Semiconductor devices are the core part of electronics circuits. Usually diodes conducts in only one direction. DIAC is made to conduct in both forward and reverse direction. The term DIAC stands for ( Diode for Alternating Current ).
What is DIAC ?
A diode that conducts electrical current only when its breakover voltage is reached is known as DIAC. The term DIAC stands for ( Diode for Alternating Current ). A ‘Diode for alternating current’ has two terminals and a member of thyristor family. This type of diode is generally used to trigger thyristor or a TRIAC.
The figure below shows the symbol of Diode for alternating current. That looks like the series connection of two diodes.
Unlike some thyristors DIAC has no gate electrodes. It has a advantage that it can simply be turned on and off by adjusting its voltage below its avalanche breakdown voltage. A DIAC is also known as ‘ a transistor without base ‘. It can be switched on and off for both positive and negative voltage polarity. It still works during avalanche breakdown.
Application of DIAC
A DIAC is widely used in TRIAC trigging circuits. It is generally connected to gate ( base ) terminal of TRIAC.
The application also include –
- In heat control circuits
- Lamp dimmer circuit
- Speed control of universal motor
A DIAC is used with TRIAC in a series combination. One terminal of DIAC is connected to the gate terminal of TRIAC. When the applied voltage across the DIAC increases above the breakover voltage ( Avalanche breakdown voltage ), it conducts current to the gate of TRIAC. When the voltage across DIAC decreases below its avalanche breakdown, it turns off ( no current flows through it ) hence, the TRIAC also remains in off state.
A DIAC is a semiconductor device that consist of two terminals and four layers. The construction is similar to that of the transistor. Some points that differentiate the construction of DIAC and transistor are –
- It has no gate terminal.
- Symmetrical switching characteristics for both polarity of voltage.
- Three regions have almost same level of doping concentration.
In the above figure, we see that a Diode for Alternating Current has three N-type material and two P-type material. Also, it has no gate terminal.
The Diode can be turned on and off for both positive and negative polarity of voltage. When A2 is more positive compared to A1, then the current flows from P2– N2– P1– N1 . When A1 is more positive compared to A2 then current flows through P1– N2– P2– N3.
The construction shows the series connection of diode.
When a very small voltage is applied in either polarity, a very small current known as leakage current flows because of drift of electrons and holes in depletion region. Although the small current is not sufficient to produce avalanche breakdown, hence the DIAC remains in non-conducting ( off ) state. When the applied voltage exceeds the breakover voltage the diode conducts as shown in V-I diagram below
The advantages of DIAC are –
- The DIAC can be turned on and off by reducing the voltage below its avalanche voltage.
- It is easily available at low cost.
- It has very small size.
The disadvantages are –
- It works above a specific voltage.
- Low power handling capacity.
- It cannot protect circuits from high voltage.
Frequently Asked Questions
- What is the difference between a DIAC and a TRIAC ?
- Both DIAC and TRIAC are from thyristor family.
- A DIAC is a two terminal device whereas a TRIAC is a three terminal device.
- A DIAC is generally connected to base pin of TRIAC that triggers it ON / OFF.
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