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Although the thyristor looks like a triode, its application in the circuit is quite different

Thyristor, as a power switching device, is often used in high-power circuit control instead of relay because of its contactless and long service life compared with relay

Thyristors control high-power devices

With the industrial control towards faster speed, more power, thyristor applications are more and more. For the study of thyristor, the author thinks it is very meaningful.​

Circuit symbol for a thyristor

High power thyristor

The thyristor has three poles, namely A pole (anode), K pole (cathode) and control pole (G pole).

Thyristors are divided into bidirectional and unidirectional thyristors

1. Unidirectional thyristor

In a unidirectional thyristor, as in A diode, current can only flow from the A to the K pole.​

Its G-pole is controlled in two ways -- when it reaches a certain voltage (P-type) and on when it falls below a certain voltage (N-type)

2. Bidirectional thyristor

Bidirectional thyristors do not distinguish between A and K poles, only G poles, current direction is bidirectional.​

Bidirectional thyristor

The key point of thyristor circuit application
1. Conduction condition -- once the G pole of the thyristor reaches the conduction condition, the thyristor will be conduction. At this time, no matter how the voltage of the G pole of the thyristor changes, it will not be cut off (except for the special backpressure cut-off thyristor).

Thyristor test circuit

Unless the current from pole A to pole K is less than the maintenance current, it will cut off and be controlled by the G pole again.​

2. Maintenance current -- Maintenance current means that the thyristor will be turned off when the current flowing through the thyristor after conduction is less than its value.​

The maintenance current of the thyristor

Thyristor application circuit analysis
The thyristor dimming speed regulating circuit can be made by using its special turn-off characteristics.​

VTH is a p-type unidirectional thyristor. After 220V ac is rectified by EL and full wave, the capacitors C1 and C2 are charged by RP. When the capacitor voltage is charged to the thyristor VTH conduction voltage, VTH conduction, EL luminous, and the thyristor is not controlled by the G pole


VTH conduction causes the capacitor to no longer get charging current, and the capacitor begins to discharge, and VTH will cut off at the lowest point of pulsating DC. However, when the capacitor voltage is still at the G-pole conduction voltage, VTH will be immediately conduction again, until the capacitor voltage drops to less than the G-pole conduction voltage, and the capacitor will be recharged, and so on.​

Pulsating dc nadir

The capacitor discharge stage is the VTH conduction stage, and the charging stage is the VTH turn-off stage. The duty cycle can be controlled by adjusting the RP to achieve the purpose of adjusting the brightness

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