Unidirectional thyristor principle

Thyristor is usually called silicon controlled rectifier (rectifier element), and its English name is Silicon Controlled Rectifier, abbreviated as SCR. It is a high-power semiconductor device. It has both unidirectional conductive rectification function and controllable switch function. It can be used to control larger power with smaller power. It is widely used in AC and DC motor speed control systems, power control systems, follow-up systems and contactless switches. As shown in the figure, it has three electrodes on the outside: anode A, cathode C, and control electrode (gate) G.

Unlike a diode, when a forward voltage is applied to both ends and no voltage is applied to the control electrode, the thyristor does not conduct, and its forward current is very small, and it is in a forward current blocking state; when a forward voltage is applied, and a forward voltage is also applied to the control electrode (between the cathode and the cathode), the thyristor enters the conduction state, and the tube voltage drop is very small (about 1V), and it can still remain in the conduction state even if the control voltage disappears. Therefore, the control voltage does not need to exist all the time. It is usually in the form of a pulse to reduce the trigger power consumption. It does not have the ability to self-shut off. To cut off the load current, the anode current can only be reduced to below the holding current, or a reverse voltage can be applied to achieve shutdown.

If used in an AC circuit, it will automatically shut down when the current passes through zero and enters the negative half cycle. In order to turn it on again, the control signal must be reapplied.

2. Bidirectional control
switch When thyristors are used for AC circuit control, as shown in the figure, two devices are connected in anti-parallel to ensure that the current can flow in both positive and negative directions. If two anti-parallel SCRs are made on the same silicon wafer, a bidirectional thyristor is formed, and the control electrode shares one, which greatly simplifies the circuit. Its characteristics are as follows:
there is no signal on the control electrode G, and there is high impedance between A1 and A2, and the tube is cut off.
When V>1.5V, regardless of the polarity, G can be used to trigger the current to control its conduction.
When A1A2 works on AC, when each half cycle alternates, the pure resistance load can generally be restored to cut off; but in the case of inductive load, the current phase lags behind the voltage. When the current passes through zero, the reverse voltage may exceed the turning voltage, causing the tube to conduct in the reverse direction. Therefore, the tube is required to be able to withstand this reverse voltage, and generally an RC absorption circuit is added.
A1 and A2 can be used interchangeably, and the trigger polarity can be positive or negative, but the trigger current is different.
Bidirectional thyristors are often used for AC voltage regulation, power regulation, temperature regulation and contactless switches. In the past, the trigger pulse was generally generated by hardware circuits, so the detection and control were not flexible enough. However, in the single-chip microcomputer control application system, the trigger pulse can often be generated by software.