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The working principle and application of thyristor dimmer

Dimmer is a kind of lighting accessories used to change the luminous flux of the electric light source and adjust the illuminance. It is widely used in home lighting, theater stages, hotel rooms, venues and exhibition halls.

In principle, all dimmers obtain different intensities of light output by changing the input current of the electric light source. The control methods include changing the voltage amplitude applied to the load and changing the time during which the current flows through the load. , The former directly changes the effective value of the current, while the latter is achieved by controlling the time and number of current conduction within the half-wave of alternating current.

1. Classification of dimmers

There are many types of dimmers. According to the nature of the power supply, they can be divided into AC dimming and DC dimming. According to the principle of the control circuit, they can be divided into amplitude dimming and phase dimming. According to the types of switching devices, they can be divided into passive dimming. Light and active dimming can be divided into segmented dimming and stepless dimming according to the level of light change. According to the load type, it can be divided into direct dimming of electric light sources and indirect dimming of lighting controllers. A comprehensive introduction to the classification of dimmers.

1.1 AM dimming

1.1.1 Variable resistor dimming

Variable resistor dimming is the earliest dimming method. By connecting a high-power variable resistor in series in the incandescent lamp lighting circuit, adjusting the variable resistor can change the current value flowing through the incandescent lamp, thereby changing The brightness of the light. This dimming method can be used in both AC and DC power circuits and will not cause radio interference. However, due to the high power consumption and high heat generation of the variable resistor, the efficiency of the system is very low, and it is generally only used as a principle demonstration.

1.1.2 Auto-voltage regulator dimming

An auto-voltage regulator is connected in series in the AC loop, and the voltage amplitude supplied to the incandescent lamp is changed by adjusting the position of the brush, thereby changing the brightness of the light. Although the auto-voltage regulator is bulky and has power frequency noise, because of the high system efficiency, increasing or decreasing the load does not affect the dimming level, and was used in large quantities for stage dimming in the early days.

1.1.3 Diode Dimming Circuit

This circuit is controlled by a three-gear switch, which is used for full-voltage power supply, half-wave power supply and shutdown control respectively. The diode here can be regarded as a one-way silicon controlled rectifier (SCR) that works in a conducting state. This dimming method is a transition type from amplitude dimming to phase dimming. Since the half-wave supply voltage of the incandescent lamp is a fixed voltage value and cannot be adjusted arbitrarily, and the incandescent lamp will flicker slightly under the half-wave voltage, the practicability of this circuit is not very good.

1.2 Phase dimming

Phase modulation dimming is to change the sine waveform by adjusting the conduction angle of each half-wave of the alternating current, thereby changing the effective value of the alternating current, so as to achieve the purpose of dimming, also known as "chopping-wave" dimming. Phase-modulated dimming includes two types: leading-edge phase control and trailing-edge phase control (also called front-cut and back-cut). The working principle is completely different from that of amplitude-modulated dimming.

1.2.1 Leading edge phase control dimmer

Cutting-edge dimmers have the advantages of high adjustment accuracy, high efficiency, small size, light weight, and easy long-distance operation. They dominate the market. Most manufacturers' products are this type of dimmer. Leading-edge phase control dimmers generally use thyristors as switching devices, so they are also called thyristor dimmers.

Although the thyristor dimmer has a simple circuit and low cost, it will produce strong radio interference when the thyristor is switched. If effective filtering measures are not taken, it will hinder the use of many electrical appliances. In addition, the thyristor dimmer has a very steep front when it is turned on, and the voltage waveform suddenly jumps from zero voltage. This has little effect on the incandescent lamp-like resistive load, but it is not suitable for the dimming of the gas discharge light source. . Because most gas discharge light sources need a driving circuit to work together, and the driving circuit is a capacitive load, the voltage jump generated by the thyristor dimmer will generate a large surge current on the capacitive load, making the circuit The work is unstable, and even causes the failure of the drive circuit to burn out.

1.2.2 Back-edge phase control dimmer

In addition to the advantages of the SCR dimmer, the trailing edge phase control dimmer has an important feature that can adapt to the dimming needs of the gas discharge lamp. With the accelerated elimination of incandescent lamps worldwide, users' demands for dimming light sources such as electronic energy-saving lamps with capacitive impedance have gradually increased, and trailing edge dimmers have just adapted to this market change. The trailing edge phase control dimmer generally uses MOSFET as a switching device, so it is also called a MOSFET dimmer.

1.3 PWM dimmer

The PWM dimmer was first used in the DC power supply to dimming linear loads such as tungsten filament bulbs. It uses a PWM signal to control the on and off of the switching device, and adjusts the current flowing through the bulb by changing the duty cycle. Dimming control.

1.4 Sine wave dimmer

The principle of the sine wave dimmer is somewhat similar to the PWM dimming method. The power switch installed in the AC line is driven by a high-frequency signal. The power switch is turned on multiple times in each half wave of the sine wave, and the conduction time Is variable. The power frequency voltage at both ends of the load is cut by the high-frequency signal, and the current flowing through the load can be adjusted by changing the frequency of the high-frequency signal, thereby realizing dimming control. Sine wave dimmers generally use IGBTs (insulated gate bipolar transistors) as switching devices, so they are also called IGBT dimmers.

The sine wave dimmer does not change the waveform characteristics of the sine wave, and has a small impact on the working state of the load, and the harmonic interference generated is not large, which makes it suitable for use with nonlinear loads. And it can reduce line loss and improve efficiency, reduce the heat generation of switching devices, and greatly improve the applicability and reliability of the circuit.

The sine wave dimmer avoids the "chopping" defect of the thyristor dimmer. It has no minimum load power limit and can adapt to various types of loads such as incandescent lamps, energy-saving lamps, fluorescent lamp ballasts, and fan motors. It is an ideal dimmer product. However, because IGBTs require special drive and protection technology, the circuit is complicated and the cost is high. Currently, it is only used in special occasions such as stage dimming, but it is still the main direction of future development.

2. Triac dimming circuit analysis

Among the commonly used incandescent lamp dimmers, triacs are the most widely used. This dimmer is turned on and off once during each half-wave period of the alternating current. When the brightness of the incandescent lamp needs to be reduced, the thyristor will turn off a part of the alternating current to reduce the current and achieve the purpose of dimming.

2.1 Circuit composition

Once the thyristor is triggered to conduct, it will continue to conduct until the AC voltage crosses zero. The thyristor is responsible for the working current flowing through the incandescent lamp. Since the resistance value of the incandescent lamp is very low when it is in the cold state, and considering the peak value of the AC voltage, in order to avoid the large current impact when starting up, the thyristor should be reserved when choosing There is a large current margin.

The trigger pulse of the trigger circuit should have sufficient amplitude and width to make the thyristor fully turn on. In order to ensure that the thyristor can be reliably triggered under various conditions, the trigger voltage and current sent by the trigger circuit must be greater than that of the thyristor. The minimum value of the trigger voltage UGT and the trigger current IGT, and the minimum width of the trigger pulse should continue until the anode current rises above the sustain current (ie, the holding current IL), otherwise the SCR will be turned off again because it is not fully turned on. The width of the trigger pulse is generally 20~50μs. For large inductive loads, the trigger pulse width should be increased because the current rises slowly. Generally, it is 300μs~1ms, which is equivalent to the 18° phase angle of a 50Hz sine wave. The capacity of C2 is generally 22nF ~ 220nF.

In the dimmer, it is a bidirectional trigger diode that realizes the trigger function, and DB3 and other types are generally used. There are also some dimmer products that use resistors or neon bulbs to replace trigger diodes, but the actual use effect is not ideal.

The protection resistor R2 is a protection resistor, used to prevent the damage of semiconductor devices caused by excessive current when POT1 is adjusted to zero resistance. If R2 is too large, the dimmable range will become smaller, so it should be selected appropriately.

The power adjustment resistor R1 determines the minimum power that the incandescent lamp can be adjusted to. If R1 is not connected, the incandescent lamp will be completely extinguished when the POT1 is adjusted to the maximum value, which will cause certain inconvenience in household applications. After R1 is connected, when POT1 is adjusted to the maximum value, due to the parallel shunt effect of R1, there is still a certain current to charge C2, so that the minimum power of the incandescent lamp can be adjusted. If R1 is replaced by a variable resistor, it can be achieved More precise adjustment to ensure the consistency of mass production. At the same time, R1 also has the function of improving the linearity of the potentiometer, making the light change more suitable for the photosensitive characteristics of the human eye.

Potentiometers Low-power dimmers generally choose potentiometers with switches, which can be linked to cut off the power when dimming to the minimum. Such potentiometers are usually divided into push type (PUSH) and rotary type (ROTARY). For dimmers with higher power, because the current through the switch contacts is too large, the potentiometer and the switch are generally installed separately to save material costs. Taking into account the requirements of the dimming characteristic curve, linear potentiometers are generally selected. The resistance band of this potentiometer is evenly distributed, and the resistance value per unit length is equal, and the resistance value changes in a linear relationship with the sliding distance or the rotation angle.

Filtering network Since the voltage chop by the SCR no longer presents a sinusoidal waveform, a large number of harmonic interferences are generated, which seriously pollutes the grid system, so effective filtering measures must be taken to reduce harmonic pollution. The filter network formed by L1 and C1 in the figure is used to eliminate the interference generated by the thyristor during operation, so that the product meets the relevant electromagnetic compatibility requirements and avoids the impact on televisions, radios and other equipment.

Thermal fuse For high-power dimmers or dimmers used for group installation, the internal temperature rise is higher than usual. Install a temperature fuse in the circuit to cut off the circuit when the temperature rises abnormally to prevent disasters. occur.

2.2 Buffer protection of SCR

When the thyristor works in the circuit, its switching state is not completed instantaneously. The equivalent impedance of the thyristor is still very large when it is just turned on. At this time, if the current rises quickly, it will cause a large turn-on loss; similarly, there will be a large current when the thyristor is close to completely turning off. At this time, if the voltage across the thyristor rises rapidly, a large turn-off loss will also occur. The switching loss will cause the heat generation of the SCR to increase, and in severe cases, it will burn down during the period. Adopting appropriate buffer measures to suppress the rate of rise of current and voltage can effectively improve the switching operating conditions of the thyristor.

There are two types of snubber circuits. One is to suppress the current rise rate by using the characteristic that the current flowing through the inductor cannot change suddenly, and the other is to suppress the voltage rise rate by using the characteristic that the voltage across the capacitor cannot change suddenly.

When thyristor is used in high-power dimming circuit, due to the increased inductance of the large current flowing through thyristor and bulb, in order to ensure the reliability of the circuit, an RC snubber network must be connected in parallel to the thyristor to limit the controllability The voltage rise rate (dv/dt) at both ends of the silicon when it is turned off. The capacitor is used to limit the dv/dt value across the triac, and the resistor is used to limit the discharge current on the capacitor when the thyristor is turned on, and attenuate the damped oscillation between the capacitor and the filter inductance.

In the aforementioned filter network, the inductor L1 is used to suppress the current rise rate dI/dt when the thyristor is turned on, and the capacitor C and the diode D form a turn-off absorption circuit to suppress the rise of the terminal voltage when the GTO is turned off. Rate dV/dt, where the resistor R provides a discharge path for the capacitor C. There are many forms of snubber circuits to be suitable for different devices and different circuits.

2.3 Hysteresis of SCR dimmer

Ordinary thyristor dimming circuit has the phenomenon of inconsistent power on and off. That is, when the potentiometer is adjusted to the maximum value of 500K, the incandescent lamp is almost extinguished. Reduce the potentiometer again, the incandescent lamp will only emit light when it is adjusted below 400K. The power when the potentiometer is adjusted to the minimum angle is greater than the power in the same position when the power is turned on, which is the phenomenon of ordinary dimmers. Hysteresis effect. The reason for the hysteresis effect is that the charging capacitor is partially discharged every time the thyristor is triggered. A small resistor in series on the trigger diode can alleviate this phenomenon. A more effective method is to use the circuit shown in the figure above and use C2 to trigger the thyristor. Due to the isolation effect of R3, the voltage drop on C2 is very small, while C3 The voltage on it remains unchanged, avoiding the hysteresis effect.

2.4 Minimum load limit of thyristor dimmer

When using a thyristor dimmer, when the load is less than a certain power, the bulb will flicker, which is caused by the insufficient minimum maintenance current of the thyristor. Since the minimum holding current of different types of thyristors is not consistent, the manufacturer will mark the applicable minimum load power limit on the product description. This problem must be paid attention to when using it.

2.5 Flicker of incandescent lamp

We know that the size of the pupil of the human eye will be adjusted with the change of the external brightness to control the intensity of the light entering the eye, but there is a time difference of about 50~200mS between the adjustment speed and the change of the scene, and the purpose is to prevent the external brightness from being fast. Eye muscle fatigue caused by changes, this characteristic of the human eye is called "persistence of vision".

The incandescent lamp uses an alternating 50Hz voltage. Since the positive and negative half waves of the alternating current will make the incandescent lamp glow, the incandescent lamp will flicker 100 times within 1s, that is, the flicker period is 10ms, which is less than the minimum vision of the human eye Temporary time, and because the thermal inertia of the tungsten lamp is relatively large, it is difficult for the human eye to feel the flicker of the incandescent lamp.

2.6 Noise of dimmer

When using a triac dimmer to control an incandescent lamp, we often feel a slight buzzing sound. This is the inherent characteristic of a triac dimmer. The reasons for it include the following two aspects:

2.6.1 Filter Oscillation

A high-quality thyristor dimmer must have an LC filter at the input end. Its function is to absorb the switching noise of the thyristor, smooth the voltage fluctuations caused by periodic switching, and prevent the dimmer from generating damage to the outside world. Harmonic interference.

The inductor in the LC filter is made of silicon steel sheet or iron powder core material. When a large current of 100Hz (take a 50Hz power supply as an example) flows through, the magnetic core will oscillate and it will make a buzzing sound. Especially when the bulb is adjusted to the brightest, the current of 100Hz is at the maximum value, and the buzzing sound from the dimmer is more obvious, which is an inevitable circuit characteristic.

Under normal circumstances, the noise emitted by the magnetic core is within the acceptable range. If the noise is too large, the problem can be solved by changing the material of the magnetic core or increasing the size of the magnetic core during the design stage.

In other words, replacing a higher power dimmer can usually eliminate this noise.

2.6.2 Filament oscillation

When the bulb is the brightest, the thyristor is almost turned on during the entire voltage cycle, and the output current is basically continuous. At this time, the bulb will not buzz. When the bulb is dimmed, the thyristor switches 100 times per second (take a 50Hz power supply as an example). The impact of this intermittent current on the filament produces a buzzing sound. Especially when the brightness of the bulb is 50%~60%, the voltage on the SCR instantly jumps from zero to the peak value of the sinusoidal voltage. At this time, the filament vibrates the most and the buzzing sound is the loudest. Changing to a thick-filament bulb or a short-filament bulb can reduce the vibration of the filament and reduce the hum.