Triacs are three-terminal silicon devices that are configured in an inverse parallel arrangement to provide load current during both halves of the AC supply voltage. They have two anodes (A1 and A2) for conducting current in both directions.
They also have two gates (G1 and G2), each of which triggers a corresponding anode. As a rule, the gate trigger voltage is the same polarity as the voltage through the triac. For example, if the voltage from A1 to A2 is positive, then the gate trigger voltage is also positive.
Once triggered, triacs continue to conduct current as long as there is current flow, even if there is no longer voltage at the gate terminal.
Typically, triacs are used to control motor speed. Because load current (armature speed) flows during both halves of the applied AC voltage, motors rotate smoothly at all rotational speeds.
Standard or four-quadrant (4Q) triacs can be triggered in all four modes. 4Q triacs must include additional protection components such as resistor-capacitor (RC) snubbers across the main terminals and an inductor that is in series with the device.
Three-quadrant (3Q) triacs can be triggered in only quadrants 1, 2, and 3. Because they do not require protection circuitry, 3Q devices are more efficient than standard triacs in applications with non-resistive loads.
Snubbers are circuits that limit the voltage and the rising ratio of off-state voltage during device turn-off. They also limit the critical current rate-of-rise during device turn-on.
Selecting triacs requires an analysis of performance specifications.
Peak repetitive off-state voltage is the maximum instantaneous value of the off-state voltage that occurs across a triac, including all repetitive transient voltages.
Peak repetitive reverse voltage is the maximum reverse voltage that may be applied continuously to the anode and cathode.
Peak cycle surge on-current is the maximum on-state current of short-term duration that can be applied for one full cycle of conduction without performance degradation.
Gate trigger current is the minimum current required to switch triacs from the off-state to the on-state at the specified off-state voltage and temperature.
Gate trigger voltage is the voltage required to produce the gate trigger current.
Root mean square (RMS) on- state current is the maximum RMS value of the principal current when devices are turned on.
Rising ratio of off-voltage is the minimum rate-of-rise value for the principal voltage that causes switching from the off-state to the on-state.
Other performance specifications include:
Repetitive peak controllable on-state voltage
Rate-of-rise off-state voltage
Critical current rate-of-rise
Integrated Circuit (IC) Package Types
Triacs are available in a variety of integrated circuit (IC) packages and with different number of pins. Basic IC package types include
Dual in-line package (DIP)
Diode outline (DO)
Transistor outline (TO)
Small outline diode (SOD)
Small outline transistor (SOT)
DIPs are available in either ceramic (CDIP) or plastic (PDIP).
IC package types for silicon controlled rectifiers include discrete packaging (DPAK), integrated packaging (IPAK), power packaging (PPAK), and metal electrode leadless face (MELF).
D2PAK is large surface mounted package that includes a heat sink.
I2PAK is a plastic package with three leads.
Thin small outline package (TSOP), a type of DRAM package that uses gull wing shaped leads on both sides, is available with both L-shaped leads (TSSOP) and J-shaped leads (TSOJ).
BS 9343 - Semiconductor devices of assessed quality: case rated bi-directional triode thyristors (triacs)
BS QC 750111 - Semiconductor discrete devices - bi-directional triode thyristors (triacs), ambient or case-rated up to 100 A