Triac triggering compliance: The basics of positive and negative supplies

November 17, 2014 // By Laurent Gonthier and Jan Dreser
Triac triggering compliance: The basics of positive and negative supplies
Laurent Gonthier and Jan Dreser consider the basics of positive and negative supplies in relation to triac triggering compliance.

Positive and negative power supplies

Non-insulated power supplies are often required when a power semiconductor, which has to be controlled using this supply, has its drive reference connected to the mains (line or neutral terminal).
This is the case, for example, for triggering an AC switch such as a Triac, ACST, ACS or SCR (Silicon Controlled Rectifier).These devices are all controlled by a gate current. This gate current has to be applied on the gate pin and circulates between the gate and the AC switch reference terminal, which is the Cathode (K) for SCR, A1 for Triac, or COM for ACST and ACS.

As the AC switch control circuit, and so its power supply, has to be referenced to the device reference terminal (connected back to the line voltage), a non-insulated power supply is required.
There are two ways to connect this drive reference to the non-insulated power supply:

  • Solution 1: connect the control circuit ground (VSS) to drive reference.
  • Solution 2: connect the control circuit voltage supply (VDD) to drive reference.


Figure 1: Supply polarity definition

Solution 1, shown in Figure 1a, is the most common solution, as the switch drive reference is also the zero voltage point (VSS). Such a topology is called a positive power supply as the voltage supply (VDD) is indeed above the mains terminal potential (line or neutral), which is connected to the drive reference (VSS).. If the supply is a 5 V power supply, then VDD is 5 V above the mains reference (neutral terminal in Figure 1a example).

This topology can be used directly only with standard Triacs and SCR, not with non-standard Triacs, ACS and ACST, as explained below. But some simple modifications can be implemented to control all these devices from a positive supply as explained at the end of this article.
Solution 2, shown in Figure 1b, is called a negative power supply. The voltage

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