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As long as you know the characteristics of silicon controlled rectifier, the use of MCU to control SCR is quite easy. In this article, we’d like to depict the detailed information about the characteristics of SCR.

Introduction

Thyristor and bi-directional silicon controlled rectifier are current controllers. They are able to use small current as trigger signals to control the large signals, which are widely used in AC system.

Thyristor is a kind of uni-directional conductive device; it generates load current only flowing to one direction (A->K). Namely, only half of energy can be transmitted in AC control. While for bi-directional SCR, the current can flow from MT1 to MT2, or vise versa. In other words, the bi-directional SCR can be regarded as a device forming by two thyristors.

Characteristic

Thyristor is a rectifier that can use the small current signal (transmit from Gate to Cathode) to trigger the large current from Anode (A) to Cathode (K).

Working principle:

When the voltage on G is larger than that on K, the Gate current is generated, and also the thyristor is conducted. After a short time, if the Gate voltage reaches Vgt and the Gate current reaches lgt, a large current from A to K will be generated. Meanwhile, the thyristor is conducted.

Additionally, when the load current reaches IL, the thyristor is still being conducted although the Gate current is very small.

Conducting conditions: G current>=lgt, load current>=IL

Cut-off conditions: Load current<IH, and maintain for a period of time.

Since the load current can flow between MT1 and MT2 freely in Bi-directional SCR, it can also be regarded as “ Bi-directional thyristor”.

Figure 4 shows the levels’ situation of MT2, G, and MT1 in 4 quadrants. By analyzing the levels of them in those 4 quadrants, we find out that:

1+ Quadrant: The triggered current (lgt) flows from G to MT1. It thereby enables load current to flow from MT2 to MT1.

1- Quadrant: The triggered current (lgt) flows from MT1 to G. But it cannot generate the load current since the level of MT2 is higher than MT1.

3- Quadrant: The triggered current (lgt) flows from MT1 to G. It thereby enables current to flow from MT1 to MT2.

3+ Quadrant: The triggered current (lgt) flows from G to MT1. But it cannot generate the load current since the level of MT2 is lower than that of MT1.

In practical application, we should avoid the SCR working at 1- and 3+ quadrants to guarantee that the levels of MT2 and G are the same. Whereas there is no need to worry about that the levels of MT2 and G are different since the controlling components only works at 1+ and 3- quadrants in AC application.

For some components with single quadrant, the characteristic of those components is almost the same as that of thyristor. They are commonly used in AC control circuit instead of mechanical solid-state relay.

Diagram of typical application

Thyratron:

Bi-directional silicon controlled rectifier:

An optical coupler must be added in the circuit of bi-directional SCR for the AC control. See the figure below:

The Control of Silicon Controlled Rectifier.rar