The triode AC Switch (TRIAC or Bidirectional Triode Thyristor) can be considered as the integration of a pair of ordinary thyristors connected in anti-parallel. The graphic symbols and volt-ampere characteristics of the triac are shown in Figure 1. It has two main electrodes T_{1} and T_{2}, and a gate electrode G. The gate allows the device to trigger conduction in both the positive and negative directions of the main electrode, so the bidirectional thyristor has symmetrical volt-ampere characteristics in the first and third quadrants. Compared with a pair of anti-parallel thyristors, the bidirectional thyristor is more economical, and the control circuit is relatively simple, so it is widely used in the fields of AC voltage regulating circuit, solid state relay and AC motor speed control.

According to the combination of the main terminal polarity and the gate polarity, there are theoretically 4 trigger modes for the bidirectional thyristor, namely:

Ⅰ_{+} trigger mode: corresponding to the volt-ampere characteristics of the Ⅰ quadrant, T_{1 }is positive relative to T_{2}, and the gate is positive relative to T_{2}.

Ⅱ_{–}_{ }Trigger mode: corresponding to the volt-ampere characteristics of the Ⅰ quadrant, T_{1} is positive relative to T_{2}, and the gate is negative relative to T_{2}.

Ⅲ_{+} trigger mode: corresponding to the volt-ampere characteristics of the Ⅲ quadrant, T_{1 }is negative relative to T_{2}, and the gate is positive relative to T_{2}.

Ⅲ_{–}_{ }Trigger mode: corresponding to the volt-ampere characteristics of the Ⅲ quadrant, T_{1} is negative relative to T_{2}, and the gate is negative relative to T_{2}.

Due to the low sensitivity of the bidirectional thyristor when using the Ⅲ+ trigger mode, the required gate power is quite large, so the Ⅲ+ trigger mode is not used for triggering in practical applications, so you can only choose between the two combinations (Ⅰ_{+}, Ⅲ_{–}) and (Ⅰ_{–}, Ⅲ_{–}). If the trigger mode (Ⅰ_{+}, Ⅲ_{+}) or (Ⅰ_{–}, Ⅲ_{+}) is wrongly selected for triggering, it may cause the bidirectional thyristor to fail to trigger conduction and cause the circuit to fail to work normally.

There are many characteristic parameters that characterize the performance of the bidirectional thyristor. Only the main technical parameters directly related to the application will be introduced here, focusing on the difference between the main technical parameters of the ordinary thyristor.

(1) Rated on-state current

Since bidirectional thyristors are usually used in AC circuits, they do not use average values but effective values to represent their rated current values.

Rated on state current refers to the maximum effective value of AC sinusoidal current allowed when the junction temperature is stable and does not exceed the rated junction temperature in the resistance load circuit with single-phase power frequency conduction angle not less than 170 ° under the ambient temperature of 40 ℃ and standard heat dissipation and cooling conditions. The maximum allowable effective value of AC sinusoidal current when the junction temperature is stable and does not exceed the rated junction temperature. Take this on-state current according to the standard and take the corresponding current level as the rated on-state current (rated current) of the device. In practical applications, it must be noted that there is a difference between the rated current I_{T (RMS)} of the bidirectional thyristor and the rated current I_{T (AV)} of the ordinary thyristor. The former uses the effective value while the latter uses the average value. The conversion relationship between the two is as follows:

It can be seen that the current capacity of a 200A bidirectional thyristor is equivalent to the current capacity of two ordinary thyristors in anti-parallel connection of 90A. The overload capacity of bidirectional thyristor is also poor, so its rated current can generally be selected according to 1.5~2 times of the actual working current.

(2) On-state voltage drop

There are two parameters indicating the on-state voltage drop performance of the bidirectional thyristor, the on-state average voltage U_{T (AV)} and the on-state peak voltage U_{TM}. The average on-state voltage U_{T(AV)} is the average value of the voltage drop between the corresponding main poles when the rated on-state current is applied. The voltage drop U_{T1} from the main terminal T_{1} to T_{2} and the voltage drop U_{T2} from the main terminal T_{2} to T_{1} of the bidirectional intertransistor should satisfy |U_{T1}-U_{T2}|≤0.5V, that is, the difference between the forward and reverse average voltages of the bidirectional thyristor shall not be greater than 0.5V. The greater the difference between the two, the worse the symmetry of the on-state volt-ampere characteristics of the bidirectional thyristor. On-state peak voltage drop U_{TM} refers to the peak voltage when the bidirectional thyristor is π times or several times the rated on-state current. When choosing a bidirectional thyristor, choose a bidirectional thyristor with smaller U_{TM} and U_{T(AV)} as much as possible, and at the same time the difference between U_{T1} and U_{T2} is smaller.

(3) Off-state repetitive peak voltage

In the volt-ampere characteristics of the bidirectional thyristor, U_{DSM} is called the off-state non-repetitive peak voltage. The standard stipulates that the off-state repetitive peak voltage U_{DRM} is 90% of the off-state non-repetitive peak voltage, and the level of the corresponding U_{DRM} is taken as the rated voltage of the bidirectional thyristor. In actual application, like the thyristor, the instantaneous peak voltage applied to the circuit cannot exceed the off-state non-repetitive peak voltage. When selecting the rated voltage of the bidirectional thyristor, it should be selected according to 2.0~2.5 times of the actual maximum voltage.

(4) Gate trigger current and trigger voltage

When the gate is triggered by DC power supply at room temperature and the main voltage is DC 12V, the minimum gate current and gate voltage that make the bidirectional thyristor fully conductive are called gate trigger current (I_{GT}) and gate trigger voltage (U_{GT}) respectively.

There are 4 trigger modes for triacs. Because the trigger power required for the Ⅲ+ trigger mode is very large, it is not used in practical applications. At present, triacs only provide Ⅰ_{+}, Ⅰ_{–}, and Ⅲ_{–}3 trigger parameters on the factory certificate.

(5) The critical rate of rise of off-state voltage du/dt

The critical rise rate of off state voltage du/dt refers to the ratio of the change value of voltage on the bidirectional thyristor to the time of change when the voltage on the bidirectional thyristor rises from 10% to 90% of udrm under the condition of open gate and rated junction temperature, applied voltage of 2/3 U_{DRM} and repetition frequency f≤50Hz. When selecting a bidirectional thyristor, the critical rate of rise of the off-state voltage du/dt is an important technical parameter. The bidirectional thyristor with a higher du/dt parameter should be selected as much as possible. Generally, the du/dt should be greater than 500V/μs.

(6) The critical rate of increase of the commutation voltage (du/dt). And the critical rate of decrease of commutation current (di/dt).

The commutation performance of the bidirectional thyristor can be used for the critical rise rate of the commutation voltage (du/dt). And the critical rate of commutation current drop (di/dt)_{c}. The larger the value of (du/dt)_{c} and (di/dt)_{c}, the better the commutation performance of the bidirectional thyristor. These two parameters are also important technical parameters when selecting a bidirectional thyristor.