Vishay OPTO Squad

PhotoTRIACs – What are they and what are they good for?

Phototriacs are solid-state AC switches whichconsist of an infrared emitter on the input and two thyristors, connected to a TRIAC, on the output. The light emitted from the infrared emitter triggers the output making it conductive.  Advantages of phototriacs include their solid-state reliability, elimination of contact bouncing and contact arcing found in mechanical relays, small size, and ability to isolate high-voltage output from the triggering input. Phototriacs are mostly used in applications as AC switches, as drivers for a power TRIAC, and DC latches in unique applications.

There are 2 different types of phototriacs based on how the output is triggered, either in a zero-crossing or non-zero crossing mode.  In non-zero crossing (NZC) phototriacs, there is no zero-crossingdetection and the output turns on immediately when the input signal triggers, when the infrared emitters turns on. They are suitable for phase angle independent control and can be used in applications such as light dimmers or motor control.

In zero crossing (ZC) phototriacs the output is only activated when the AC load voltage crosses the zero point.  The zero-detection circuitry inhibits the output to turn on first when the voltage is below a certain value, almost “zero”. These phototriacs can be used in applications such as heater control or solenoid drivers whereby the current and voltage spikes resulting in electromagnetic interference (EMI) and radio frequency interference (RFI) should be minimized and a snubber network should or must be avoided.

Two of the key performance parameters of a phototriac are the static dv/dt and commutating dv/dt.  Static dv/dt is the rated parameter of rise in voltage without any triggering signal I(FT). Exceeding this parameter will lead the phototriac to be triggered on, however it will turn off again at the next zero crossing of the load voltage.

Commutating dv/dt rating should be taken into consideration when the forward current is changing. When the load voltage and current are not in phase with each other and the phototriac tries to turn off at zero current, the triac might turn on again due to a sudden rise in the voltage which exceeds the commutating dv/dt rating. For a successful turn-off, the current should drop at a rate slow enough to prevent the device from retriggering on.

We recommend the IL410 product family for zero crossing and the IL420 product family for non-zero crossing. Vishay offers industry leading 10 kV/μs performance TRIACs.

For more on Phototriacs, download our FAQs:

Additional Resources:



Application Note:


Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.


This entry was posted on November 7, 2018 by in Articles.
%d bloggers like this: