Standard Triacs vs. Alternistors in the features


Triacs are semiconductor devices widely used in switching and power control applications, like phase control , switching, Fan speed control, motors and etc. In Digi-Key, you may usually find 2 different Triacs types - Standard and Alternistor.

Standard Triacs

Standard Triacs are full-featured AC power switching with simple, low current gate drive. They offer the same non-sparking and non-aging properties of semiconductors, making them ideal for a wide range of supply voltage devices.

However, the use of triacs is limited by their switching behavior. Often, the slope of the current reduction is too high, and/or the slope of the reapplied voltage (dV / dt) is too high, which may cause “spurious Triggering”. For example, the following shows STMicroelectronics’s BTA26-600BRG decreasing main current versus (dV/dt).


One solution is to add additional components operated with the triac (i.e. snubber) on the circuit, e.g. the following decoupled with an RC network is one common method to improve the gate triggering behavior. Note that this compromise increases the PCB loading cost.



Alternistors are an enhancement on semiconductor designs for voltage transients changing tolerance. This enhancement eliminates the need for an external RC Network and is therefore more effective for use of triacs without any voltage changing concerns. In the market, you may find these by the name “Snubberless Triac” which is STMicroelectronics 's trademark for alternistors, like T835-600B-TR.

The following figure shows the correlation between commutation behavior and sensitivity in STMicroelectronics’ traditional triacs and its snubberless triacs.