Quiz: Electrical Engineering
Topic: Power Electronics
Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minutes
Q1. The DIAC is primarily used as:
(a) Pulse generator
(b) Surge protector
(c) Triggering device
(d) Power thyristor
Q2. An inverter whose input voltage remains constant is called:
(a) Power source inverter
(b) Current source inverter
(c) Pulse-width modulated inverter
(d) Voltage source inverter
Q3. A Silicon-controlled rectifier (SCR) conducts appreciable current when its …………. with respect to cathode.
(a) Gate is negative and anode is positive
(b) Gate is positive and anode is negative
(c) Gate and anode are both negative
(d) Gate and anode are both positive
Q4. The minimum value of current required to maintain conduction in a silicon-controlled rectifier (SCR) is called its……………current.
(b) Gate trigger
Q5. An SCR is a
(a) four-layer, four junction Device
(b) four-layer, three junction Device
(c) four-layer, two junction Device
(d) three-layer, single junction Device
Q6. Choose the false statement.
(a) SCR is a bidirectional device
(b) SCR is a controlled device
(c) In SCR the gate is the controlling terminal
(d) SCR are used for high-power applications
Q7. A single-phase full bridge inverter is fed from a 314 V battery and is delivering power to a pure resistance load what is the value of fundaments output voltage?
(a) 200 V
(b) 282.8 V
(c) 100 V
Q8. RC snubber circuit is used to limit the rate of
(a) Rise of current in SCR
(b) Rise of voltage across SCR
(c) Conduction period
(d) All of the above
Q9. The thyristor is turned -off when the anode current falls below_____
(a) Forward current
(b) Latching current
(c) Holding current
(d) Breakover current
Q10. A Triac is a
(a) 2-terminal switch
(b) 2-terminal bilateral switch
(c) 3-terminal bilateral switch
(d) 3-terminal bidirectional switch
Sol. DIAC symbol:
A DIAC is a three-layer, two-terminal bidirectional device. used as a triggering device to control the gate current of a triac.
When the input DC voltage remains constant, then it is called Voltage Source Inverter (VSI) or Voltage Fed Inverter (VFI).
When input current is maintained constant, then it is called Current Source Inverter (CSI) or Current Fed Inverter (CFI).
VSI is mainly used to convert a constant DC voltage into an AC voltage with variable magnitude and frequency.
Sol. A Silicon-controlled rectifier (SCR) conducts appreciable current when its Gate and anode are both positive with respect to cathode.
Sol. Holding Current I_H :
This is the minimum anode current required to maintain the thyristor in the on state. To turn off a thyristor, the forward anode current must be reduced below its holding current for a sufficient time for mobile charge carriers to vacate the junction.
Latching Current I_L:
This is the minimum anode current required to maintain the thyristor in the on-state
immediately after a thyristor has been turned on and the gate signal has been removed.
Sol. SCR is a four-layer p-n-p-n device which forms three p-n junctions.
Sol. SCR is a unidirectional device, current only flows from anode to cathode.
Sol. The output dc voltage of single-phase full bridge inverter is given as
Eo =(2√(2 ))/π E_dc
E_dc = Input DC voltage = 314 V
Eo = 2√2 × 314/π
Eo = 282.8 V
Sol. The thyristors can be protected against transient voltages by an RC network This RC network is connected in parallel across the thyristor. It is called a snubber circuit.
Whenever there is a large spike or voltage transient across the thyristor, it is absorbed by the RC circuit. The RC circuit (snubber) acts as a lowpass filter for this voltage transient.
Sol. latching current is associated with turn-on process and holding current that of turn-off process.
The latching current is the minimum value of anode current to trigger or turn ON the thyristor from its OFF state to ON state even after the trigger pulse is removed. To trigger an SCR, the anode current must be built up to the latching current before the gate pulse is removed.
The holding current is the minimum value of the anode current to hold the thyristor in ON state. During turn OFF, the anode current should be below the holding current.
Sol. The major drawback of an SCR is that it can conduct current in one direction only.
The triac also differs from the SCR in that either a positive or negative gate signal trigger it into conduction. Thus, the triac is a three terminal, four layers, directional, the semiconductor device that controls ac power whereas an SCR controls dc power or forward biased half cycles of ac in a load. Because of its bidirectional conduction property, the triac is widely used in the field of power electronics for control purposes.