Deck 14: Time Response of Reactive Circuits

Question 1

Only the number of turns in the primary and secondary of a transformer determines the actual secondary Voltage.

Answer 1

False

Question 2

The impedance matching characteristic for a transformer is needed for a situation Where maximum power transfer to the load is desired.

Answer 2

True

Question 3

A typical transformer fault Would be an open Winding.

Answer 3

True

Question 4

An ideal 10:1 transformer With 20 VA of power has less than 2 VA on the output.

Question 5

The phrase ʺMaximum power is delivered to the loadW hen the load resistance equals the source resistanceʺ is a definition of ________.

A)Lenzʹs Law
B)the maximum power transfer theorem
C)Kirchhoffʹs Law
D)Ohmʹs Law

Question 6

What is the secondary voltage in Figure 14-1 if the turns ratio is 7:1?

A) 840 V_RMS
B) 8.59 V_RMS
C) 17.1 V_RMS
D) 420 V_RMS

Question 7

Eddy current loss in a transformer is ________.

A)due to current flowing in the core
B)caused by rapid reversal of the magnetic field
C)another name for flux leakage loss
D)caused by the resistance of the Wire

Question 8

An ideal transformer has no power loss.

Question 9

A core material is not always necessary for proper operation of a transformer.

Question 10

A step-down transformer could have a primary-secondary turns ratio of 4:1.

Question 11

The hysteresis loss in a transformer is ________.

A)caused by the resistance of the Wire
B)caused by rapid reversal of the magnetic field
C)another name for flux leakage loss
D)caused by current flowing in the core

Question 12

If a dc voltage is input to a transformerʹs primary, then an ac voltage is induced in its secondary.

Question 13

The primary to secondary resistance should be relatively low.

Question 14

What is the secondary voltage in Figure 14-1 if the turns ratio is 1: 3 ?

A) 40 V_RMS
B) 240 V_RMS
C) 360 V_RMS
D) 80 V_RMS

Question 15

A transformer can be used as an impedance matching device.

Question 16

Autotransformers cannot be used for isolation purposes since there is only one Winding.

Question 17

Transformer cores are made from laminated iron to reduce losses.

Question 18

A transformer With a turns ratio of 1:7 is a step down transformer.

Question 19

The efficiency of all transformers is very low.

Question 20

A transformer With a turns ratio of 1:1 is often used to isolate a load from a source.

Question 21

What is the primary current in Figure 14-1 if I_S = 40 mA and the turns ratio equals 4:1 ?

A)160 mA
B)4 mA
C)40 mA
D)10 mA

Question 22

What is the secondary current in Figure 14-1 with a 50 load resistor and 4: 1 turns ratio?

A) 600 mA
B) 1.66 A
C) 9.6 A
D) 4.8 A

Question 23

What is the reflected resistance seen by the primary in Figure 14-1 if the turns ratio is 4: 1 and IS is 40 mA ?

A) 16
B) 4
C) 8
D) 12

Question 24

What is the secondary voltage in Figure 14-1 if the turns ratio is 4.5:1?

A)26.7V
B)4.72V
C)540V
D)5.92V

Question 25

When a 1 $\mathrm { k } \Omega$ load resistor is connected across the secondary winding of a transformer with a turns ratio of 1: 2 , the source "sees" a reflected load of______________

A) 1 $\mathrm { k } \Omega$
B) 2 $\mathrm { k } \Omega$
C) 4 $\mathrm { k } \Omega$
D) 250 $\Omega$

Question 26

In Figure 14-1, if the secondary current is 50 mA, the primary current is ________.

A)300 mA
B)150 mA
C)48.7 mA
D)50 mA

Question 27

In Figure 14-1, the primary to secondary turns ratio is 1:3 and the secondary current equals 120 mA.What is the primary current?

A)180 mA
B)40 mA
C)360 mA
D)cannot compute because theVoltage is not given

Question 28

What is the secondary voltage in Figure 14-1 if the turns ratio is 9:1?

A)26.6V
B)106V
C)13.3V
D)53.2V

Question 29

In Figure 14-1, the primary to secondary turns ratio is changed to 2.5:1 and the secondary current equals 100 mA . What is the reflected resistance seen by the primary?

A) 3
B) 5
C) 1.2
D) 2.5

Question 30

$$\text { Figure } 14 < 1$$

-In Figure 14-1, if the primary to secondary turns ratio is 7.5:1, the output voltage equals________

A) 32 V_RMS
B) 16 V_RMS
C) 900 V_RMS
D) 9 V_RMS

Question 31

What is the secondary current in Figure 14-1 with a 100 load resistor and 3:1 turns ratio?

A) 33 mA_RMS
B) 400 mA_RMS
C) 40 mA_RMS
D) 330 mA_RMS

Question 32

In Figure 14-1, if the primary to secondary turns ratio is changed to 4:1 and a 1 kΩ load resistor is in the secondary, the secondary current equals ________.

A)30 mA
B)240 mA
C)120 mA
D)480 mA

Question 33

$$\text { Figure } 14 < 1$$

-In Figure 14-1, if the secondary current is 50 mA , the reflected resistance seen by the primary equals_____

A) 7.2 $\mathrm { k } \Omega$
B) 14.4 $\mathrm { k } \Omega$
C) 800 $\Omega$
D) 2.4 $\mathrm { k } \Omega$

Question 34

$$\text { Figure } 14 < 1$$

-In Figure 14-1, if the primary to secondary turns ratio is 9:1 and a 1 $\mathrm { k } \Omega$ load resistor is placed in the secondary, the reflected resistance seen by the primary equals_______

A) 111 $\Omega$
B) 162 $\mathrm { k } \Omega$
C) 81 $\mathrm { k } \Omega$
D) 1 $\mathrm { k } \Omega$

Question 35

$$\text { Figure } 14 < 1$$

-What is the reflected resistance seen by the primary in Figure 14-1 if the turns ratio is 3: 1 and I_S is 240 mA ?

A) 9000 $\Omega$
B) 1500 $\Omega$
C) 167 $\Omega$
D) 500 $\Omega$

Question 36

What is the primary current in Figure 14-1 if the turns ratio equals 1: 3 and Is equals 240 mA ?

A) 1.2 A
B) 720 mA
C) 120 mA
D) 60 mA

Question 37

In Figure 14-1, if there are five times more turns in the primary than in the secondary, then What is the secondary voltage?

A)120V
B)12V
C)24V
D)600V

Question 38

When a 1 $\mathrm { k } \Omega$ load resistor is connected across the secondary winding of a transformer with a turns ratio of 2: 1 , the source "sees" a reflected load of________

A) 4 $\mathrm { k } \Omega$
B) 500 $\Omega$
C) 2 $\mathrm { k } \Omega$
D) 1 $k \Omega$

Question 39

If 10W of power are applied to the primary of an ideal transformer With a turns ratio of 1:5, the power delivered to the secondary load is ________.

A)0.5W
B)0W
C)10W
D)50W

Question 40

$$\text { Figure } 14 < 1$$

-In Figure 14-1, if a load resistor of 75 $$\Omega$$ is placed in the secondary circuit,What is the secondary current?

A)533 mA
B)16 A
C)4.8 A
D)5.33 A

Question 41

The reflected load in a transformer is defined as the actual load as it appears to the source and results from:

A)theVoltage ratio.
B)the power ratio.
C)the current ratio.
D)the turns ratio.

Question 42

A)150
B)12.2
C)3.06
D)The primary should have less turns than the secondary.

Question 43

WhichW indingW ill have the lowest measured resistance in a transformerW ith four secondary Windings?

A)the 5 VSecondary
B)the 12 VSecondary
C)the 6.3 VSecondary
D)the 550 VSecondary

Question 44

A step-up transformerW ill increase ________ and decrease ________.

A)power, current
B)current, impedance
C)voltage, impedance
D)voltage, power

Question 45

What is the likeliestW ay to fix a center-tapped transformerW hich outputs unequal voltages on each half of the secondary?

A)Replace the transformer.
B)Remove some turns from the half with the higherVoltage winding.
C)Add some turns to the half wiIth the lowerVoltage winding.
D)Do not concern yourself; the customer will probably not notice the problem.

Question 46

A matching transformer's primary needs______ more turns than its secondary to match a 600 $\Omega$ audio signal distribution line to an 8 $\Omega$ speaker.

A) 0.013
B) 8.66
C) 75
D) 0.115

Question 47

The small dot placed on schematics of transformers indicates:

A)the phase of theVoltages.
B)the polarity of theVoltages.
C)the direction of the winding.
D)all of these.

Question 48

The turns ratio required to match a 50 $\Omega$ source to a 200 $\Omega$ load is_____

A) 4: 1
B) 1: 2
C) 1: 4
D) 2: 1

Question 49

If a 12 V battery is connected across the primary of a transformerW ith a turns ratio of 1:4, then the secondary voltage is ________.

A)3V
B)0V
C)12V
D)48V

Question 50

A step-up transformerW ill increase ________ and decrease ________.

A)impedance, current
B)current,Voltage
C)voltage, impedance
D)current, impedance

Question 51

For a transformer to operate properly, it must:

A)be supplied with the proper AC signal.
B)work within itsVoltage, current and power ratings.
C)be connected to a load.
D)all of these.

Question 52

What is the problem if a power transformer is delivering a low output voltage even though the primary voltage is correct?

A)a partially shorted secondary
B)a shorted primary
C)an open primary
D)an open secondary

Question 53

The turns ratio of a transformer (n)is defined as:

A)the number of turns in the secondary divided by the number of turns in the primary.
B)the number of turns in the primary divided by the number of turns in the secondary.
C)the number of turns in the primary multiplied by the number of turns in the secondary.
D)the number of turns in the secondary multiplied by the number of turns in the primary.

Question 54

The relationship for the voltage ratio $\left( \mathrm { V } _ { \mathrm { sec } } / \mathrm { V } \text { pri } \right)$ in a transformer is:

A) the same as the turns ratio $\left(\mathrm{Nsec}/{\mathrm{Nspri}}\right)$
B) opposite to the turns ratio $\text { (Nsec/Npri) }$
C) more than the turns ratio $\left( \mathrm { N } _ { \mathrm { sec } } / \mathrm { N } _ { \mathrm { pri } } \right)$
D) less than the turns ratio $\left( N _ { \mathrm { sec } } / \mathrm { N } _ { \text {pri } } \right)$

Question 55

The relationship for the current ration in a transformer is:

A) less than the turns ratio
B) the inverse of the turns ratio
C) more than the turns ratio
D) the same as the turns ratio

Question 56

The relationship for the impedance matching ratio in a transformer is:

A) equal to the turns ratio
B) opposite to the turns ratio squared.
C) equal to the turns ratio squared.
D) opposite to the turns ratio

Question 57

When the turns ratio of a transformer is 1:10 and the primary AC voltage in 6 V, then the secondary voltage is ________.

A)6V
B)36V
C)60V
D)0.6V

Question 58

Transformers ________.

A)match the impedance of a source to the impedance of a load
B)convert a higherVoltage into a lowerVoltage
C)convert a lower current into a higher current
D)all of these

Question 59

To couple two circuits togetherW ith no change in either voltage or current, use ________.

A)a step-down transformer
B)a step-up transformer
C)a power transformer
D)an isolation transformer

Question 60

Although the input voltage to the transformerʹs primary is correct, the transformer is outputting zero volts. A probable trouble is ________.

A)an open primary
B)a partially shorted primary
C)a shorted primary
D)a partially shorted secondary

Question 61

The measured voltage on the secondary of a given transformer is 0 V.W hich of the following could have caused this?

A)open secondary
B)open primary
C)shorted load
D)any of the above

Question 62

36 V of dc is applied to aW indings ratio of 1:2.W hat is the secondary voltage?

A)something slightly less than 72V ince transformers do have some loss
B)144V
C)72V
D)none of the above

Question 63

How can theW indings ratio be safely determined for an unmarked transformer?

A)StandardizedW
Ire color codes indicate Windings ratio.
B)The turns ratio is the same as the ratio of primary resistance to secondary resistance ratio.
C)Apply known lowVoltage ac to the primary and measure the secondary output. The ratio of
TheVoltages is the same as the Windings ratio.
D)either A or C

Question 64

An autotransformer differs from most transformers in that:

A)it does not isolate the primaryVoltage from the secondary Voltage.
B)primary and secondary Windings are the same.
C)source current flows through both the Windings.
D)all of these.

Question 65

A center tap transformer has a connection at the midpoint of the secondaryW inding resulting in:

A)two 180 degree out of phase secondaryVoltages.
B)secondaryVoltages half that of a non-center tapped transformerW
Ith the same turns ratio.
C)two equal magnitude secondaryVoltages.
D)all of these.