Line A-6 Flashcards
The BJT is considered _______________-controlled while the FET is considered_______________-controlled.
Current, voltage
In contrast to the base-emitter junction of the BJT, the gate-to-source connection of the FET is connected in _______________ bias.
Reverse
Regarding FETs, the resistance between the _______________ and the _______________ is controlled by applying a bias to the _______________
Source (drain), drain (source), gate
When a FET’s gate voltage is zero, the drain current is at _______________
Maximum
As the voltage between a FET’s gate and source increases, the drain current _______________
Decreases
The charge carriers in an N-channel FET are _______________, while in a P-channel FET they are _______________.
Electrons, holes
The IGBT combines the insulated gate aspect of a _______________ with the output performance of a _______________.
MOSFET, BJT
Like the MOSFET, the IGBT is a _______________-controlled device.
Voltage
The IGBT experiences far lower _______________ losses for a given current when in its “on” state than a comparable MOSFET
I^2 R
Unlike the BJT, both the MOSFET and the IGBT can generate complex _______________ through pulse-width modulation.
Waveforms
Often, discrete IGBTs are paralleled together in ______________________________ configurations to form IGBT modules
Darlington-like
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
Variable frequency drives (VFDs)
How is an SCR normally turned on?
By applying a small amount of power to the gate terminal
How does the power-handling ability of the SCR compare to that of a transistor?
SCRs can handle much more power.
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?
An SCR requires only a small fraction of the power a transistor would require.
Name two advantages that an SCR has over a mechanical switch.
No contact wear, pitting or corrosion. No contact bounce.
In a stud-type SCR, how does the gate terminal compare in physical size to the anode and cathode terminals?
Gate terminal is the smallest terminal.
In general, how does the physical size of an SCR relate to its current rating?
As current rating increases, physical size increases.
What is likely to happen if the SCR’s current rating is exceeded?
The SCR will overheat and will likely be permanently damaged.
What happens if the SCR’s reverse-blocking voltage is exceeded?
The SCR will conduct in the reverse direction and will be permanently damaged.
What happens if the SCR’s forward-blocking voltage is exceeded?
The SCR will be turned on without a gate signal.
What terminal is commonly tied to the heat sink in the SCR?
Anode
T/F
An SCR is a thyristor, but a thyristor is not necessarily an SCR
True
What happens to an SCR if the current drops below the holding current?
SCR will switch off
To turn on the SCR, what must the polarity of the gate be with respect to the cathode?
The gate must be positive with respect to the cathode.
Once an SCR has been turned on, how can it be turned off?
By opening the load circuit or reducing the load current below the SCR holding current rating
What words are used to describe the action of turning an SCR on?
Firing or triggering; sometimes gating is also used
Why is it not required to maintain the gate signal once the SCR is on?
The gate signal has no effect once the SCR is turned on.
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
Positive, negative
What range of current would approximate the typical holding current magnitude in an SCR?
Milliamps
When an SCR is conducting, how many volts (to the nearest round number) would appear
across the terminals A–K?
1V
Why must you connect a resistor (load) in series with an SCR?
An SCR in the on mode would represent a short-circuit.
If an SCR is triggered at 10° into the cycle, for how many degrees will it conduct?
170°
What is meant by the term phase control with reference to an SCR?
The ability to change the shape of the voltage waveform and, therefore, magnitude of voltage across a load
If the firing angle is large, the average DC output voltage across the load is ___________
Small
The maximum number of degrees that the firing angle can have is _______
180°
When used with a single-phase bridge rectifier, how many SCRs are needed to give complete phase control for a load?
One
Name one common application of phase control using the SCR
Varying the speed of a DC motor
What are the two common forms of triggering used with SCRs?
Resistor triggering and resistor-capacitor triggering
If resistor triggering is used, the SCR can be fired between ________ degrees up to ________ degrees in the positive half-cycle
0 to 90
If resistor-capacitor triggering is used, the SCR can be fired from ________ degrees up to ________ degrees in the positive half-cycle
0 to 180
Regardless of the type of triggering circuit, the SCR will always block for at least ________ degrees of the AC cycle
180
What determines the rate at which the capacitor charges to the required SCR positive gate voltage in the resistor-capacitor phase-control circuit?
The ohms setting of the rheostat in series with the capacitor (assumes a non-variable capacitor)
Whereas the SCR is a unidirectional device, a triac is a ________ device
Bidirectional
In general, triacs have ________ current ratings than SCRs
Lower
Name one common application for a triac.
Lamp dimming; domestic range hot plate control; smaller motor speed control; applications that do not need high current
In some (but not all) triacs, how is the gate terminal readily identified?
smaller than the other two leads
The voltage drop across a triac, when conducting rated current, would be closest to:
1 V to 2 V
Can a triac be triggered into conduction without a gate signal?
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)
What is meant by the triac holding current?
Current required to keep the triac on without the gate signal
The action of a triac could be likened to two ________ connected back-to-back
SCR’s
Triacs are available in rms current ratings up to a maximum of about ________ A
45
A diac is a ________ sensitive switch
Voltage
When triggered, a diac has a ________ resistance in ________ direction(s)
Low, both
What is the significance of diac breakover voltage?
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
When the diac breaks down and conducts, the voltage across it while it is conducting is
typically in the region of ________ V to ________ V
10, 15
For what is a diac commonly used?
Triggering a triac
In a diac oscillator circuit: What determines the output frequency
Values of R and C
In a diac oscillator circuit: What component is normally used to vary the output frequency?
Resistance (rheostat) setting
Because of the shape of the output voltage pulses, the diac oscillator is sometimes called a ________ generator
Sawtooth
Typical current rating of UJTs would be:
Milliamperes
What factors determine the firing voltage of a UJT?
Voltage across B1 and B2 and a constant for the UJT called intrinsic standoff ratio
Firing voltages of UJTs are normally _________ than those of diacs
Lower
T/F
The UJT must operate from a DC source only
True
How can you change the voltage at which the UJT switches on?
By changing the voltage across B2–B1 terminals
Name a common use for the UJT.
Triggering circuits for SCR’s or triacs
How does the current rating of LASCRs compare with those of standard SCR’s?
Much lower
Why does an LASCR need a transparent or translucent lens?
Because the gate trigger is light-activated
An LASCR is switched on by a light source. What happens now if the light source is removed?
The LASCR will stay on, provided the anode-cathode current does not drop below the
required holding current
How is an LASCR turned off?
by decreasing the anode-cathode current to zero or to a low value below the holding current level
T/F
All LASCRs have three terminals.
False
The largest LASCR available would have a current rating of about how many amperes?
About 3 amps
State five applications of thyristors.
Lamp dimming; speed control; rectification; AC motor starting; timing circuits; pulse generators
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
Higher, higher
What type of supply is required for the following three-phase: half-wave rectifier
Wye-connected supply (with neutral connection)
What type of supply is required for the following three-phase: full-wave bridge rectifier
Any three-phase supply (wye or delta)
For three-phase rectifiers, what value of voltage must the individual diodes be rated for in the half-wave rectifier
Peak value of the line-to-line AC voltage
For three-phase rectifiers, what value of voltage must the individual diodes be rated for in the full-wave rectifier
Peak value of the line-to-line AC voltage
What “minimum” value of voltage does the rectified DC wave reach in the half-wave rectifier
50% of the peak value
What “minimum” value of voltage does the rectified DC wave reach in the full-wave rectifier
86.6% of the peak value
What is the “form factor” commonly used for the three-phase half-wave rectifier
1.17 × AC phase (rms) value
What is the “form factor” commonly used for the three-phase full-wave rectifier
1.35 × AC line (rms) value
What advantage is offered by using SCRs instead of diodes in three-phase rectifier circuits?
SCRs can be phase-controlled to offer a variable DC output.
___________ 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
Electrostatic discharge (ESD)
Under normal circumstances, the atoms comprising matter remain electrically _______
Neutral
List the four common sources of static electricity
Contact-induced charge separation (the triboelectric effect)
Pressure-induced charge separation (the piezoelectric effect)
Heat-induced charge separation (the pyroelectric effect)
Electrostatic induction
Of the four common sources of static electricity, which one poses the greatest risk to solid-
state devices?
Contact-induced charge separation (the triboelectric effect)
Air has a dielectric strength of approximately _______________
30 kV/cm
Electronic devices deemed particularly susceptible to electrostatic discharge are termed ________________
Electrostatic-sensitive devices (ESDs)
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 Faraday shield
What are three common precautions that personnel can take to protect electrostatic-sensitive devices from the effects of electrostatic discharge?
Stand on anti-static floor mats
Wear anti-static shoes and clothing
Wear anti-static wrist straps
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 ___________
High
