TRIAC
TRIODE IN ALTERNATING CURRENT
- A TRIAC (Triode for Alternating Current) is a 3-terminal AC semiconductor switch.
- Composed of 2 thyristors facing opposite directions such that it can conduct current in either direction.
- MT1 and MT2 are current carrying terminals while the Gate terminal is used for triggering by applying a small voltage signal.
- Once triggered, it continues to conduct current until the current falls below a threshold value.
- Used in power switching because it has high level of voltage and current.
- Conduct both directions of AC current flow.
STRUCTURE
▫ A four-layer NPNP device in parallel with a PNPN device.
▫ Two SCR connected in antiparallel with common gate.
▫ Designed to respond to a gating current through a single gate.
▫ Gate forms ohmic contacts with p ad n regions in either +ve or –ve direction.
▫ Terminals
MT1
MT2
Gate
WORKING
• TRIAC is turned on by a finite gate current.
• TRIAC conducts even without gate. It starts conducting after reaching break over voltage.
• Gate current determines the supply voltage. Larger gate current smaller supply voltage.
• Four modes of operation. (with respect to MT1)
▫ MT2 and Gate positive
▫ MT2 positive and Gate negative
▫ MT2 and Gate negative
▫ MT2 negative gate positive
• Fourth mode of operation should b avoided in circuits where di/dt (rate of change of current) is high.
Specifications
• Instantaneous ON state Voltage = 1.5 V
• Instantaneous ON state current = 25A
• Holding current = 75mA
• Average triggering current = 5mA
VI CHARACTERISTICS:
Applications
• Light dimmers
• Heat control circuits
• Speed control of electric fans and motors
Advantages
• Can conduct in both direction –Bidirectional device
• Gets OFF when voltage is reversed
• Has high current and voltage ratings
• Suitable for AC power conditioning.
Disadvantages
• Very small frequency range to operate as switching device
• Gate is unable to control when ON
• Not compatible with DC
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