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Diacs are bidirectional diodes that switch AC voltages and trigger silicon controlled rectifiers (SCRs) and triacs. SCRs are four-layer (PNPN) thyristors with an input terminal (gate), an output terminal (anode), and a common terminal (cathode) for both the input and output. Triacs are three-terminal silicon devices that function as two SCRs configured in an inverse, parallel arrangement, so as to provide load current during both halves of the AC supply voltage. Diacs, which are similar to open base NPN transistors, exhibit a high-impedance blocking state up to a voltage breakover point above which negative resistance is achieved. Except for a small leakage amount, diacs do not conduct current until a breakover voltage is attained. Because they are bidirectional, diacs are used as firing devices in phase control such as light dimmers and motion speed controls. 

Performance specifications for diacs include breakover voltage, breakover voltage symmetry, breakover current, output voltage, repetitive peak on-state current, and power dissipation. Breakover voltage (VBO), the voltage at which diacs begin to conduct, is measured between the input and output terminals when diacs switch on. Breakover voltage symmetry ( VBO) is the maximum breakover voltage range with a specified capacitance when diacs are connected in parallel. Measured during the “on” state, output voltage (VO) is the voltage across a 20-ohm resistor in series with a diac during the discharge of a specified capacitor. Repetitive peak on-state current (ITRM) is the maximum limiting peak on-state current, including all repetitive transient currents, for which diacs are rated. Power dissipation (Pd) is the power dissipated by diacs during the “on” state.

Diacs are available in a variety of IC package types with different numbers of pins and diodes. Basic IC packages types for diacs include discrete packaging (DPAK), power packaging (PPAK), and in-line packaging (IPAK). Other package types include diode outline (DO), transistor outline (TO), and small outline transistor (SOT). Diacs that use metal electrode leadless face (MELF) packaging have metallized terminals at each end of a cylindrical body. Other available package types for diacs include thin small outline package (TSOP), thin shrink small outline L-leaded package (TSSOP), and thin small outline J-lead (TSOJ) package.


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