Performance analysis of four dimming circuits

Performance analysis of four dimming circuits

  The so-called random dimming means that when the trigger signal is input, no matter what the direction and value of the power supply voltage are, the load is immediately turned on. And it is turned off at zero crossing. It dims by changing the conduction angle of the thyristor. Figures 1, 2, 3, and 4 are all random dimming circuits. 　Figure 1 is the simplest dimming circuit. Its advantage is that there are few components, only three. R is a current limiting resistor to prevent the components from being damaged by excessive current when the RP is at a low resistance value when the power is just turned on. Although this circuit is the simplest, it also has the most disadvantages. One is the selection of RP in the circuit. Because different types of thyristors have different trigger currents, especially thyristors with different currents, the trigger current can differ by more than multiple times. When using thyristors with different trigger currents, the resistance value of RP should also be different to obtain a better adjustment range; the second is that there is a phenomenon of inconsistent conduction voltage in both directions. This phenomenon occurs in the range above 50% power. The closer the power is to 50%, the more obvious it is. When it is lower than 50%, even unidirectional conduction occurs. These phenomena are caused by the thyristor itself. Because the bidirectional trigger current of the bidirectional thyristor is inconsistent, the small one turns on first and the large one turns on later. When the large one cannot turn on at the voltage peak, it becomes unidirectional conduction. This is a common problem of bidirectional thyristors, because their trigger current cannot be consistent, which is a disadvantage that is difficult to overcome; third, there is flickering. There is serious flickering at 50% power. This is because the one with a large trigger current can only turn on at the voltage peak. Due to the fluctuation of the voltage, the unstable factor of the thyristor trigger current changes slightly, causing one direction of the thyristor to turn on and off. There is also flickering in weak light. Figure 1 also has an advantage, that is, when the RP resistance is large enough, the power switch can be used without turning off the light. When turning off the light, adjust the light to the darkest until it goes out. The circuit of Figure 2 can use a smaller RP. In addition 　　to more components, Figure 3 has the same advantages and disadvantages as Figure 1. The component parameters of this circuit must be properly matched to achieve better results. When changing the parameters of one component, the parameters of other components must also be adjusted. This is the circuit with the highest difficulty in making. If this circuit uses a unidirectional thyristor (powered by full-wave rectification), the inconsistency of the bidirectional conduction voltage or the unidirectional conduction phenomenon can be reduced. The unidirectional conduction phenomenon only occurs in weak light. This phenomenon is not caused by the thyristor, but by the circuit itself. Because one-way voltage is loaded on the K pole. It is equivalent to connecting a resistor in series with the K pole, thereby increasing the trigger voltage. 　　The above circuit is not suitable for speed regulation due to the unidirectional conduction phenomenon. 　　Figure 4 is the most ideal circuit. It will not flicker, and there is no unidirectional conduction or bidirectional asymmetry phenomenon (unless the DB3 trigger voltage is asymmetric). This circuit has the most stable performance. Therefore, it is often used for speed regulation. It also does not have the problem of selecting thyristors, so thyristors with different currents can be used. Unless the trigger current is too large, the instantaneous current provided by the capacitor cannot make it conduct. If so, the capacitor capacity can only be increased to increase the trigger current (RP should be reduced to avoid affecting the adjustment range).