When we talk about dimming LED lighting, many articles mix the dimming control and dimming methods, such as combining PWM dimming and other dimming control methods. These confusing articles are very easy to mislead people, so I plan to completely explain the dimming method(working principle) and dimming control method.
The light-emitting principle of LED
LED (Light Emitting Diode) is a solid-state semiconductor device that can convert electrical energy into visible light. It is composed of P-type semiconductor conductors. When current acts on this chip through a wire, electrons are pushed to the P region, and the electrons in the N-type semiconductor and the holes in the P-type semiconductor collide violently in the light-emitting layer to produce photons, and then energy is emitted in the form of photons. This is the principle of LED light emission.
2 types of dimming methods about LED(determined by LED's Volt-ampere characteristics)
DC dimming ( power dimming / direct drive dimming)
DC dimming controls the brightness by adjusting the current of the LED lamp or changes the brightness by adjusting the circuit power (power = voltage x current), so changing the voltage or current can change the LED lighting brightness.
1. Why can adjusting the current of the LED adjust its light brightness?
An LED is a diode that can emit light. It relies on electrons to flow to the P region, where electrons collide with holes and then emit energy in the form of photons. Therefore, its light-emitting principle determines that the brightness of the LED is proportional to its forward current. Most LEDs will give a rated current, but this does not mean that the LED can only work at these rated currents.
For example, a 1-watt and 3-watt LED are the same model. If the current increases from 350mA to 700mA, the power increases from 1W to 3W, so this LED can work at any value between 350 and 700mA.
Therefore, adjusting the current passing through the LED can adjust its light intensity.
2. How to adjust the LED's current? Directly adjust the voltage?
Volt-ampere characteristics refer to the relationship between the voltage applied to both ends of a conductor and the current flowing through the conductor.
Because according to the diode volt-ampere characteristics, we will find that when the forward voltage is added above the turn-on voltage, a slight voltage fluctuation will cause the current flow through the light-emitting diode LED to fluctuate exponentially.
The nonlinearity of the LED volt-ampere characteristics means that a small voltage change will cause a large current change. From the volt-ampere characteristics, a 10% change in the power supply voltage will cause several times the change in the forward current.
It is not that the LED voltage and current are fixed. In fact, the volt-ampere characteristics of LEDs are not fixed but vary with temperature.
Assuming that a 3.3V constant voltage source is used to work 20mA at room temperature, and when the temperature rises to 85 degrees, the current will increase to 35-37mA, and its brightness will not increase. The increase in current will only make its temperature rise higher, increasing the light decay and reducing life.
3. Why does LED use a wide voltage constant current dimming power supply?
From the LED volt-ampere characteristics, we know that LED cannot be dimmed by simply adjusting the voltage and current. Generally, a wide voltage constant current dimming power supply is used. The constant current source means that the power supply determines the current size of the LED, and the wide voltage means that the power supply will automatically match the appropriate power supply voltage according to the LED volt-ampere characteristics.
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, so the 8 series is 26.4V, so a 26.4V constant current source should be used. When dimming, the current is reduced to 100mA. Currently, the forward voltage is only 2.8V, and the 8 series is 22.4V, so the load voltage should also be lower.
4. Disadvantage
If the buck-type (wide voltage) constant current source is adjusted to a low forward voltage, the LED's load current will also become very low. Hence, the step-down ratio is very large, which may exceed the normal working range of this buck-type (wide voltage) constant current source, making it unable to work and flickering.
In addition, if the buck-type (wide voltage) constant current source works at low brightness for a long time, its efficiency will be reduced, and the temperature rise will increase. It will not work because the efficiency of the buck-type (wide voltage) constant current source is related to the step-down ratio. The larger the step-down ratio, the lower the efficiency and the greater the power consumption on the chip, which will damage the life of the constant current source and the LED light source.
PWM dimming(determined by LED's fast switching)
1. What is PWM dimming?
PWM dimming achieves the dimming effect by controlling LED lamps' on/off time and duty cycle.
LED is a diode that can achieve fast switching. Its allowable switching speed can be as high as microseconds, unmatched by any light-emitting device (so traditional lamps cannot be dimmed if they are too slow to switch on and off). Therefore, as long as the power supply is changed to a pulse-constant current source, the brightness can be changed by changing the pulse width. This method is called pulse width modulation (PWM) dimming.
2. How does PWM dimming work?
PWM dimming does not adjust the brightness by changing the power but by alternating the screen on and off. When PWM dimming is on, it does not emit light continuously but keeps on lighting and turning off the screen. When alternating on and off is fast enough, the naked eye will think the LED is always on.
During the process of lighting and turning off, the longer the off state lasts, the lower the brightness of the LED will be to the naked eye. The longer the lighting time, the shorter the off time, and the brighter the LED will be. Most of them are fixed frequency, and changing the brightness is to adjust the duty cycle.
3. Advantages of LED PWM (pulse width modulation) dimming:
- No LED color spectrum offset will be generated because the LED always works between full amplitude current and 0.
- It has extremely high dimming accuracy because the pulse waveform can be completely controlled to very high accuracy, so it is easy to achieve an accuracy of one ten-thousandth.
- Even if dimming is performed over a large range, flickering will not occur. Because the working conditions of the constant current source (boost ratio or buck ratio) will not be changed, overheating and other problems are even less likely to occur.
- It can be combined with digital (DALI/DSI/DMX 512) control technology for control because the digital control signal can be easily converted into a PWM signal.
4. Disadvantage of the PWM dimming method
The choice of pulse frequency: Because the LED is in a fast-switching state, if the operating frequency is very low, the human eye will feel flickering. To fully use the visual residual phenomenon of the human eye, its operating frequency should be higher than 100Hz, preferably 200Hz.
- The whistling sound caused by dimming is not perceptible to the human eye above 200Hz, but it is within the range of human hearing up to 20kHz. At this time, you may hear a slight hissing sound.
LED lighting dimming control methods
Based on the characteristics of LED, there are two dimming methods. LED lightings are dimmed by LED power supply. LED power supply can be understood as a part of the LED lamp itself. Just like the lighting and extinguishing of the lamp requires a switch control, the lamp also needs to be dimmed according to the instructions received by the power supply. The different uses of this instruction form various dimming control methods for the lamp. Let's talk about the six common LED lighting equipment control methods on the market:
1. Leading edge phase cutting (FPC), triac dimming
Leading-edge dimming uses a thyristor circuit. Starting from AC phase 0, the input voltage is chopped, and it is input only when the thyristor is turned on.
The working principle is to adjust the conduction angle of each half-wave of the AC to change the sine waveform, thereby changing the effective value of the AC current to achieve the purpose of dimming. Leading-edge phase control dimmers generally use thyristors as switching devices, so they are also called triac dimmers.
Advantages: Low dimming cost, compatible with existing lines, no need for rewiring (other dimming methods require signal lines).
Disadvantages: Many LED triac dimming drivers find it difficult to fully match dimmers, so the dimming process will not be smooth, cannot be turned off, and will flicker slightly.
2. Trailing edge phase cutting (RPC) MOS tube dimming
The trailing edge phase-cut control dimmer is very similar to the leading edge phase-cut; it is just generally made of field effect transistor (FET) or insulated gate bipolar transistor (IGBT) devices. The trailing edge phase-cut dimmer generally uses MOSFET as the switching device, so it is also called a MOSFET dimmer, commonly known as an "MOS tube."
MOSFET is a fully controlled switch that can be controlled to open or close, so there is no phenomenon that the thyristor dimmer cannot be completely turned off.
Advantage: A MOSFET dimming circuit is more suitable for capacitive load dimming than a thyristor. There is no minimum load requirement, and it can achieve better performance on a single lighting device or a very small load. MOS tubes are rarely used in dimming systems and are generally only made into knob-type single-lamp dimming switches.
Disadvantage: The MOS tube dimming method has not been developed due to its high cost, relatively complex dimming circuit, and difficulty in achieving stability.
So, the triac dimmer still occupies most of the dimming system market.
3. 1-10V dimming
The 1-10V dimming device has two independent circuits: a normal voltage circuit, which turns on or off the power to the lighting equipment, and a 1-10V signal circuit, which provides a reference voltage to tell the lighting equipment the dimming level.
0-10V dimming controllers were previously commonly used for dimming control of fluorescent lamps. Now, because a constant power supply is added to the LED driver module and there are dedicated control circuits, 0-10V dimmers can also support many LED lighting lamps.
However, the application's disadvantages are also very obvious. The low-voltage control signal requires an additional set of lines, which greatly increases the construction requirements.
4. DALI (digital addressable lighting interface)
DALI is the acronym for "Digital Addressable Lighting Interface", which means "digital addressable lighting interface". It is an internationally open lighting control communication protocol IEC62386, which specifies the digital communication control method between electronic ballasts, control units, and various sensors. In other words, DALI is a protocol specifically used for lighting control, which can control each lamp individually, realize brightness, color temperature, color, etc., group control of lamps, set different scenes, etc.
a. The origin of DALI
In the 1990s, Europe began to develop and research digital fluorescent lighting control systems. Major European electronic ballast manufacturers (such as Halvar, Hüco, Philips, Osram, Tridonic, Trilux, etc.) have joined the formulation of DALI standards. The DALI definition standard aims to establish a simple lighting system with a clear structure;
It is used for intelligent and high-performance lighting management in indoor areas, and the controlled object is clearly the ballast. It can be interfaced with a building management system (BMS), such as the KNX system. DALI is usually used as a small part of building electrical control. For example, it can be integrated into the KNX system through the KNX-DALI gateway to achieve a wide range of lighting control.
b. DALI network
The DALI standard defines a DALI network as including a maximum of 64 units (independently addressable), 16 groups, and 16 scenes. Different lighting units on the DALI bus can be flexibly grouped to achieve different scene control and management. In a DALI network, 30 to 40 control instructions can be processed per second.
DALI is not a true point-to-point network. It replaces the 1-10V voltage interface to control the ballast. Compared with the traditional 1-10V dimming, the advantage of DALI is that each node has a unique address code and has feedback. The longer distance dimming will not have signal attenuation like 1-10V, but in engineering practice, this distance should not exceed 200 meters.
c. DALI is not suitable for large-scale LED lighting control
A DALI network can only control 21 (64/3 colors) full-color LED lamps. DALI is oriented to traditional lighting control, focusing on the system's static control, reliability, stability, and compatibility. The scale of an LED lighting system is much larger than that of a DALI system, which requires the system to be connected to a larger bus network. Therefore, the DALI system is often incorporated into other bus systems as a subsystem in large lighting projects.
d. Advantages of DALI dimming:
-Digital dimming, precise, stable and smooth dimming;
-DALI can communicate bidirectionally and can provide feedback on the status of lamps to the system;
-Single lamp control, more flexible control;
-DALI has strong anti-interference ability.
e. Disadvantages of DALI dimming
-Complex signal line layout
-High price.
-The DALI dimming driver still needs standby power consumption when turning off the lights to ensure that the microcontroller is in a standby state at any time.
5. DMX512 (or DMX) dimming
DMX (Digital Multiplex) means multi-channel digital transmission.
The DMX512 control protocol is an industrial standard for data transmission between lighting controllers and lighting equipment released by the American Stage Lighting Association (USITT) in 1990. It covers electrical characteristics, data protocols, data formats, and other aspects.
A DMX512 interface can connect up to 512 channels, so we know this device is a digital transmission-dimming device with 512 dimming channels. It is an integrated circuit chip that separates control signals such as brightness, contrast, and chromaticity, processes them separately, and changes the analog output level value by adjusting the digital potentiometer to control the brightness and hue of the video signal. It divides the light level from 0 to 100% into 256 levels. The control system can achieve full color by achieving R, G, B, and 256 grayscales.
Its output signal transmission method is RS485 differential signal transmission, which effectively guarantees the reliability of its signal. The ideal transmission distance can reach 200m. Still, in actual application, due to signal interference and attenuation, adding a repeater to strengthen the signal at about 40 meters is generally recommended. Although the dimming principle seems complicated, we can simply summarize the DMX512 system into three parts: DMX master control --- DMX decoder --- LED lamps.
Compared with the traditional analog dimming system, the digital lighting system of the DMX512 control protocol has brought earth-shaking changes to architectural lighting, night scene lighting, and the lighting effects of large and medium-sized film and television studios and variety shows with its powerful control function.
However, in actual applications, multiple controllers are interconnected to control complex lighting solutions, and the operating software design will be more complicated. The controller has many wirings, and the construction is relatively cumbersome, which has a high cost for later maintenance.
6. Wireless dimming (Bluetooth, wifi, ZigBee, radio frequency)
We can see that LED lighting dimming is all dimmed by the lamp's power supply. The main function of the dimming system is to give a command signal to the power supply. In addition to triac dimming, a special signal line needs to be set up to transmit the signal, which is very inconvenient. So, can wireless networks be used to transmit dimming signals? The answer is, of course, yes, and they are being used more and more, among which the most used is the Bluetooth Mesh solution.
6.1 Bluetooth Mesh Lighting Control Solution
The Bluetooth Mesh network is a new Bluetooth Low Energy (Bluetooth LE) network topology for establishing many-to-many (many: many) device communications. It allows you to create a large network based on multiple devices. The network can contain dozens, hundreds, or even thousands of Bluetooth Mesh devices, which can transmit information to each other.
Bluetooth Mesh has five major advantages
- The biggest advantage is that Bluetooth Mesh can communicate directly with smart terminals without a gateway, which can fully utilize the advantages of a good human-computer interaction experience;
- There can be no gateway, which is low-cost, or a gateway can be set up to provide more network control methods.
- Very convenient installation and the user experience is particularly good;
- Strong low-power support capability.
- Strong networking capability: 250 nodes can be grouped in two directions, and 1,000 can be grouped in one direction.
The best indoor scene for a Bluetooth mesh network application is the smart lighting system. The layout range of lights is generally very close, and the mutual broadcast between Bluetooth modules can be contacted. This is better than the ZigBee mode, which must be connected through a gateway to achieve on-site interoperability.
With Bluetooth Mesh, you only need to use one control device to control all functions in the smart home system simultaneously, easily, and efficiently. The powerful architecture of Bluetooth Mesh can also be expanded to meet the needs of offices, factories, industrial environments, and even cities, connecting millions of nodes. The disadvantage is that the lamps cannot be physically started if the system fails.
6.2 RF or "Radio Frequency" Dimming Light Control Solution
RF or "Radio Frequency" dimming provides a way to adjust the brightness of a strip remotely. Unlike wired plug-in LED dimmers, LED strips can be dimmed by remote control at distances up to 20 meters in diameter. This is generally used for manual control or as a backup control for automatic, wireless control (when Bluetooth or Wi-Fi lighting control systems fail).
6.3. WIFI dimming lighting control solution Dimming
WIFI networked smart dimmers allow you to remotely control LEDs via a WiFi connection. When setting up a "smart" office lighting system, these dimmers can be controlled through an online system. The disadvantage of this method is that it requires a good network distribution in the lighting area, that is, the price of smart dimming controllers is slightly expensive, and a dedicated system needs to be developed to control lighting dimming, or access cloud platform management.
Professional knowledge may be a bit complicated. In actual application, professional designers can design specific solutions according to actual conditions. If you want to know more about dimming lamps, please send an email to
[email protected]
