As is well known, resistors are ubiquitous in daily life. We can use resistors to turn on and off small currents, and convert small currents into larger ones. But when dealing with larger currents, they are not very effective, and there is also a disadvantage that once the switch current is removed, they cannot work, which means they are not as useful in devices such as alarms.

Thyristor, also known as thyristor; Thyristors are high-power devices that can handle high voltage and high current. With the advancement of design and manufacturing technology, their capacity is becoming increasingly larger. So far, several main types have emerged, including unidirectional thyristors, bidirectional thyristors, photo controlled thyristors, reverse conducting thyristors, switchable thyristors, fast thyristors, and so on.

Thyristors are divided into two types: bolt shaped and flat plate shaped. Bolt shaped structures are convenient for replacing components and are used for components below 100A. Flat plate shape, this structure has better heat dissipation effect and is used for components above 200A. Thyristors are composed of four layers of semiconductors. The internal structure of the bolt
shaped thyristor shown on the right is composed of four layers of semiconductor materials, P1, N1, P2, and N2, stacked on a single crystal silicon wafer, forming three PN junctions.

Thyristors are semi controlled power electronic devices, and their operating conditions are as follows:

When a thyristor is subjected to reverse anode voltage, regardless of the voltage applied to the gate, the thyristor is in a reverse blocking state.

When a thyristor is subjected to a positive anode voltage, it only conducts when the gate is subjected to a positive voltage. At this point, the thyristor is in a forward conducting state, which is the thyristor's thyristor current characteristic, that is, the controllable characteristic.

3. When the thyristor is conducting, as long as there is a certain positive anode voltage, regardless of the gate voltage, the thyristor remains conducting, that is, after the thyristor is conducting, the gate loses its function. The gate only serves as a trigger.

4. When the main circuit voltage (or current) decreases to near zero while the thyristor is  conducting, the thyristor will turn off.

The working principle of thyristor:
During the operation of a thyristor, its anode (A) and cathode (K) are connected to the power supply and load, forming the main circuit of the thyristor. The gate (G) and cathode (K) of the thyristor are connected to the device that controls the thyristor, forming the control circuit of the thyristor.

(1) Working principle of unidirectional thyristor
Unidirectional thyristor is a PNPN flash layer structure, forming three PN junctions with three external electrodes: anode, cathode K, and control electrode G. A unidirectional thyristor can be equivalent to a composite transistor composed of PNP and NPN transistors.

After applying a positive voltage between anode A, the thyristor does not conduct. Only when the trigger voltage is applied to the control electrode G, VT1 and VT2 quickly conduct successively and provide base current to each other to maintain the conduction of the thyristor. At this point, even if the trigger voltage on the control electrode is removed, the thyristor remains in a conducting state until the current passing through is less than the holding current of the thyristor, at which point the thyristor will turn off

(2) Working principle of bidirectional thyristor
A bidirectional thyristor can be equivalent to two unidirectional thyristors connected in reverse parallel. A bidirectional thyristor can control bidirectional conduction, so the other two electrodes except for the control electrode G are no longer divided into anode and cathode, but are called main electrodes T1 and T2.

When a trigger voltage is applied to the control electrode G, the bidirectional thyristor conducts, and the well remains conductive even after the trigger voltage disappears. Current can flow from T1 through VS2 to T2, and from T2 through VS1 to T1. When the current is less than the holding current of the thyristor, the thyristor turns off.


(3) Working principle of switchable thyristor
After the ordinary unidirectional or bidirectional thyristor is turned on, the control pole does not work. To turn off the thyristor, the power must be cut off, so that the forward current flowing through the thyristor is less than the holding current I. The characteristic of the switchable thyristor is that it can be turned off by the control pole, overcoming the above defects. When a
positive pulse voltage is applied to the switchable thyristor control electrode G, the thyristor conducts, and when a negative pulse voltage is applied to the control electrode G, the thyristor turns off.