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Electrical Level 3 > Line A-6 > Flashcards

Line A-6 Flashcards

1
Q

The BJT is considered _______________-controlled while the FET is considered_______________-controlled.

A

Current, voltage

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2
Q

In contrast to the base-emitter junction of the BJT, the gate-to-source connection of the FET is connected in _______________ bias.

A

Reverse

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3
Q

Regarding FETs, the resistance between the _______________ and the _______________ is controlled by applying a bias to the _______________

A

Source (drain), drain (source), gate

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4
Q

When a FET’s gate voltage is zero, the drain current is at _______________

A

Maximum

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5
Q

As the voltage between a FET’s gate and source increases, the drain current _______________

A

Decreases

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6
Q

The charge carriers in an N-channel FET are _______________, while in a P-channel FET they are _______________.

A

Electrons, holes

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7
Q

The IGBT combines the insulated gate aspect of a _______________ with the output performance of a _______________.

A

MOSFET, BJT

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8
Q

Like the MOSFET, the IGBT is a _______________-controlled device.

A

Voltage

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9
Q

The IGBT experiences far lower _______________ losses for a given current when in its “on” state than a comparable MOSFET

A

I^2 R

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10
Q

Unlike the BJT, both the MOSFET and the IGBT can generate complex _______________ through pulse-width modulation.

A

Waveforms

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11
Q

Often, discrete IGBTs are paralleled together in ______________________________ configurations to form IGBT modules

A

Darlington-like

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12
Q

Because of its ability to modulate the waveforms of large quantities of power, the IGBT is often a key component of the _________________________________ used to control the large motors of high-traction and high-inertia applications

A

Variable frequency drives (VFDs)

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13
Q

How is an SCR normally turned on?

A

By applying a small amount of power to the gate terminal

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14
Q

How does the power-handling ability of the SCR compare to that of a transistor?

A

SCRs can handle much more power.

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15
Q

How does the quantity of control power needed at the gate of an SCR compare to that required for the base of a transistor controlling the same load?

A

An SCR requires only a small fraction of the power a transistor would require.

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16
Q

Name two advantages that an SCR has over a mechanical switch.

A

No contact wear, pitting or corrosion. No contact bounce.

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17
Q

In a stud-type SCR, how does the gate terminal compare in physical size to the anode and cathode terminals?

A

Gate terminal is the smallest terminal.

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18
Q

In general, how does the physical size of an SCR relate to its current rating?

A

As current rating increases, physical size increases.

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19
Q

What is likely to happen if the SCR’s current rating is exceeded?

A

The SCR will overheat and will likely be permanently damaged.

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20
Q

What happens if the SCR’s reverse-blocking voltage is exceeded?

A

The SCR will conduct in the reverse direction and will be permanently damaged.

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21
Q

What happens if the SCR’s forward-blocking voltage is exceeded?

A

The SCR will be turned on without a gate signal.

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22
Q

What terminal is commonly tied to the heat sink in the SCR?

A

Anode

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23
Q

T/F
An SCR is a thyristor, but a thyristor is not necessarily an SCR

A

True

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24
Q

What happens to an SCR if the current drops below the holding current?

A

SCR will switch off

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25
Q

To turn on the SCR, what must the polarity of the gate be with respect to the cathode?

A

The gate must be positive with respect to the cathode.

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26
Q

Once an SCR has been turned on, how can it be turned off?

A

By opening the load circuit or reducing the load current below the SCR holding current rating

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27
Q

What words are used to describe the action of turning an SCR on?

A

Firing or triggering; sometimes gating is also used

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28
Q

Why is it not required to maintain the gate signal once the SCR is on?

A

The gate signal has no effect once the SCR is turned on.

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29
Q

Normally, for an SCR to conduct, the anode must be ____________ with respect to the cathode, and the cathode must be ________ with respect to the gate

A

Positive, negative

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30
Q

What range of current would approximate the typical holding current magnitude in an SCR?

A

Milliamps

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31
Q

When an SCR is conducting, how many volts (to the nearest round number) would appear
across the terminals A–K?

A

1V

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32
Q

Why must you connect a resistor (load) in series with an SCR?

A

An SCR in the on mode would represent a short-circuit.

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33
Q

If an SCR is triggered at 10° into the cycle, for how many degrees will it conduct?

A

170°

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34
Q

What is meant by the term phase control with reference to an SCR?

A

The ability to change the shape of the voltage waveform and, therefore, magnitude of voltage across a load

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35
Q

If the firing angle is large, the average DC output voltage across the load is ___________

A

Small

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36
Q

The maximum number of degrees that the firing angle can have is _______

A

180°

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37
Q

When used with a single-phase bridge rectifier, how many SCRs are needed to give complete phase control for a load?

A

One

38
Q

Name one common application of phase control using the SCR

A

Varying the speed of a DC motor

39
Q

What are the two common forms of triggering used with SCRs?

A

Resistor triggering and resistor-capacitor triggering

40
Q

If resistor triggering is used, the SCR can be fired between ________ degrees up to ________ degrees in the positive half-cycle

A

0 to 90

41
Q

If resistor-capacitor triggering is used, the SCR can be fired from ________ degrees up to ________ degrees in the positive half-cycle

A

0 to 180

42
Q

Regardless of the type of triggering circuit, the SCR will always block for at least ________ degrees of the AC cycle

A

180

43
Q

What determines the rate at which the capacitor charges to the required SCR positive gate voltage in the resistor-capacitor phase-control circuit?

A

The ohms setting of the rheostat in series with the capacitor (assumes a non-variable capacitor)

44
Q

Whereas the SCR is a unidirectional device, a triac is a ________ device

A

Bidirectional

45
Q

In general, triacs have ________ current ratings than SCRs

A

Lower

46
Q

Name one common application for a triac.

A

Lamp dimming; domestic range hot plate control; smaller motor speed control; applications that do not need high current

47
Q

In some (but not all) triacs, how is the gate terminal readily identified?

A

smaller than the other two leads

48
Q

The voltage drop across a triac, when conducting rated current, would be closest to:

A

1 V to 2 V

49
Q

Can a triac be triggered into conduction without a gate signal?

A

Yes, by raising the voltage across the main terminals high enough (breakover), or by a rapid
rise in voltage across the main terminals (dv/dt)

50
Q

What is meant by the triac holding current?

A

Current required to keep the triac on without the gate signal

51
Q

The action of a triac could be likened to two ________ connected back-to-back

A

SCR’s

52
Q

Triacs are available in rms current ratings up to a maximum of about ________ A

A

45

53
Q

A diac is a ________ sensitive switch

A

Voltage

54
Q

When triggered, a diac has a ________ resistance in ________ direction(s)

A

Low, both

55
Q

What is the significance of diac breakover voltage?

A

The diac acts like an open switch when the voltage across its terminals is below its breakover voltage rating, but it acts like a closed switch once the voltage across its terminals is raised to its nominal breakover voltage

56
Q

When the diac breaks down and conducts, the voltage across it while it is conducting is
typically in the region of ________ V to ________ V

A

10, 15

57
Q

For what is a diac commonly used?

A

Triggering a triac

58
Q

In a diac oscillator circuit: What determines the output frequency

A

Values of R and C

59
Q

In a diac oscillator circuit: What component is normally used to vary the output frequency?

A

Resistance (rheostat) setting

60
Q

Because of the shape of the output voltage pulses, the diac oscillator is sometimes called a ________ generator

A

Sawtooth

61
Q

Typical current rating of UJTs would be:

A

Milliamperes

62
Q

What factors determine the firing voltage of a UJT?

A

Voltage across B1 and B2 and a constant for the UJT called intrinsic standoff ratio

63
Q

Firing voltages of UJTs are normally _________ than those of diacs

A

Lower

64
Q

T/F
The UJT must operate from a DC source only

A

True

65
Q

How can you change the voltage at which the UJT switches on?

A

By changing the voltage across B2–B1 terminals

66
Q

Name a common use for the UJT.

A

Triggering circuits for SCR’s or triacs

67
Q

How does the current rating of LASCRs compare with those of standard SCR’s?

A

Much lower

68
Q

Why does an LASCR need a transparent or translucent lens?

A

Because the gate trigger is light-activated

69
Q

An LASCR is switched on by a light source. What happens now if the light source is removed?

A

The LASCR will stay on, provided the anode-cathode current does not drop below the
required holding current

70
Q

How is an LASCR turned off?

A

by decreasing the anode-cathode current to zero or to a low value below the holding current level

71
Q

T/F
All LASCRs have three terminals.

A

False

72
Q

The largest LASCR available would have a current rating of about how many amperes?

A

About 3 amps

73
Q

State five applications of thyristors.

A

Lamp dimming; speed control; rectification; AC motor starting; timing circuits; pulse generators

74
Q

Comparing a three-phase, half-wave rectifier to a three-phase, full-wave bridge rectifier, the
ripple, frequency is __________ for the bridge rectifier.
b. voltage is _________ for the half-wave rectifier

A

Higher, higher

75
Q

What type of supply is required for the following three-phase: half-wave rectifier

A

Wye-connected supply (with neutral connection)

76
Q

What type of supply is required for the following three-phase: full-wave bridge rectifier

A

Any three-phase supply (wye or delta)

77
Q

For three-phase rectifiers, what value of voltage must the individual diodes be rated for in the half-wave rectifier

A

Peak value of the line-to-line AC voltage

78
Q

For three-phase rectifiers, what value of voltage must the individual diodes be rated for in the full-wave rectifier

A

Peak value of the line-to-line AC voltage

79
Q

What “minimum” value of voltage does the rectified DC wave reach in the half-wave rectifier

A

50% of the peak value

80
Q

What “minimum” value of voltage does the rectified DC wave reach in the full-wave rectifier

A

86.6% of the peak value

81
Q

What is the “form factor” commonly used for the three-phase half-wave rectifier

A

1.17 × AC phase (rms) value

82
Q

What is the “form factor” commonly used for the three-phase full-wave rectifier

A

1.35 × AC line (rms) value

83
Q

What advantage is offered by using SCRs instead of diodes in three-phase rectifier circuits?

A

SCRs can be phase-controlled to offer a variable DC output.

84
Q

___________ is the sudden transfer of electrons between two electrically charged objects, or between one charged object and another that is capable of receiving its charge

A

Electrostatic discharge (ESD)

85
Q

Under normal circumstances, the atoms comprising matter remain electrically _______

A

Neutral

86
Q

List the four common sources of static electricity

A

Contact-induced charge separation (the triboelectric effect)

Pressure-induced charge separation (the piezoelectric effect)

Heat-induced charge separation (the pyroelectric effect)

Electrostatic induction

87
Q

Of the four common sources of static electricity, which one poses the greatest risk to solid-
state devices?

A

Contact-induced charge separation (the triboelectric effect)

88
Q

Air has a dielectric strength of approximately _______________

A

30 kV/cm

89
Q

Electronic devices deemed particularly susceptible to electrostatic discharge are termed ________________

A

Electrostatic-sensitive devices (ESDs)

90
Q

What is the term used to describe the layer within ESD protective packaging that is made of conductive material and that cancels out electric charges?

A

A Faraday shield

91
Q

What are three common precautions that personnel can take to protect electrostatic-sensitive devices from the effects of electrostatic discharge?

A

Stand on anti-static floor mats

Wear anti-static shoes and clothing

Wear anti-static wrist straps

92
Q

To lower the magnitudes of triboelectric voltages generated and discharged in environments containing electrostatic sensitive devices, one should strive to keep the environment’s relative humidity ___________

A

High

Electrical Level 3 (9 decks)

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