Power Electronics

Topics: Diode, Rectifier, Sine wave Pages: 2 (451 words) Published: August 23, 2012
Power electronics
Objective exam 2011

1.When UJT is used for triggering an SCR, the wave shape of the voltage obtained from UJT firing circuit is a
a) Sine wave (b) saw tooth wave (c)trapezoidal wave (d) Square wave 2. In a thyristor circuit, the angle of conduction can be changed by changing (a) anode Voltage (b) Anode Current (c) Forward Current rating (d)gate Current 3. Freewheeling diodes are mainly used for

(a) Better utilization factor (b) di/dt protection (c) Quick turn on (d) None 4.The value of dc load voltage for a 1-( semi converter under continuous conduction mode is
(a) Vm/((1+cos() (b) Vm/2((1+cos() (c) Vm/((1-cos() (d) Vm/2((1-cos() 5.For continuous conduction each thyristor pair of a two pulse full converter should conduct for
(a) ( (b) (- ( (c) ( + ( (d) (
6.Snubber circuit is used for
(a) Over Voltage protection (b) dv/dt protection
(c) Over current protection (d) None
7.In general the circuit turn off time (tc) must be
(a) Equal to thyristor turn off time tq (b) greater than tq (c) less than tq (d) None 8.Once SCR starts conducting a forward current, its gate loses control over (a) anode circuit voltage only (b) anode circuit current only (c) both anode circuit voltage and current (d) anode circuit voltage, current and time 9.A thyristor mat be turned as

(a) Dc switch (b) Ac switch (c) Either a or b (d0 square wave switch 10. In UJT,with VBB as the voltage across two base terminals the emitter potential at peak point is given
(a) ( VBB (b)(VD (c) ( VBB + VD (d) (VD + VBB

Fill in the blanks
11. The most efficient triggering signal for SCR is a __________________ 12. A 1-( half wave controlled rectifier has sin314t as the input voltage and R as load.For a firing angle of 300 for the thyristor,the average output voltage...
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