Deck 11: Thyristors

Question 1

A thyristor can be used as

A)a switch.
B)a power source.
C)a resistor.
D)an amplifier.

Answer 1

A

Question 2

If an SCR starts to conduct when a gate current is established, when the gate circuit is interrupted the SCR

A)gate current will increase.
B)will turn off.
C)will continue to conduct.
D)current will increase.

Answer 2

C

Question 3

A triac acts like

A)two diacs in parallel.
B)two SCRs in series.
C)a normal transistor.
D)two SCRs in parallel.

Answer 3

D

Question 4

If the gate circuit is open on a triac used for ac motor control, the motor

A)will constantly run at half speed.
B)will run at 60 RPM.
C)can never turn on.
D)will constantly run at full speed.

Question 5

Refer to d)in the figure above. The symbol is for

A)a triac.
B)a UJT.
C)a diac.
D)a PUT.
E)an SCR.

Question 6

The minimum input current that can turn on a thyristor is called the

A)breakdown current.
B)switching current.
C)low- current dropout current.
D)holding current.

Question 7

A triac

A)cannot be triggered with gate voltages.
B)can be triggered by either a positive or a negative gate voltage.
C)can trigger only on negative gate voltages.
D)can trigger only on positive gate voltages.

Question 8

An SCR will turn off when the voltage across it is

A)decreased by the value of the gate voltage.
B)decreased to near 0 V.
C)increased to the supply voltage.
D)timed out.

Question 9

Once a diac is conducting, the only way to turn it off is with

A)low- current dropout.
B)a positive gate voltage.
C)breakover.
D)a negative gate voltage.

Question 10

A conducting SCR can be turned off by

A)temporarily shorting across the SCR's anode to cathode.
B)reducing the source voltage to zero.
C)opening the anode circuit.
D)All of the above.

Question 11

The only way to make a four- layer diode conduct is with

A)low- current dropout.
B)forward breakover voltage.
C)a trigger input applied to the gate.
D)None of the above.

Question 12

An SCR is used to control the speed of a dc motor by the _ of the pulse delivered to the motor.

A)decreasing, gate width
B)varying, width
C)increasing, amplitude
D)None of the above.

Question 13

Disconnecting the gate lead on an SCR

A)destroys the device.
B)will result in maximum gate current.
C)means that the SCR cannot be triggered on.
D)results in the SCR staying on all the time.

Question 14

The diac is equivalent to

A)two triacs in parallel.
B)two four- layer diodes in series.
C)two SCRs in parallel.
D)two four- layered diodes in parallel.

Question 15

The trigger voltage of an SCR is closest to

A)0 V.
B)4 V.
C)0.7 V.
D)breakover voltage.

Question 16

A UJT has

A)one emitter lead.
B)two base leads.
C)two emitter and one base leads.
D)both A and B above.

Question 17

A device is needed to trigger an SCR. One possible device to use might be an)

A)Schockley diode.
B)SCR.
C)UJT.
D)PUT.

Question 18

A silicon- controlled switch

A)has two gate leads.
B)has only 1 gate lead.
C)can only be turned off with low- current dropout.
D)is bidirectional.

Question 19

The best choice to shut down a dc power supply in case of an abnormally high voltage condition is an)

A)diac.
B)PUT.
C)triac.
D)SCR.
E)UJT.

Question 20

The diac is an)

A)unidirectional device.
B)transistor.
C)bidirectional device.
D)three- layer device.

Question 21

The voltage across a conducting SCR

A)equals the source voltage.
B)equals the breakover voltage.
C)is approximately zero.
D)decreases with more anode current.

Question 22

A good choice to trigger a triac would be an)

A)diac.
B)Schockley diode.
C)4- layer diode.
D)SCR.

Question 23

Of the following applications, the likely one to use a diac is an)

A)high frequency amplifier.
B)lamp dimmer.
C)battery charger.
D)oscillator.

Question 24

The only way to stop an SCR that is conducting is by

A)low- current drop out.
B)a positive trigger.
C)breakover.
D)reverse- bias triggering.

Question 25

The best device to be used to control a dc motor is the

A)PUT.
B)diac.
C)triac.
D)SCR.

Question 26

A triac is used for ac because

A)it is turned off when the ac voltage reaches zero.
B)it can deliver more power to the load in a cycle.
C)it conducts on both alternations.
D)All of the above.

Question 27

Refer to a)in the figure above. The symbol is for

A)a triac.
B)a UJT.
C)a diac.
D)a PUT.
E)an SCR.

Question 28

An SCS has a unique ability to

A)control a PUT.
B)be turned off only with a pulse.
C)be turned on or off with a pulse.
D)All of the above.

Question 29

Refer to b)in the figure above. The symbol is for

A)a triac.
B)a UJT.
C)a diac.
D)a PUT.
E)an SCR.

Question 30

The PUT Programmable Unijunction Transistor)is actually a type of

A)triac.
B)thyristor.
C)FET device.
D)SCR.

Question 31

To avoid confusing the schematic symbols for a JFET and a UJT, keep in mind that the UJT symbol

A)has four terminals.
B)has no arrow.
C)has an arrow at an angle.
D)has two circles.
E)uses a dot.

Question 32

A silicon controlled rectifier has

A)two external leads.
B)three external leads.
C)three doped regions.
D)four external leads.

Question 33

The most likely device to be used in an oscillator is an)

A)PUT.
B)UJT.
C)SCR.
D)triac.
E)diac.

Question 34

Each of the following has a gate except the

A)SCR.
B)triac.
C)PUT.
D)diac.

Question 35

The best choice to control a small variable speed ac motor is an)

A)SCR.
B)PUT.
C)diac.
D)triac.
E)UJT.

Question 36

For a UJT, the intrinsic standoff ratio is

A)always less than 1.
B)always greater than 1.
C)equal to 1.
D)usually around 1 k▲ or so.

Question 37

The minimum current that keeps a latch closed is called the

A)holding current.
B)pick- up current.
C)trigger current.
D)critical rate of current rise.

Question 38

An SCR crowbar circuit is used to

A)control three phase ac motors.
B)protect against overvoltage conditions.
C)detect when a circuit is overheating.
D)protect against undervoltage conditions.

Question 39

SCRs are

A)bidirectional.
B)four- layer diodes.
C)high- current devices.
D)low- power devices.

Question 40

An SCR is usually turned on by

A)breakover.
B)holding current.
C)a gate trigger.
D)breakdown.