Short notes
Objective questions (Answers are given below)
- A unijunction transistor has
(a) anode, cathode and a gate
(b) two bases and one emitter
(c) two anodes and one gate
(d) anode, cathode and two gates. - Which semiconductor device acts like a diode
and two resistors ?
(a) SCR (b) triac
(c) diac (d) UJT. - A UJT has RBB = 10 K and RB2 = 4 K. Its intrinsic
stand-off ratio is
(a) 0.6
(b) 0.4
(c) 2.5
(d) 5/3. - An SCR conducts appreciable current when its
………………….. with respect to cathode.
(a) anode and gate are both negative
(b) anode and gate are both positive
(c) anode is negative and gate is positive
(d) gate is negative and anode is positive. - After firing an SCR, the gating pulse is removed.
The current in the SCR will
(a) remains the same
(b) immediately fall to zero
(c) rise up
(d) rise a little and then fall to zero. - An SCR may be turned OFF by
(a) interrupting its anode current
(b) reversing polarity of its anode- cathode
voltage
(c) low-current dropout
(d) all of the above. - A triac behaves like two
(a) inverse parallel-connected SCRs with
common gate
(b) diodes in series
(c) four-layer diodes in parallel
(d) resistors and one diode. - A triac can be triggered into conduction by
(a) only positive voltage at either anode
(b) positive or negative voltage at either anode
(c) positive or negative voltage at gate
(d) both (b) and (c).
- A diac is equivalent to a
(a) pair of SCRs
(b) pair of four-layer SCRs
(c) diode and two resistors
(d) triac with two gates. - An SCS has
(a) four layers and three terminals
(b) three layers and four terminals
(c) two anodes and two gates
(d) one anode, one cathode and two gates. - An SCS may be switched ON by a
(a) positive pulse at its anode
(b) negative pulse at its cathode
(c) positive pulse at its cathode gate G2
(d) positive pulse at its anode gate G1. - The dv / dt effect in an SCR can result in
(a) high rate-of-rise of anode voltage
(b) increased junction capacitance
(c) false triggering
(d) low capacitive charging current. - The di / dt effect in an SCR leads to the formation
of
(a) local hot spots
(b) conduction zone
(c) charge spreading zone
(d) none of the above. - SCR turns OFF from conducting state to blocking
state on
(a) reducing gate current
(b) reversing gate voltage
(c) reducing anode current below holding
current value
(d) applying ac to the gate - When a thyristor is negatively biased,
(a) all the three junctions are negatively
biased
(b) outer junctions are positively biased and
the inner junction is negatively biased
(c) outer junctions are negatively biased
and the inner junction is positively
biased
(d) the junction near the anode is negatively
biased and the one near the cathode is positively
biased
- A LASCR is just like a conventional SCR except
that it
(a) cannot carry large current
(b) can also be light-triggered
(c) has no gate terminal
(d) cannot be pulse-triggered. - The minimum value of current required to maintain
conduction in an SCR is called its ……………..
current.
(a) commutation
(b) holding
(c) gate trigger
(d) breakover - Diacs are primarily used as
(a) pulse generators
(b) triggering devices
(c) surge protection devices
(d) power thyristors.
- (b) 2. (d) 3. (a) 4. (b) 5. (a) 6. (d) 7. (a) 8. (d) 9. (b) 10. (d) 11. (c) 12.(c) 13. (a) 14. (c) 15. (c) 16. (b) 17. (b) 18. (b)
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