. The typical derating percentage should be 25%-30% of the maximum rated load of the dimmer circuit.

 

2) CMOS Dimming

CMOS Dimming Circuit Diagram

Figure 4. CMOS Dimming Circuit Diagram

✅Advantages: There is no minimum load requirement, so that better performance can be achieved on a single LED lighting device or a very small load.
❎Disadvantages: High cost, complicated dimming circuits, lack of high-power products, and poor stability.

 

3) 0-10V Dimming

0-10V Dimming Circuit Diagram

Figure 5. 0-10V Dimming Circuit Diagram

✅Advantages: Simple application, good compatibility, high precision, better dimming effect than phase-cut dimming.
❎Disadvantages: Need to add additional control lines and controllers. The dimming effect is related to the wire diameter, cable material, power current, and power supply quantity of 0-10V dimming.

 

4) DALI Dimming

DALI Dimming Circuit Diagram

Figure 6. DALI Dimming Circuit Diagram

✅Advantages: Accurate and smooth dimming, two-way communication, and strong anti-interference ability, mainly used in single lamp control.
❎Disadvantages: Like 0-10V products, additional control circuits and controllers need to be added.

 

5) DMX512 Dimming

DMX512 Dimming Circuit Diagram

Figure 7. DMX512 Dimming Circuit Diagram

✅Advantages: Powerful control functions bring rich lighting effects to architectural lighting, night lighting, studios and variety shows.
❎Disadvantages: Special wiring layout and types are required, and certain programming is required to set the basic colors and scenes, which is more costly for later maintenance. The ideal transmission distance of DMX signal is less than 200 meters. And meanwhile, in actual use, the signal is greatly interfered by the outside world.

 

6) SLC and Ready2mains Dimming

ready2mains dimmer signal

Figure 8. LED Dimming via Ready2mains

✅Advantages: The digital dimming signal is transmitted through the AC wire, without additional signal wires and wiring. Digital signal transmission has good anti-interference performance and excellent dimming effect.
❎Disadvantages: At present, there are relatively few products using this type of digital dimming technology, so there are relatively few compatible products.

 

Frequently Asked Questions about LED Dimmer

1. What are LED dimmers?
An LED dimmer is the term for a device that performs a dimming control operation within such an LED lighting device. LEDs react instantaneously to alterations in power input, making solid state lighting especially suitable for dimming scenarios.

 

2. Why do my LED dimmer lights flicker?
LED bulb flickering can be traced in almost every instance to a non-compatible dimmer switch in the lighting circuit. ... LED bulbs don't have glowing filaments. When the dimmer switch goes off and on many times per second, the LED bulb becomes a flickering strobe light.

 

3. Do you need a special dimmer for LED lights?
Use an LED Dimmer switch
A standard dimmer switch cannot be used with an LED light as you will never be able to dim the LED light either completely or not very well. LED lights need their own special electronic dimmer switch to have a fully functioning and dimming light.

 

4. How do LED dimmer switches work?
In the case of PWM, dimmable LEDs work by creating a dimming effect. Unlike traditional lighting such as incandescent, dimmable LED bulbs don't rely on voltage to dictate their level of brightness. Instead, they essentially rely on a cycle of being on and off.

 

5. What is the best dimmer switch for LED lights?
Best Overall: Lutron Toggler Single-Pole/3-Way Light Dimmer.
Best Budget: GE Slide Dimmer Rocker Wall Switch, Single Pole.
Best Smart: Kasa Smart Dimmer Switch HS220.
Best for Bedrooms: Lutron Maestro LED+ Dimmer Switch, Single-Pole or Multi-Location.
Best for LED: Lutron Diva LED+ Dimmer Switch, Single-Pole or 3-Way.

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