Nowadays, LED technology has become the mainstream in lighting systems, and various types of dimming methods are emerging one after another. SCR dimming is one of the methods. In this article, Xiaobian will introduce an example to illustrate the problem that the brightness of the lamp in the circuit cannot be adjusted when using the thyristor for control.
In this example, the IO port of the MCU is connected to the OUT1 of the MOC3021. The purpose is to control the brightness of the lamp by adjusting the output of the IO port to change the duty cycle. However, the brightness of the lamp is found to be unchanged during the actual test, even if the parameters are changed. There is no difference between the brightness and the brightness when the MOC3021 is always on. Why?
Figure 1 thyristor dimming
Once the thyristor is turned on, it cannot be turned off with the control signal, and it needs to wait until the voltage is reversed before it can be turned off. Therefore, it is necessary to synchronize with the commutation signal when triggering.
The correct approach is as follows: After the commutation signal, delay 9mS to provide the trigger signal. The holding current does not refer to the trigger pin, but to the current in the main circuit of the switch. To understand this, refer to the thyristor material.
The zero point is detected. When the zero crossing occurs, the thyristor will turn off by itself. The brightness of the lamp is controlled according to the brightness of the lamp during the two zero points. It is not necessary to consider turning off the thyristor during the two zero points, otherwise it can be turned off. After that, the thyristor can only be turned off when the zero crossing occurs. On the other hand, if you want to use zero trigger, detect the position of the zero point, etc., you must first trigger the thyristor with zero point to realize the voltage regulation.
In summary, the fundamental understanding of the principle of thyristor voltage regulation can smoothly solve the problem that dimming cannot be achieved. But friends who have similar problems in the lighting circuit, before looking for a solution, may wish to carefully recall the characteristics of the thyristor dimming circuit. Perhaps the problem can be solved.
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