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Home > Mall Dynamic > Introduction OF Dimming Principle OF Thyristor Application
Dimmer is a kind of lighting accessories used to change the luminous flux of the electric light source and adjust the illuminance, which is widely used in home lighting, theater stages, hotel rooms, venues and exhibition halls. In principle, all dimmers obtain light output of different intensity by changing the input current of the electric light source, and the control method includes changing the voltage amplitude added to the load and changing the time when 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 in the half wave of alternating current.
There are many types of dimmers, according to the nature of the power supply can be divided into AC dimming and DC dimming, according to the principle of the control circuit can be divided into amplitude dimming and phase dimming, according to the type of switching device can be divided into passive dimming and active dimming, according to the level of light change can be divided into segmented dimming and endless dimming. According to the load type, it can be divided into the direct dimming of the electric light source and the indirect dimming of the lighting controller, etc., and the classification of the dimmer is comprehensively introduced below.
Variable resistor dimming the earliest dimming method, by connecting a high-power variable resistor in series in the incandescent lighting circuit, adjusting the variable resistor can change the current value flowing through the incandescent lamp, thus changing the brightness of the light. This dimming method can be used in AC and DC power supply circuits, and does not produce radio interference, but due to the high power consumption and high heat of the variable resistor, the efficiency of the system is very low, and it is generally only used as a principle demonstration.
An autoregulator is connected in a series in the AC circuit to change the voltage amplitude supplied to the incandescent lamp by adjusting the position of the brush, thus changing the brightness of the light. Although the autoregulator body is large and bulky, and there is power frequency noise, due to the high efficiency of the system, the increase or decrease of the load does not affect the dimming level, and it has been widely used for stage dimming in the early days.
This circuit is controlled by a three-speed switch for full voltage supply, half wave supply and off control. The diode here can be regarded as a unidirectional thyristor (SCR) operating in the on-state, and this dimming mode is the transition type of AM dimming to phase dimming. Since the half wave voltage supply of the incandescent lamp is a fixed voltage value, which can not be adjusted arbitrarily, and the incandescent lamp will flicker slightly under the half wave voltage, the practicality of this circuit is not very good.
Phase-modulated dimming changes the sinusoidal waveform by adjusting the conduction Angle of each half wave of alternating current, so as to change the effective value of alternating current, so as to achieve the purpose of dimming, also known as "chopper" dimming. Phase-modulated dimming includes two types of front phase control and back edge phase control (also known as front cut and back cut), and the working principle is completely different from amplitude-modulated dimming.
Frontier dimmer has the advantages of high adjustment accuracy, high efficiency, small size, light weight, easy remote control, etc., dominates the market, and most manufacturers' products are this type of dimmer. Front phase control dimmer generally uses thyristor as a switching device, so it is also called thyristor dimmer.
Although the circuit of thyristor dimmer is simple and low-cost, but because the thyristor switch will produce strong radio interference, if the 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 opened, and the voltage waveform suddenly jumps from zero voltage, which has little impact on the resistive load of incandescent lamps, but it is not suitable for the dimming use of gas discharge light sources. Because most gas discharge light sources need to work with the drive circuit, and the drive circuit is a capacitive load, the voltage jump generated by the thyristor dimmer will produce a large surge current on the capacitive load, making the circuit work unstable, and even causing the failure of the drive circuit to burn.
In addition to the advantages of thyristor dimmers, an important feature of the back edge phase control dimmer is that it can adapt to the dimming needs of gas discharge lamps. As the worldwide elimination of incandescent lamps continues to accelerate, the demand for dimming light sources such as capacitive impedance electronic energy-saving lamps has gradually increased, and then dimmers just adapt to this market change. The back edge phase control dimmer generally uses MOSFET as a switching device, so it is also called MOSFET dimmer.
PWM dimmer was first used for linear load dimming such as tungsten bulb in DC power supply, it uses a PWM signal to control the on-off and cutoff of the switching device, and adjusts the current flowing through the bulb by changing the duty cycle, so as to achieve dimming control
The principle of the sine wave dimmer is somewhat similar to the PWM dimming way, the power switch installed in the AC line is driven by a high frequency signal, the power switch is switched on several times in each half wave of the sine wave, and the switching time is variable. The power frequency voltage at both endsof 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, so as to achieve dimming control. Sine wave dimmer generally uses IGBT (insulated gate bipolar transistor) as a switching device, so it is also called IGBTdimmer.
The sine wave dimmer does not change the waveform characteristics of the sine wave, has little influence on the working state of the load, and produces little harmonic interference, which makes it can be used well with nonlinear loads. And can reduce the line loss and improve efficiency, reduce the heat of the switching device, greatly improve the applicability and reliability of the circuit.
Sine wave dimmer avoids the "chopper" defect of thyristor dimmer, it has no minimum load power limit, can adapt to incandescent lamp, energy-saving lamp, fluorescent lamp ballast and fan motor and other types of load, is a more ideal dimmer product. However, because IGBT requires special drive and protection technology, resulting in complex circuits and high costs, it is currently only used in special occasions such as stage dimming, but it is still the main direction of future development
Among the incandescent dimmers commonly used at present, the bidirectional thyristor is the most widely used. This dimmer is switched on and off once during each half wave of alternating current, and 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 dimming purposes.
The figure above is a typical bidirectional thyristor dimmer circuit, potentiometer POT1 and resistors R1, R2 and capacitor C2 constitute a phase-shift trigger network, when the terminal voltage of C2 rises to the blocking voltage of bidirectional trigger diode D1, D1 breakdown, bidirectional thyristor TRIAC is triggered and the lamp is lit. Adjusting POT1 can change the charging time constant of C2, and the voltage conduction Angle of TRAIC will change accordingly, which will also change the current flowing through the bulb, resulting in the brightness of the incandescent lamp changing with the adjustment of POT1. The linkage switch SW1 on POT1 can turn off the input power when the brightness is dimmed to the lowest level to realize the switching control of the dimmer.
Once the thyristor is triggered, it will continue to be switched on until the AC voltage is zero. The thyristor bears the working current flowing through the incandescent lamp, because the resistance value of the incandescent lamp in the cold state is very low, and then taking into account the peak of the AC voltage, in order to avoid the large current impact at the time of startup, the selection of thyristor should leave a large current margin.
The trigger pulse of the trigger circuit should have sufficient amplitude and width to make the thyristor fully switched 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 the minimum value of the trigger voltage UGT and the trigger current IGT of the thyristor. And the minimum width of the trigger pulse should continue until the anode current rises above the maintenance current (that is, holding the current IL), otherwise the thyristor will be turned off again because it is not fully switched on. The width of the trigger pulse is generally 20~50μs, for large inductive loads, due to the slow rise of the current, the trigger pulse width should also be increased, generally 300μs~ 1ms, equivalent to the 18° phase Angle of the 50Hz sine wave. The capacity of C2 can be generally selected from 22nF to 220nF.
In the dimmer, the trigger function is achieved by a two-way trigger diode, which is generally selected DB3 and other models. There are also some dimmer products that use components such as resistors or neon bubbles to replace trigger diodes, but the actual use effect is not ideal.
The protection resistance R2 is a protection resistance used to prevent the damage of semiconductor devices caused by excessive current when POT1 is adjusted to zero resistance. R2 is too large and will cause the dimmable range to 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 some inconvenience in home applications. After accessing R1, 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 changed into a variable resistor, more accurate adjustment can be achieved to ensure the consistency of mass production. At the same time, R1 also has the effect of improving the linearity of the potentiometer, so that the light changes are more suitable for the light-sensitive characteristics of the human eye.
Potentiometer Small power dimmer generally choose a potentiometer with a switch, in the dimming to the lowest hour can be linked to cut off the power supply, this potentiometer is usually divided into PUSH and ROTARY two. For dimmers with large power, because the current through the switch contact 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, the linear potentiometer is generally selected, the resistance band of this potentiometer is evenly distributed, the resistance value per unit length is equal, and the resistance value changes in a straight line relationship with the sliding distance or Angle.
Because the voltage of the filter network is no longer sinusoidal waveform after being chopped by thyristor, a lot of harmonic interference is generated, which seriously pollunes the power grid system, so effective filtering measures should be taken to reduce harmonic pollution. The filter network composed of L1 and C1 in the figure is used to eliminate this interference when the thyristor is working, in order to make the product meet the relevant electromagnetic compatibility requirements and avoid the impact on TV, radio and other equipment.
Temperature fuse For high-power dimmers or dimmers used for group installation, the internal temperature rise is higher than usual, install a temperature insurance in the circuit, you can cut off the circuit when the abnormal temperature rise, to prevent disaster accidents.
When the thyristor is working in the circuit, its switching state is not instantaneous. The equivalent impedance of thyristor is still large when it is just switched on, and if the current rises quickly, it will cause great opening loss; Similarly, when the thyristor is close to complete shutdown, there is also a large current, and if the voltage at both ends of the thyristor rises rapidly, it will also produce a large shutdown loss. Switching loss will lead to an increase in the calorific value of thyristor, which will cause a period of burnout in severe cases. The working conditions of thyristor switching can be effectively improved by using appropriate buffer measures to suppress the rising rate of current and voltage.
There are two forms of buffer circuit, one is to use the current flowing through the inductor can not change the characteristic to inhibit the current rise rate, the other is to use the capacitor at both ends of the voltage can not change the characteristic to inhibit the voltage rise rate.
When the thyristor is used in high-power dimming circuit, due to the increase of the sensitivity of the thyristor and the lamp, in order to ensure the reliability of the circuit, a parallel RC buffer network must be connected to the thyristor to limit the voltage rise rate (dv/dt) at both ends of the thyristor when the thyristor is turned off. The capacitor is used to limit the dv/dt value across the bidirectional thyristor, the resistor is used to limit the discharge current on the capacitor when the thyristor is switched on, and the damping oscillation between the capacitor and the filter inductor is attenuated.
In the filter network described above, the inductor L1 is used to suppress the current rise rate dI/dt when the thyristor is on, the capacitor C and the diode D constitute a turn-off absorption circuit to suppress the terminal voltage rise rate dV/dt when the GTO is off, where the resistor R provides a discharge path for the capacitor C. Buffer circuits come in many forms to suit different devices and different circuits
The ordinary thyristor dimming circuit has the phenomenon of inconsistent startup power and shutdown power, that is, when the potentiometer is adjusted to the maximum 500K, the incandescent lamp is almost extinguished. The potentiometer is turned down again, only when it is adjusted below 400K, the incandescent lamp will emit light, and the power of the potentiometer is adjusted to the minimum Angle when the power is larger than that of the same position when the power is turned on, which is the hysteresis effect of the ordinary dimmer. The reason for the hysteresis effect is that the charging capacitor is partially discharged every time the thyristor is triggered. A small resistance in the trigger diode can reduce this phenomenon, a more effective way is to use the circuit above, with C2 to trigger the thyristor, due to the isolation of R3, C2 voltage drop is very little, while C3 voltage is unchanged, to avoid the emergence of hysteretic effect.
When the thyristor dimmer is used, when the load is less than a certain power, the bulb will flicker, which is caused by the minimum maintenance current of the thyristor. Because the minimum maintenance current of different types of thyristors is not consistent, the manufacturer will mark the applicable minimum load power limit on the product description, which must be paid attention to when using.
We know that the pupil of the human eye will adjust the size with the change of external brightness to control the intensity of light entering the eye, but the adjustment speed and the change of the scene have a time difference of about 50~200mS, the purpose is to prevent eye muscle fatigue caused by rapid changes in external brightness, this feature of the human eye is called "visual retention"
The incandescent lamp uses an alternating 50Hz voltage, because the positive and negative half waves of alternating current will make the incandescent lamp glow, so the incandescent lamp should blink 100 times within 1s, that is, the flicker cycle is 10ms, this time is less than the minimum visual temporary time of the human eye, and because the thermal inertia after the lamp tungsten glow is large, it is difficult for the human eye to feel the flicker of the incandescent lamp.
When using bidirectional thyristor dimmers to control incandescent lamps, we often feel a slight buzz, which is an inherent characteristic of bidirectional thyristor dimmers, and the reasons for it include the following two aspects:
A high-quality thyristor dimmer must have an LC filter at the input end, which is used to absorb the switching noise of the thyristor, smooth the voltage fluctuation caused by the periodic switch, and prevent the dimmer from generating harmonic interference to the outside world.
The inductor in the LC filter is made of silicon steel sheet or iron powder core material, and when a large current of 100Hz (taking 50Hz power supply as an example) flows through, it will buzz due to the vibration of the magnetic core. Especially when the bulb is adjusted to the brightest, the current of 100Hz is the
Under normal circumstances, the noise emitted by the magnetic core is within the acceptable range, if the noise is too large, in the design stage can be replaced by replacing the magnetic core material or increasing the size of the magnetic core to solve the problem, for the user, replacing a more powerful dimmer usually can also eliminate this noise.
maximum, and the buzz emitted by the dimmer is more obvious, which is an inevitable circuit characteristic.
When the bulb is brightest, the thyristor is almost on throughout the voltage cycle, and the output current is basically continuous, and the bulb will not buzz at this time. When the light bulb is dimmed, the thyristor is switched on and off 100 times per second (in the case of a 50Hz power supply), and this intermittent current shocks the filament and produces a buzzing sound. In particular, when the brightness of the bulb is 50% to 60%, the voltage on the thyristor instantly jumps from zero to the peak of the sine voltage, this time the filament vibration is the largest, and the buzz is the loudest. Switch to thick filament bulbs or short filament bulbs, you can reduce the vibration of the filament and thus reduce the buzz.
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