Deck 40: Thyristors

Question 1

The SCR of Fig. 40-23 has V GT =1.0 V, I GT = 2 mA, and I H = 12 mA. What is the output voltage when the SCR is off What is the input voltage that triggers the SCR If V CC is decreased until the SCR opens, what is the value of V CC

Answer 1

Refer to circuit diagram of SCR in Figure 40-23 in the textbook.
The SCR is off means no current flows through the circuit. In such cases, input voltage is equal to the output voltage. Here, is the output voltage of the circuit is the input voltage of the circuit
Write the expression for the input voltage needed to trigger an SCR. Here, is the gate triggering voltage is the gate triggering current is the gate resistance
After the SCR is triggered on, it can be turned off only when the supply voltage is reduced to a lower value and it is the turn off voltage of the thyristor and is given by, Here, is the holding current is the load resistance
Convert the units for gate triggering current from mA to A. Convert the units for holding current from mA to A. Convert the units for gate resistance from to . Input voltage is equal to the output voltage when the SCR is off. Substitute 12 V for in the equation. Therefore, the output voltage, when the SCR is off is .
Write the expression for the input voltage needed to trigger an SCR. Substitute, 1.0 V for , for , and for in the equation. Therefore, the input voltage, needed to trigger the SCR is .
Calculate the decreased value of . Substitute, for , and for in the equation. Therefore, the decreased value of is .

Question 2

A thyristor can be used as

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

Answer 2

A Thyristor is a semi-conducting device which can be used as a switch. The different types of thyristors are silicon controlled rectifier, triac etc.
Thus, a thyristor can be used as a switch.
Hence, the correct option is .
A thyristor is not a resistor, it does not amplify and it is not a power source. Hence, the other options are not correct.

Question 3

What is the peak output voltage in Fig. 40-24 if R is adjusted to 500

Answer 3

NO ANSWER

Question 4

Positive feedback means that the returning signal

A) opposes the original change.
B) aids the original change.
C) is equivalent to negative feedback.
D) is amplified.

Question 5

If the SCR of Fig. 40-23 has a gate trigger voltage of 1.5 V, a gate trigger current of 15 mA, and a holding current of 10 mA, what is the input voltage that triggers the SCR The supply voltage that resets the SCR

Question 6

A latch always uses

A) transistors.
B) negative feedback.
C) current.
D) positive feedback.

Question 7

In Fig. 40-24, R is adjusted to 750 . What is the charging time constant for the capacitor What is the Thevenin resistance facing the gate

Question 8

To turn on a four-layer diode, you need

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

Question 9

The resistor R 2 in Fig. 40-25 is set to 4.6 k . What are the approximate firing and conduction angles for this circuit How much ac voltage is across C

Question 10

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

A) holding current.
B) trigger current.
C) breakover current.
D) low-current dropout.

Question 11

Using Fig. 40-25, when adjusting R 2 , what are the minimum and maximum firing angle values

Question 12

The only way to stop a four-layer diode that is conducting is by

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

Question 13

What are the minimum and maximum conduction angles of the SCR in Fig. 40-25

Question 14

The minimum anode current that keeps a thyristor turned on is called the

A) holding current.
B) trigger current.
C) breakover current.
D) low-current dropout.

Question 15

Calculate the supply voltage that triggers the crowbar of Fig. 40-26.

Question 16

A silicon controlled rectifier has

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

Question 17

If the zener voltage in Fig. 40-26 is changed from10 to 12 V, what is the voltage that triggers the SCR

Question 18

An SCR is usually turned on by

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

Question 19

The diac of Fig. 40-27 has a breakover voltage of 20 V, and the triac has a V GT of 2.5 V. What is the capacitor voltage that turns on the triac

Question 20

SCRs are

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

Question 21

What is the load current in Fig. 40-27 when the triac is conducting

Question 22

The usual way to protect a load from excessive supply voltage is with a

A) crowbar.
B) zener diode.
C) four-layer diode.
D) thyristor.

Question 23

An RC snubber protects an SCR against

A) supply over voltages.
B) false triggering.
C) breakover.
D) crowbarring.

Question 24

When a crowbar is used with a power supply, the supply needs to have a fuse or

A) adequate trigger current.
B) holding current.
C) filtering.
D) current limiting.

Question 25

The diac is a

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

Question 26

The triac is equivalent to

A) a four-layer diode.
B) two diacs in parallel.
C) a thyristor with a gate lead.
D) two SCRs in parallel.

Question 27

Any thyristor can be turned on with

A) breakover.
B) forward-bias triggering.
C) low-current dropout.
D) reverse-bias triggering.

Question 28

The trigger voltage of an SCR is closest to

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

Question 29

Any thyristor can be turned off with

A) breakover.
B) forward-bias triggering.
C) low-current dropout.
D) reverse-bias triggering.

Question 30

Exceeding the critical rate of rise produces

A) excessive power dissipation.
B) false triggering.
C) low-current dropout.
D) reverse-bias triggering.

Question 31

A four-layer diode is sometimes called a

A) unijunction transistor.
B) diac.
C) pnpn diode.
D) switch.

Question 32

An SCR can switch to the on state if

A) its forward breakover voltage is exceeded.
B) I GT is applied.
C) the critical rate of voltage rise is exceeded.
D) all of the above.

Question 33

The maximum fi ring angle with a single RC phase control circuit is

A) 45°.
B) 90°.
C) 180°.
D) 360°.

Question 34

A triac is generally considered most sensitive in

A) quadrant I.
B) quadrant II.
C) quadrant III.
D) quadrant IV.

Question 35

An IGBT is essentially a

A) BJT on the input and MOSFET on the output.
B) MOSFET on the input and MOSFET on the output.
C) MOSFET on the input and BJT on the output.
D) BJT on the input and BJT on the output.

Question 36

The maximum on-state output voltage of an IGBT is

A) V GS(on ).
B) V CE(Sat).
C) R DS(on).
D) V CES.