Component identification in thyristor families: Which description best matches a DIAC?

Introduction / Context:
DIACs are small, inexpensive trigger devices that provide symmetrical turn-on for AC control circuits. Being able to describe one accurately prevents confusion with SCRs, TRIACs, UJTs, and zener diodes.

Given Data / Assumptions:

• Two-terminal device with no gate lead.
• Breakover occurs at approximately equal magnitudes for positive and negative polarities.
• Commonly placed in series with a TRIAC gate.

Concept / Approach:
The defining trait of a DIAC is bidirectional breakover. It remains off until |V| reaches V_BO, then conducts sharply, providing a clean, symmetrical trigger pulse to a TRIAC in either half-cycle. This symmetry reduces waveform distortion in phase-control applications.

Step-by-Step Solution:
Identify terminals: Only two—no gate.Identify conduction: Symmetric in both polarities at breakover.Application: Series with TRIAC gate to produce consistent firing angle both halves.Therefore, it is a two-terminal bidirectional trigger diode.

Verification / Alternative check:
Manufacturer curves show nearly mirrored V–I characteristics in quadrants I and III, confirming bidirectional behavior and trigger functionality.

Why Other Options Are Wrong:

• SCR: Has a gate and is unidirectional.
• UJT: Three-terminal negative-resistance device, used in triggering but different physics.
• Zener diode: Unidirectional breakdown for voltage reference.
• Four-terminal bilateral switch: Not a standard DIAC description.

Common Pitfalls:

• Assuming DIACs can control power flow directly; they primarily trigger other devices.
• Confusing DIAC symbols with TRIAC symbols; TRIACs have three terminals.

Final Answer:
A two-terminal bidirectional trigger diode used to fire TRIACs; breaks over in either polarity