How does the LED SCR dimming (Triac dimming) work?

1.SCR is the abbreviation of Silicon Controlled Rectifier, which is a kind of high-power electrical component, also called thyristor.

It has the advantages of small size, high efficiency, and long life. In the automatic control system, it can be used as a high-power drive device to realize the control of high-power equipment with low-power controls. It has been widely used in AC and DC motor speed control systems, power control systems, and follow-up systems.

2.The Triac is equivalent to the connection of two thyristors in structure.

It has the function of bidirectional conduction, the control circuit is simple, and there is no reverse withstand voltage problem, so it is especially suitable for use as an AC contactless switch.

3.SCRs are mainly divided into three types: spiral type, flat type, and flat bottom type. The spiral SCR is more widely used.
The thyristor has three poles-anode (A), cathode (C), and control pole (G). The die is a four-layer structure composed of P-type conductors and N-type conductors. There are three PN junctions in total. It is very different in structure from a silicon rectifier diode with only one PN junction. The four-layer structure of the thyristor and the introduction of the control electrode laid the foundation for its excellent control characteristics of “controlling big by small”. In the application of thyristor, as long as a small current or voltage is added to the control electrode, a large anode current or voltage can be controlled. At present, SCR components with a current capacity of several hundred amperes or even thousands of amperes can be manufactured. Generally, SCRs below 5 amperes are called low-power SCRs, and SCRs above 50 amperes are called high-power SCRs.

4. LED scr dimmer switch（triac dimmer module) dimming principle

We can view the first, second, and third layers from the cathode as an NPN transistor, and the second, third, and fourth layers form another PNP transistor. The second and third layers are overlapped and shared by two tubes. The equivalent circuit diagram of Figure 1 can be drawn. When a positive voltage E is applied between the anode and the cathode, and a positive trigger signal is input between the control electrode G and the cathode C (equivalent to the base-to-emitter interval of BG2), BG2 will generate a base current Ib2. Amplify, BG2 will have a collector current IC2 that is amplified β2 times. Because the collector of BG2 is connected to the base of BG1, IC2 is the base current Ib1 of BG1. BG1 amplifies Ib1 (Ib2) and returns the collector current IC1 of β1 to the base of BG2. Amplify in this cycle until BG1 and BG2 are completely turned on. In fact, this process is “on the fly”. For the SCR, the trigger signal is applied to the control electrode, and the scr dimmer switch（triac dimmer circuit& triac dimming led driver) is turned on immediately. The turn-on time is mainly determined by the performance of the thyristor.

Once the thyristor is triggered and turned on, due to cyclic feedback, the current flowing into the base of BG2 is not only the initial Ib2, but the current amplified by BG1 and BG2 (β1*β2*Ib2). This current is huge In Ib2, it is enough to keep BG2 continuously on. At this time, even if the trigger signal disappears, the thyristor remains in the conducting state. Only when the power supply E is disconnected or the output voltage of E is reduced to make the collector current of BG1 and BG2 less than the minimum value for maintaining conduction, the thyristor can Shut down. Of course, if the polarity of E is reversed, BG1 and BG2 will be in the cut-off state under the action of reverse voltage. At this time, even if the trigger signal is input, the thyristor cannot work. Conversely, E is connected to the positive direction, and the trigger signal is negative, and the thyristor cannot be turned on. In addition, if the trigger signal is not applied, and the positive anode voltage is too large to exceed a certain value, the thyristor will also be turned on, but it is already an abnormal working condition.

The controllable characteristic of the thyristor that controls the conduction (large current through the thyristor) through a trigger signal (small trigger current) is an important feature that distinguishes it from ordinary silicon rectifier diodes.