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