Electronics Devices & Circuits

Thyristor Family Viva Questions

Thyristor Family Viva Questions

Thyristor Family Viva Questions, Short Questions on Thyristor Family, Viva Questions on SCR, Triac Viva Questions, Gate Turn ON (GTO) Viva Questions, SCR Viva Questions, Diac Viva Questions, Viva Questions on IGBT, Electronics Device Viva Questions, Engineering Viva Questions

Sinusoidal Oscillator Viva Questions

Short Questions with Answers

Q.1. Write the important features of SCR.

Ans. Important Features of SCR

  • It is a latching type device.
  • It can handle a very large power.
  • It is a current controlled device, because the gate current controls SCR.
  • It acts as an open or closed switch.
  • The on state voltage drop is very low.
  • It can handle thousands of amperes of current.

Q.2. Write the difference between holding and latching currents.

Ans. Difference between Holding and Latching Currents

  • The latching current is important only at the time of SCR turn on, whereas holding current is important only at the time of SCR turn off.
  • If the anode current goes below the holding current then the conducting SCR turns off. The holding current is defined for IG = 0, whereas if anode current at the time of turn on is higher than latching current then only SCR will latch into its on state.
  • If IA < IL, then SCR does not latch. It will remain on as long as IG is being supplied. As soon as IG is removed, SCR will turn off.
  • Latching current is always higher than the holding current.
  • The values of IL and IH are constant. They do not depend on the gate current magnitude.

Q.3. What are SCR turn-on methods?

Ans. SCR is turned on by increasing the anode current. This can be achieved with one of the following methods:

  1. By raising the temperature
  2. By focusing light
  3. By applying high voltage
  4. By applying voltage at high dv/dt
  5. By increasing the gate current.

Q.4. What are the salient features of Gate Turn ON?

Ans. The gate turn on has the following salient features:

  • Less power consumption.
  • It is possible to turn on the SCR precisely at the desired value of anode to cathode voltage.
  • If the supply voltage is AC, then it is possible to adjust the firing angle a to the desired value.
  • Triggering circuits are easy to design.
  • Turn on gain is very high. Turn on gain is the ratio of anode current to the gate current.
  • Pulsed triggering circuit provide greater flexibility.

Q.5. What should be the precautions to Avoid SCR damage?

Ans. Precautions to Avoid SCR Damage

  • In order to avoid SCR damage or failure, the rate of anode current rise diF/dt should he kept less than the maximum value given in SCR specification sheet. The failure of the device may take place due to excessive power dissipation that takes place during the time interval tr.
  • This power dissipation can be reduced by applying large values of gate current during the td(on) and tr intervals. For this reason, the gate current applied is often large at the start of the turn on interval and is gradually reduced as time proceeds.

Q.6. Write the Drawbacks of SCR.

Ans. Drawbacks of SCR

  • It can conduct only in one direction. Hence, it can control power only during one half cycle of ac.
  • It can turn on accidentally due to high dv/ dt of the source voltage.
  • It is not easy to turn off the conducting SCR. We have to use special circuits called commutation circuits to turn off SCR.
  • SCR cannot be used at high frequencies. The maximum frequency of its operation is 400 Hz.
  • Gate current cannot be negative.

Q.7. Write the advantages of SCR.

Ans. Advantages of SCR

  • It can handle large voltages, currents and power.
  • The voltage drop across conducting SCR is small. This will reduce the power dissipation in the SCR.
  • Easy to turn on.
  • Triggering circuits are simple.
  • It can he protected with the help of a fuse.
  • We can control the power delivered to the load.

Q.8. Write the applications of SCR?

Ans. Applications of SCR

  • Controlled rectifiers
  • DC to DC converters or choppers
  • DC to AC converters or inverters
  • As static switch
  • Battery charger
  • Speed control of DC and AC motors
  • Lamp dimmers
  • AC voltage stabilizers.

Q.9. Write the salient features of Triac.

Ans. Salient Features of Triac

  • It is a bi-directional device. So, we can control the power delivered to load in both the half cycles of ac supply.
  • It is equivalent to two SCRs connected back to back.
  • We can turn it on by using a positive as well as negative gate current.
  • It is more suitable for resistive loads.
  • It is more economical than SCRs, since inside the same package, we get two SCRs connected back to back.
  • It can control power delivered to ac loads such as fan motor.
  • It is not necessary to use a protection diode across the Triac.
  • We can use a single heat sink.

Q.10. Write the drawbacks of Triac.

Ans. Drawbacks of Triac

  • We cannot use it as a controlled rectifier.
  • Low dv/dt rating than SCR. So possibility of accidental turn on is higher than that of SCR.
  • Low di/dt rating.
  • Suitable for resistive loads only. Not suitable for controlling power to highly inductive loads.
  • Its power rating is lower than that of SCR.
  • Triggering circuits require to be designed more carefully.

Q.11. Write the applications of Triac.

Ans. Applications of Triac

  • As a static switch
  • In the AC voltage stabilizers
  • Fan speed regulator
  • Lamp dimmer
  • Flasher circuit
  • Proximity detector

Q.12. What is meant by Diac?

Ans. Diac is one of the most important devices used to trigger the Triac. It is sometimes called a Triac without gate terminal.

Diac is basically used as a triggering device for Triac. The following control systems are Diac for this purpose:

  1. Lamp Dimmer
  2. Fan speed regulator
  3. Temperature controller

Q.13. Write the drawbacks of Diac.

Ans.  Drawbacks of Diac

  • It is a low power device.
  • It does not have a control terminal.

Q.14. Write the salient features of IGBT.

Ans. Salient features of IGBT

  • Low on state voltage drop.
  • Low on state power loss.
  • It has a higher switching frequency than that of a power BJT.
  • IGBT has the best qualities of BJT and MOSFET.  

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