A DIAC is a two-terminal, bidirectional trigger diode primarily used to trigger TRIACs, while a thyristor (specifically, an SCR - Silicon Controlled Rectifier) is a three-terminal, unidirectional switch capable of controlling much larger currents, and generally triggered by a gate signal.
Here's a breakdown of the key differences:
| Feature | DIAC | Thyristor (SCR) |
|---|---|---|
| Terminals | 2 (A1, A2) | 3 (Anode, Cathode, Gate) |
| Directionality | Bidirectional (conducts in both directions) | Unidirectional (conducts in one direction only) |
| Function | Triggering device for TRIACs | Power control switch |
| Triggering | Voltage-based (breakover voltage) | Current-based (gate current) |
| Applications | Light dimmers, motor speed controls | High-power switching, phase control, rectification |
| Current Handling | Low | High |
In-Depth Explanation:
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DIAC (Diode for Alternating Current): A DIAC is essentially a two-terminal, three-layer semiconductor device. It operates as a switch that remains off until the voltage across it exceeds its breakover voltage (VBO). Once this voltage is reached, the DIAC conducts in either direction. It doesn't have a gate terminal, so you can't directly control when it turns on, only the voltage at which it turns on. Its primary use is to provide a sharp, reliable trigger pulse for TRIACs.
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Thyristor (SCR - Silicon Controlled Rectifier): A thyristor, specifically an SCR, is a three-terminal, four-layer semiconductor device. It acts as an electronic switch. It blocks current in both directions until a small current is applied to its gate terminal. Once triggered by a gate current, the SCR latches on and conducts current from anode to cathode, even if the gate current is removed (provided the anode current remains above the holding current). It turns off when the anode current falls below the holding current or when the voltage across it is reversed. Thyristors are used in high-power applications such as motor control, phase control, and rectification.
Why the DIAC is Used to Trigger the TRIAC:
DIACs are well-suited for triggering TRIACs in AC circuits because:
- Bidirectional Triggering: The TRIAC needs a trigger pulse of either polarity to turn on in either half-cycle of the AC waveform. The DIAC's bidirectional nature fits this requirement perfectly.
- Sharp Triggering: The DIAC provides a quick, sharp pulse when its breakover voltage is reached, ensuring reliable triggering of the TRIAC.
Analogy
Think of a DIAC like a simple pressure-activated valve that bursts open when a certain pressure is achieved, releasing a triggering blast (to the TRIAC). The Thyristor (SCR) is more like a tap controlled with a handle (the gate).
