DIAC PPT

DIACDiode for  alternating current

ByUpama Bose13MPE0002

INTRODUCTION

•A diac is an important member of the thyristor family.

•A diac is a two-electrode bidirectional avalanche diode which can be switched from off-state to the on-state for either polarity of the applied voltage.

The main characteristics are of the DIAC are as follows:

(i) Break over voltage (ii) Voltage symmetry

(iii) Break-back voltage (iv) Break over current

(v) Lower power dissipation Although most DIACs have symmetric switching voltages,

asymmetric DIACs are also available. Typical DIACs have a power dissipations ranging from 1/2 to 1 watt.

Construction of a DiacA diac is a P-N-P-N structured four-layer, two-terminal semiconductor device.

MT2 and MT1 are the two main terminals of the device. There is no control terminal in this device.

This is just like a triac without gate terminal.

The doping level at the two end P-layers is the same.

Operation of a Diac

When T1 is positive,current flow path is p-n-p-n For positive polarity of T2,current flow path is p-n-p-n+

When applied voltage in either polarity is small (less than breakover voltage)leakage current, flows through the device

However, when the magnitude of the applied voltage exeeds the avalanche breakdown voltage, breakdown takes place and the diac current rises sharply

Characteristics of a DiacOnce the diac goes into conduction, the diac maintains an almost continuous negative resistance characteristic.

It remains "in conduction" until the current through it drops below the holding current, IH.

Most DIACs have breakover voltage around 30 V

MATLAB SIMULATION MODEL(1ST QUAD. CHARACTERISTICS)

MATLAB SIMULATION MODEL(3rd QUAD. CHARACTERISTICS)

Vd Id0.1 00.5 00.8 00.9 0.00951 0.019051.1 0.028671.2 0.03811.3 0.047621.6 0.07471.7 0.8571

0-0.1 0-0.5 0-0.8 0-0.9 -0.0095-1 -0.01905-1.1 -0.02867-1.2 -0.0381-1.3 -0.04762-1.6 -0.0747-1.7 -0.8571

TABLE-VALUES

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Series1

PLOT of Vd vs Id

Diac Applications

The diacs are used as triggering devices in triac phase control circuits,employed for:

lamp dimmer

heat control

universal motor speed control etc.

THANK YOU……