Brief introduction of thyristor
Silicon controlled rectifier (SCR) is a kind of high-power electrical components, also known as thyristor. It has the advantages of small volume, high efficiency and long service life. In the automatic control system, it can be used as a high-power driver to control high-power equipment with small power control. It is widely used in AC / DC motor speed regulation system, power regulation system and servo system.
SCR is divided into unidirectional SCR and bidirectional SCR. Triac is also called triac. Bidirectional thyristors are equivalent to two unidirectional thyristors connected reversely in structure, which have bidirectional conduction function. Its on-off state is determined by the control pole G. Adding positive pulse (or negative pulse) to the control pole G can make it conduct forward (or reverse). This device has the advantages of simple control circuit and no reverse voltage withstand problem, so it is especially suitable for AC contactless switch.
Characteristics of thyristor
SCR is divided into unidirectional SCR and bidirectional SCR. The unidirectional SCR has three pins: anode a, cathode K and control pole G. Bidirectional thyristor has three pins: the first anode A1 (T1), the second anode A2 (T2) and the control pole G. Only when positive voltage is applied between anode a and cathode K of unidirectional thyristor, and the required positive trigger voltage is applied between control electrode G and cathode, can it be triggered to turn on. The voltage drop between anode a and cathode K is about 1V. After the unidirectional thyristor is turned on, even if the control electrode G loses the trigger voltage, as long as the positive voltage between anode a and cathode K remains, the unidirectional thyristor will continue to be in the low resistance conduction state. Only when the voltage of anode a is removed or the polarity of voltage between anode a and cathode K is changed (AC zero crossing), the unidirectional thyristor will change from low resistance conduction state to high resistance cutoff state. Once the unidirectional thyristor is cut off, even if the positive voltage is applied again between anode a and cathode, the positive trigger voltage still needs to be applied again between control electrode G and cathode K to conduct. The on and off states of unidirectional thyristor are equivalent to the on and off states of the shape. Contactless switch can be made with it.
Between the first anode A1 and the second anode A2 of the bidirectional silicon controlled rectifier, no matter whether the applied voltage polarity is forward or reverse, as long as the trigger voltage with different positive and negative polarity is applied between the control electrode G and the first anode A1, the trigger conduction can be in a low resistance state. At this time, the voltage drop between A1 and A2 is about 1V. Once the bidirectional thyristor is turned on, it can continue to turn on even if the trigger voltage is lost. Only when the current of the first anode A1 and the second anode A2 decreases and is less than the maintenance current or when the voltage polarity between A1 and A2 changes and there is no trigger voltage, the bidirectional thyristor will be cut off. At this time, only when the trigger voltage is applied again, the bidirectional thyristor can be turned on.
Classification of thyristors
（1） According to turn off, conduction and control mode classification: according to its turn off, conduction and control mode, thyristor can be divided into ordinary thyristor, bidirectional thyristor, reverse conduction thyristor, gate turn off thyristor (GTO), BTG thyristor, temperature controlled thyristor and light controlled thyristor.
（2） Classification by pin and polarity: silicon controlled rectifier can be divided into two pole silicon controlled rectifier, three pole silicon controlled rectifier and four pole silicon controlled rectifier according to its pin and polarity.
（3） Classification by packaging form: silicon controlled rectifier can be divided into metal packaging silicon controlled rectifier, plastic packaging silicon controlled rectifier and ceramic packaging silicon controlled rectifier. Among them, metal encapsulated thyristors are divided into bolt shape, flat shape, round shell shape and so on; plastic encapsulated thyristors are divided into two types: with heat sink type and without heat sink type.
（4） Classification by current capacity: thyristors can be divided into high-power thyristors, medium power thyristors and low-power thyristors according to current capacity. Generally, high-power thyristors are mostly packaged in metal shell, while medium and low-power thyristors are mostly packaged in plastic or ceramic.
（5） Classification by turn off speed: SCR can be divided into ordinary SCR and high frequency (fast) SCR according to its turn off speed.
（6） Zero crossing trigger – it is generally power regulation, that is, when the sinusoidal AC voltage phase zero crossing trigger, it must be zero crossing to trigger and turn on the thyristor.
（7） Non zero crossing trigger – no matter what phase the AC voltage is in, the thyristor can be triggered. The common trigger is phase-shifting trigger, that is, the output percentage can be changed by changing the conduction angle (angular phase) of the sinusoidal AC.
Main parameters of SCR
1. Rated on state current (it) is the maximum stable working current, commonly known as current. Commonly used it thyristor is generally one to dozens of amperes.
2. Reverse repetitive peak voltage (VRRM) or off state repetitive peak voltage (Vdrm), commonly known as withstand voltage. The VRRM / Vdrm of commonly used thyristors are generally several hundred to one thousand volts.
3. Control pole trigger current (IGT), commonly known as trigger current. The IGT of commonly used thyristors is generally several to tens of milliamperes.
4. The average current allowed to pass through the cathode and anode under the specified ambient temperature and heat dissipation conditions