Question

The diac is used in triggering of:-
1. SCR
2. TRIAC
3. UJT
4. PUT

Answer 1

Correct Answer - Option 2 : TRIAC

The DIAC:

• The DIAC is a diode that conducts electrical current only after its break overvoltage (VBO) has been reached momentarily.
• The term DIAC is obtained from capital letters, DIode that can work on AC (alternating current).
• It is a two-terminal three-layer device.
• DIAC is a bidirectional device that can conduct the electric current in both directions.
• The DIAC can be switched from the off state to the ON state for either polarity of the applied voltage.
• It is equivalent to two PN junction diodes connected in an anti-parallel combination.
• The DIAC break-over voltage cannot be controlled and it is about 40V.
• The DIAC is commonly used as a solid-state triggering device for the other semiconductor switching devices, mainly for TRIACs.
• DIAC is a low power device that is not suitable for high-power applications.

 

SCR:

• Silicon controlled rectifier is the first member of the thyristor family, it is called SCR because silicon is used for its construction and its operation as a rectifier and can be controlled.
• It is a three-terminal (anode, cathode, gate), a four-layer device.
• SCR is a unidirectional device. It blocks the current flow from the cathode to the anode.
• It is a semi-controlled device because we can only control the ON state, but not the OFF state.  
• SCR break over voltage can be controlled by adjusting the gate current.
• SCR is a large power device that can be used in large power control applications.
• SCR mainly used to control DC power, such as phase-controlled rectifiers. and also used for the protection of power supplies.

 

FET:

• The field-effect transistor is a type of transistor that uses an electric field to control the current flow. So it is a voltage-controlled device.
• FET is a three-terminal device and the terminals are namely source, gate, and drain.
• These are also known as unipolar transistors since they involve single carrier type operation.
• The most widely used field-effect transistor is MOSFET (metal-oxide-semiconductor field-effect transistor).
• FETs usually have a high switching frequency leads to having fewer switching losses and the ON-state resistance of the FETs is more due to these more conduction losses.
• FETs are generally used as amplifiers.

 

​THE TRIAC:

• TRIAC is a bidirectional thyristor with three terminals.
• TRIAC is the word derived by combining the capital letters from the word TRIode and AC (alternating current).
• It is used extensively for the control of power in AC circuits such as AC voltage regulators.
• It is equivalent to two thyristors connected in an anti-parallel combination by connecting their gates together.
• TRIACs are widely used in applications such as light dimmers and motor speed control.