Controllable silicon dimming (LED section)

Ordinary incandescent and halogen lamps usually use thyristors for dimming. Because incandescent bulbs and halogen bulbs are pure resistive devices, they do not require the input voltage to be a sine wave because their current waveform is always the same as the voltage waveform. Therefore, no matter how the voltage waveform deviates from the sine wave, as long as the effective value of the input voltage is changed, dimming can be achieved. The use of thyristors is to cut the sine wave of alternating current in order to change its effective value. The load is connected in series with a thyristor switch.

Changing the voltage division ratio of a variable resistor can alter its conduction angle, thereby achieving the goal of changing its effective value. Usually, this potentiometer comes with a switch connected to the input of n, used to turn on and off the light. In addition to thyristors, there are also technologies such as transistor trailing edge dimming, etc. Because their basic problems are the same, they will not be introduced here.

3.1 Disadvantages and problems of thyristor dimming

However, there are a series of issues with thyristor dimming.

1. The thyristor disrupts the waveform of the sine wave, thereby reducing the power factor value. Typically, the PF is below 0.5, and the smaller the conduction angle, the worse the power factor (only 0.25 at 1/4 brightness).

Similarly, non sinusoidal waveforms increase the harmonic coefficient.

3. Non sinusoidal waveforms can generate severe interference signals (EMI) on the line

4. It is easy to become unstable at low loads, so a leakage resistor must be added. And this leakage resistor consumes at least 1-2 watts of power.

5. Unexpected problems may occur when the ordinary thyristor dimming circuit outputs to the LED driving power supply, which is that the LC filter at the input end will cause the thyristor to oscillate. This oscillation is not significant for incandescent lamps because the thermal inertia of incandescent lamps makes it impossible for the human eye to see this oscillation. However, the driving power supply for LED will generate audio noise and flicker.

3.2 Advantages of thyristor dimming

Although thyristor dimming has many disadvantages and problems, it has certain advantages, that is, it has formed an alliance with incandescent halogen lamps and occupies a large dimming market. If LED wants to replace the position of incandescent and halogen lamps with thyristor dimming, it also needs to be compatible with thyristor dimming.

Specifically, in some places where controllable silicon dimming incandescent or halogen lamps have been installed, controllable silicon dimming switches and knobs have been installed on the wall, and two connecting wires to the lamps have also been installed in the wall. It is not easy to replace the thyristor switch on the wall and increase the number of connecting wires. The simplest method is to keep everything the same, just unscrew the incandescent lamp on the lamp head and replace it with an LED bulb with compatible thyristor dimming function. This strategy is like LED fluorescent lamps, it is best to make them exactly the same size as current T10 and T8 fluorescent lamps, without the need for professional electricians. Ordinary people can directly replace them, which can quickly become popular. Therefore, many foreign manufacturers of LED driver ICs have developed ICs that are compatible with existing thyristor dimming.
The difference between them and general flyback ICs is that they can detect the conduction angle of the thyristor to determine the current of the LED for dimming. We are not going to go into detail about their working principle and performance because we do not believe this is the direction of LED dimming.

3.4 Issues and drawbacks of compatible thyristor dimming

Despite multiple multinational chip companies launching chips and solutions compatible with existing thyristor dimming. However, such solutions are not recommended for the following main reasons:

1. Controllable silicon technology is an outdated technology that has been around for over half a century and has many drawbacks as mentioned earlier. It is a technology that should have been phased out long ago. It should exit the historical stage at the same time as incandescent and halogen lamps.

2. Many of these chips claim to have PFC, which can improve power factor. However, in reality, they only improve the power factor of the thyristor load, making them appear similar to pure resistance incandescent and halogen lamps, without improving the power factor of the entire system including thyristors.

3. The overall efficiency of all LED dimming systems compatible with thyristors is very low, and some have not yet considered the loss of leakage resistors required for stable operation. Valley LED dimming systems also regulate the forward current of LEDs, which has drawbacks such as chromatographic shift as mentioned earlier.

The proportion of installing dimmable thyristor incandescent lamps and halogen lamps is less than one in ten thousand, and the proportion of installing thyristor switches in walls is even less than one in ten thousand among thyristor dimmable lamps, because the vast majority of those installing thyristor dimmable lamps are desk lamps, bedside lamps, and standing lamps. Moreover, there are dozens of different specifications of thyristors and transistor dimmer switches on the market, and in reality, the developed IC cannot be compatible with all thyristors switches, only a small part of them can be compatible.

6. LED is a brand new Genesis technology with unparalleled advantages. There is no need to sacrifice the advantages of LED in order to take care of outdated thyristors. It is even more inappropriate to install a thyristor switch on the wall to achieve LED dimming.
The difference between them and general flyback ICs is that they can detect the conduction angle of the thyristor to determine the current of the LED for dimming. We are not going to go into detail about their working principle and performance because we do not believe this is the direction of LED dimming.