Thyristor is divided into two kinds: one-way thyristor and two-way thyristor. Bidirectional thyristor is also called TRIAC, or Triac for short. Bidirectional thyristor is equivalent in structure to two one-way thyristor reverse connection, this thyristor has the function of double guide pass. Its on-off state is determined by the control pole G. Positive (or negative) pulse is added to the control pole G to make it positive (or reverse) conduction. The advantage of this device is that the control circuit is simple, no reverse voltage problem, so it is particularly suitable for AC contactless switch use.

The main parameters of thyristor are:

1. Rated on-state average current IT Under certain conditions, the average 50 Hz sinusoidal half-wave current that can be passed continuously between anode and cathode.

2, positive blocking peak voltage VPF in the open circuit of the control pole without trigger signal, anode positive voltage has not exceeded the conductance voltage, can be repeated at the thyristor ends of the peak voltage. The peak forward voltage of the thyristor cannot exceed the parameter value given in the manual.

3, reverse blocking peak voltage VPR when the thyristor plus reverse voltage, in the reverse off state, you can repeat the reverse peak voltage at both ends of the thyristor. Do not exceed the value specified in the manual.

4, trigger voltage VGT in the specified ambient temperature, anode - cathode plus a certain voltage, thyristor from the off state to the on-state required minimum control electrode current and voltage.

5, the maintenance current IH at the specified temperature, the control pole break, the minimum anode positive current necessary to maintain the thyristor conduction. Many new thyristor components have been introduced, such as fast thyristor suitable for high frequency applications, can use positive or negative trigger signal to control the two direction of the bidirectional thyristor, can use positive trigger signal to make it on, with negative trigger signal to make it off the thyristor and so on.

Commonly used thyristor packaging forms are TO-92, TO-126, TO-202AB, TO-220, TO-220ABC, TO-3P, SOT-89, TO-251, TO-252, SOT-23, SOT23-3L, etc.