Deck 11: Thyristors

Question 1

The purpose of an SCR crowbar circuit is to protect a load in case of

A)an open circuit
B)overvoltage
C)power failure
D)all of the above

Answer 1

B

Question 2

Assume that a UJT has y = 0.70 when VBB = 10 V. The peak- point emitter voltage is

A)7.0 V
B)7.7 V
C)6.3 V
D)0.7 V

Answer 2

B

Question 3

In an SCR, reverse- breakdown voltage VBR(R)is defined at the start of the

A)zener region
B)forward blocking region
C)avalanche region
D)reverse blocking region

Answer 3

C

Question 4

Figure 2
Refer to Figure 2. The symbol is for a

A)a diac
B)a UJT
C)an SCS
D)none of the above

Question 5

Figure 5
Refer to Figure 5. The signal you expect to see on the oscilloscope is a

A)sine wave
B)a square wave
C)modified sawtooth wave
D)series of positive trigger pulses

Question 6

To stop conduction in an SCR, you could

A)remove gate current
B)apply a negative pulse to the gate
C)both A and B
D)none of the above

Question 7

The internal base1 resistance for a UJT (r'B1)is controlled by the

A)emitter voltage
B)base2 current
C)emitter current
D)peak point voltage

Question 8

The voltage drop across a conducting SCR

A)is half the load voltage
B)is a small negative voltage
C)follows the input voltage
D)is low

Question 9

Figure 4
Refer to Figure 4. The symbol for an SCS is

A)(a)
B)(b)
C)(c)
D)(d)

Question 10

A diac is equivalent to two back- to- back inverted

A)triacs
B)four- layer diodes
C)SCRs
D)SCSs

Question 11

Figure 5
Refer to Figure 5. If the value of R3 is reduced, the output amplitude will _ and the frequency will .

A)decrease, increase
B)decrease, decrease
C)increase, decrease
D)increase, increase

Question 12

Figure 4
Refer to Figure 4. The symbol for a diac is

A)(a)
B)(b)
C)(c)
D)(d)

Question 13

Figure 5
Refer to Figure 5. Q1 is a

A)PUT
B)UJT
C)SCR
D)SCS

Question 14

The forward breakover voltage for an SCR

A)is a constant quantity specified by the manufacturer
B)is proportional to VAK
C)is dependent on the maximum specified anode current
D)decreases for increasing IG

Question 15

A device that does not have a gate terminal is

A)a triac
B)a diac
C)a LASCR
D)an SCS

Question 16

A device that can be triggered on or off is

A)a triac
B)an SCS
C)both A and B
D)none of the above

Question 17

The number of pn junctions in a UJT is

A)zero
B)one
C)two
D)three

Question 18

If a SCR is triggered near 45° of the ac cycle, it will conduct for approximately

A)135°
B)225°
C)315°
D)90°

Question 19

Figure 1
Refer to Figure 1. The type of device that is illustrated by this structure is

A)an SCS
B)a UJT
C)a diac
D)none of the above

Question 20

A diac will stop conducting if

A)the peak of the applied waveform is reached
B)low- current dropout occurs
C)it receives a negative gate voltage
D)all of the above

Question 21

A device that can be used to control a dc motor from an ac source is

A)a UJT
B)a diac
C)an SCR
D)none of the above

Question 22

Figure 3
Refer to Figure 3. The characteristic is for

A)a diac
B)a UJT
C)a triac
D)an SCR

Question 23

LASCR stands for low- amplitude silicon controlled rectifier.

Question 24

Increasing the firing angle in an SCR control circuit

A)increases the SCR current
B)increases the holding current
C)increases the output current
D)none of the above

Question 25

An equivalent circuit for a UJT is back- to back npn and pnp transistors.

Question 26

A diac can conduct current in either direction.

Question 27

The characteristic of a UJT that determines its turn- on point is called the

A)blocking point
B)standoff ratio
C)forward breakover voltage
D)trigger current

Question 28

A device that has a negative resistance region is a

A)a diac
B)an SCS
C)a UJT
D)none of the above

Question 29

A SCR can be turned on

A)by exceeding the forward breakover voltage
B)by forced commutation
C)both A and B
D)none of the above

Question 30

The conduction angle of an SCR is controlled by the

A)anode voltage
B)timing of gate current pulse
C)load voltage
D)none of the above

Question 31

An SCR can conduct current in either direction.

Question 32

The gate terminal of a PUT always is biased on.

Question 33

The forward- blocking region of a four- layer diode is from 0 V to VAK = VBR(F).

Question 34

A triac is essentially a bidirectional electronic switch.

Question 35

The IH specification is the highest allowed current in a thyristor.