DIAC/TRIAC

A DIAC (Diode for Alternating Current) and a TRIAC (Triode for Alternating Current) are two types of bidirectional thyristors used in alternating current (AC) circuits to control power in electrical devices. Here’s an explanation of each:

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DIAC

Definition:

A DIAC is a two-terminal bidirectional diode that conducts electric current only after its breakover voltage (VBO) is momentarily reached. It belongs to the thyristor family but has no gate terminal.

Operation:

• The DIAC remains in a non-conducting state as long as the voltage across its terminals is below its breakover voltage.
• When the voltage exceeds this value in either direction, the DIAC suddenly starts conducting.
• It continues to conduct until the current flowing through it drops below a certain value called the holding current (IH), at which point it returns to its non-conducting state.

Key Characteristics:

✔ Bidirectional – Conducts current in both directions.
✔ No gate terminal – Activation depends solely on the voltage across its terminals.
✔ Symmetrical breakover voltage – The triggering voltage is typically the same in both directions.
✔ Primarily used as a triggering device for other thyristors, especially TRIACs.

Main Applications:

• Triggering circuits for TRIACs (e.g., in light dimmers, motor speed controls).
• Overvoltage protection circuits.
• Neon lamp triggering.

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TRIAC

Definition:

A TRIAC is a three-terminal bidirectional thyristor (two main terminals and a gate) used for AC power control. It can be thought of as two SCRs (unidirectional thyristors) connected in inverse parallel with their gates tied together.

Operation:

• The TRIAC can conduct current in both directions between its two main terminals when an appropriate gate current is applied.
• Once triggered, it remains conducting even if the gate signal is removed, as long as the main current does not drop below the holding current.
• It turns off when the main current approaches zero at the end of each half-cycle of the AC voltage.

Key Characteristics:

✔ Bidirectional – Can control current in both AC voltage directions.
✔ Gate terminal – Activation is controlled by a signal applied to the gate.
✔ Can be triggered by positive or negative gate signals, regardless of the polarity of the main terminal voltage.
✔ Handles higher power levels than DIACs.

Main Applications:

• Light dimmers.
• AC motor speed control (fans, small motors).
• Heating controls.
• AC solid-state relays.
• AC switching circuits.

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Comparison Summary

In Short:

• DIAC = Triggering device (no gate, voltage-controlled switching).
• TRIAC = Power control device (gate-controlled, used in dimmers & motor controls).

Both are essential for AC power regulation, with the DIAC often acting as the starter for the TRIAC in many circuits.

See other components:

• Resistor.
• Capacitor.
• Transistor.
• Thyristor.
• Inductor.
• Opto-coupler.
• Regulator.
• Diode.
• Transformer.
• Ceramic resonator.
• Relay.
• Fuse.
• Electret micro.
• Quartz.
• NE555(IC timer).
• TL494(IC PWM).
• TDA2822(IC audio).
• LM386(IC audio).
• TDA3886(IC audio).
• 74HCxx (IC Logic).