Triac three terminal semiconductor

TRIAC
A triac is three terminal semiconductor switching device which can control alternating current in a load.

It is an abbreviation for triode ac switch  tri indicates that the device has three terminals and ac means that the device controls alternating current or can contact current in either direction.

Operation
The ac supply to be controlled across the main terminals of triac through a load resistance Rl the gate circuit consists of battery a current limiting resistor R and switch S .
With switch S open there will be no gate current and the triac is cut off. Even with no gate current, the triac can be turned on provided supply voltage become equal to the breakover voltage of traic the noramal way to turn on triac is by introducing  a proper gate current.

When switch S is closed the gate current starts flowing in the gate circuit in a similar manner to scr , the breakover voltage of the triac can be varied by making proper gate current to flow.with a few milliamperes introduced at the gate , the triac will start conducting wheter terminal MT2 is positive or negative  with respect to MT1if terminal MT2 is positive w.r.t. MT1 the triac turns on and the conventional current will flow from MT2 to MT1. if the terminal MT2 is Negative w r to MT1. the traic is again tured on  but this time the conventional current flows from MT1 to MT2

Characteristics

The V-I characteristics for triac in the Ist and III rd quadrants are essentially identical   to those of an SCR in the first quadrant. The triac can be operated with either positive or negative gate controlled voltage but in normal operation usually the gate voltage is positive in Quadrant I and Negative in Quadrant III.

The supply voltage at which the triac is turned  on depends upon the gate current. the grater the gate current , the smaller supply voltage at which the triac is turned on .this permits to use a triac to control ac power in a load 0 to full power in a smooth and continues manner with no loss in the controlling device.

Applications of the Triac.

As a high power lamp switch
Electronic change over of transformer taps.

GATE TURN OFF THYRISTOR (GTO):

• The gate turn off thyristor is a PN PN device can be turned like an ordinary thyristor by a pulse of Positive gate current.
• It can be easily turned off by a negative gate pulse of approximate amplitude.
• As no forced commutation circuitry is required for GTO’s .these devices are compact and less cost.
• The negative gate current required to turn off a GTO is quite a large percentage(20 to 30%) of anode current 800A.
• GTO require a negative current Pulse of 200A peak for turning it off.

VI characteristics :
      

• The latching current for large power GTO’s is several amperes(2A) as compared to 100-500mA for conventional thyristors of the same rating.
• If Gate current is not able to turn on the GTO,it behaves like a high voltage low gain transistor with considerable anode current.