introduction to other electronic devices

8/6/2019 Introduction to Other Electronic Devices

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INTRODUCTION TO OTHER

ELECTRONIC DEVICES(DIAC)


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INTRODUCTION

A diac is an important member of the thyristor familyand is usually employed for triggering triacs. A diac is atwo-electrode bidirectional avalanche diode which canbe switched from off-state to the on-state for either

polarity of the applied voltage. This is justlike a triac without gate terminal, as shown in figure. Itsequivalent circuit is a pair of inverted four layer diodes.Two schematic symbols are shown in figure. Againthe terminal designations are arbitrary since the diac,

like triac, is also a bilateral device. The switching fromoff-state to on-state is achieved by simply exceeding theavalanche break down voltage in either direction.


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CONSTRUCTION OF DIAC

A diac is a P-N-P-N structured four-layer, two-terminal

semicon ductor device, as shown in figure.A. MT2 and

MTX are the two main terminals of the device. There isno control terminal in this de vice. From the diagram, a

diac unlike a diode, resembles a bipolar junctiontransistor (BJT) but with the following exceptions.

there is no terminal attached to the mid dle layer (base),

the three regions are nearly identical in size,

the doping level at the two end P-layers is the same so

that the device gives sym metrical switchingcharacteristics for ei ther polarity of the applied voltage.


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OPERATION OF A DIAC

When the terminal MT2 is positive, the current flow path isP1-N2-P2-N3 while for positive polarity of terminal MT1 thecurrent flow path is P2-N2-P1-N1. The operation of the diaccan be explained by imagining it as two diodes connected in

series.W

hen applied voltage in either polarity is small (lessthan breakover voltage) a very small amount of current,called the leakage current, flows through the device.Leakage current caused due to the drift of electrons andholes in the depletion region, is not sufficient to causeconduction in the device. The device remains in non-

conducting mode. However, when the magnitude of theapplied voltage exeeds the avalanche breakdown voltage,breakdown takes place and the diac current rises sharply, asshown in the characteristics shown in figure.


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CHARACTERISTIC OF A DIAC

Volt-ampere characteristic of a diac is shown in figure. It resemblesthe English letter Z because of the symmetrical switchingcharacteristics for either polarity of the applied voltage.

The diac acts like an open-circuit until its switching or breakovervoltage is exceeded. At that point the diac conducts until its current

reduces toward zero (below the level of the holding current of thedevice). The diac, because of its peculiar construction, does notswitch sharply into a low voltage condition at a low current level likethe SCR or triac. Instead, once it goes into conduction, the diacmaintains an almost continuous negative resistance characteristic,that is, voltage decreases with the increase in current. This means

that, unlike the SCR and the triac, the diac cannot be expected tomaintain a low (on) voltage drop until its current falls below a holdingcurrent level.


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APPLICATION OF DIAC

The diacs, because of their symmetrical

bidirectional switching characteristics, are

widely used as triggering devices in triac phase

control circuits employed for lamp dimmer,

heat control, universal motor speed control etc.


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TYPE OF APPLICATION OF DIAC

1. Triac Lamp Dimmer Circuit.

The circuit for a triac controlled by an R-C phase-shift network and a diac is given in figure. This

circuit is an example of a simple lamp dimmer. Thetriac conduction angle is adjusted by adjusting thepotentiometer R. The longer the triac conducts,the brighter the lamp will be. The diac acts like an

open-circuit until the voltage across the capacitorexceeds its breakover or switching voltage (andthe triacs required gate trigger voltage).


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2. Heat Control Circuit.

A typical diac-triac circuit used for smooth control ofac power to a heater is shown in figure. The capacitor

C1 in series with choke L across the triac slows-up the

voltage rise across the device during off-state. The

resistor R4 across the diac ensures smooth control atall positions of potentiometer R2. The triac conduction

angle is adjusted by adjusting the potentiometer R2.

The longer the triac conducts, the larger the output

will be from the heater. Thus a smooth control of theheat output from the heater is obtained.


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CIRCUIT IN DIAC

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