Question 1

A DIAC is a two-terminal, bi-directional trigger device with no gate lead. Once a DIAC is conducting, what is the only way to switch it off?

Accepted Answer

Introduction / Context: Unlike TRIACs and SCRs, DIACs have no gate terminal. They are used to provide symmetrical trigger pulses, commonly in phase-control circuits. Understanding how conduction starts and stops in a DIAC is key to designing dimmers and trigger networks. Given Data / Assumptions: Two-terminal DIAC device. Bi-directional conduction once breakover voltage is reached. No external gate control is available. Concept / Approach: Conduction in a DIAC begins when the applied voltage magnitude exceeds its breakover voltage. Once conducting, it remains on until the current falls below the holding current. Because there is no gate, you cannot command turn-off with a gate signal; you must rely on the circuit current decreasing sufficiently—often naturally at AC zero crossings. Step-by-Step Reasoning: 1) Turn-on: |V| exceeds breakover → DIAC switches to low-impedance state.2) On-state persists while device current ≥ holding current.3) Turn-off occurs only when current drops below holding current (low-current dropout).4) In AC circuits, this is normally achieved each half-cycle at the zero crossing. Verification / Alternative check: Review typical lamp dimmer circuits: DIAC triggers a TRIAC; both devices naturally commutate off when the AC load current passes through zero and falls below their holding currents. Why Other Options Are Wrong: Positive/negative gate voltages: DIAC has no gate terminal. Increase voltage above breakover: that turns it on, not off. Reverse with a gate pulse: again, no gate; polarity reversal alone does not guarantee turn-off unless current drops below holding current. Common Pitfalls: Confusing DIAC behavior with TRIAC/SCR gate control. Always design the surrounding network to guarantee current fall below holding current for reliable commutation. Final Answer: reduce current below its holding value (low-current dropout)

Question 2

Hall-effect sensors are widely used in embedded systems and automotive electronics. Which physical quantity do they directly detect and convert into an electrical signal?

Accepted Answer

Introduction / Context: Hall-effect sensors are solid-state transducers that exploit the Hall effect: a transverse voltage appears across a conductor or semiconductor carrying current when subjected to a perpendicular magnetic field. These devices are ubiquitous in brushless motors, proximity switches, and current sensors. Given Data / Assumptions: A current flows through the Hall element. A magnetic field perpendicular to the current path is present. The resulting Hall voltage is measurable and proportional to field strength (within range). Concept / Approach: The Hall voltage VH is approximately proportional to the magnetic flux density B, the current through the element, and inversely proportional to its thickness, with a material-dependent Hall coefficient. Sensor ICs integrate the Hall plate, amplifiers, and often comparators or digital outputs to produce clean logic-level signals or analog outputs proportional to magnetic field. Step-by-Step Explanation: 1) Drive a current through the Hall plate.2) Apply a magnetic field perpendicular to the plate.3) Charge carriers deflect due to the Lorentz force, building a transverse voltage (Hall voltage).4) Condition the Hall voltage to produce a usable output indicating magnetic field magnitude and/or polarity. Verification / Alternative check: Applications like BLDC motor commutation use Hall sensors to sense rotor magnets and report position. Current sensors route the conductor near a Hall element so B ∝ current, enabling isolated current measurement. Why Other Options Are Wrong: Temperature, moisture, pressure, light: These require thermistors/RTDs, hygrometers, pressure transducers, or photodiodes/phototransistors, not Hall elements. Common Pitfalls: Confusing Hall sensors with magnetoresistive sensors; both detect magnetic fields, but operating principles differ. Hall sensors are robust and economical for many applications. Final Answer: magnetic fields

Question 3

In thyristor terminology (SCR), “closing” the device means turning it OFF (commutating it). What is the only reliable way to force an SCR to turn off in normal operation?

Accepted Answer

Introduction / Context: The silicon-controlled rectifier (SCR) is a unidirectional PNPN thyristor that latches ON after it is triggered. Unlike a transistor or a MOSFET, an SCR cannot be driven OFF by its gate once it is conducting. This question checks whether you know the correct mechanism for turning an SCR OFF (often called “closing” it or commutation). Given Data / Assumptions: Device: SCR (thyristor) operating in its normal region. Gate can trigger conduction but does not provide a turn-OFF path. Standard definitions: latching current and holding current. Concept / Approach: An SCR turns ON when the anode-cathode current exceeds the latching current after a gate trigger or breakover. Once ON, it remains conducting until the anode current falls below the device’s holding current. This forced reduction of current—often by natural AC current zero or by commutation networks—is called low-current dropout or current starvation below the holding current. Step-by-Step Solution: Recognize: Gate triggering enables ON-state; it cannot force OFF-state.Identify OFF condition: I_AK (anode current) must drop below the holding current I_H.Methods: natural commutation in AC at current zero, or forced commutation using external circuitry.Therefore, the only reliable “close” method is low-current dropout below I_H. Verification / Alternative check: Datasheets specify I_L (latching current) and I_H (holding current). Practical circuits for DC use explicit commutation (capacitor/inductor networks) to interrupt anode current, confirming that OFF control is by reducing current, not by the gate. Why Other Options Are Wrong: a) Gate trigger: Turns ON, not OFF. b) Forward breakover: Another turn-ON mechanism at high forward voltage. d) Valley voltage: A term more relevant to UJT characteristics, not SCR turn-OFF. e) Reverse-biasing the gate only: Gate control cannot unlatch an SCR once conducting. Common Pitfalls: Assuming gate works like a transistor base for turn-OFF—it does not. Ignoring holding current; even small load currents can keep the SCR latched. Final Answer: low-current dropout

Question 4

Photovoltaic (solar) cells generate electrical power when illuminated. Which application most directly uses a photovoltaic cell as its energy source?

Accepted Answer

Introduction / Context: Photovoltaic (PV) cells convert light into electricity. They are ideal where grid power is unavailable or inconvenient, and where small to large amounts of DC power are needed without moving parts. The question asks you to identify the application that uses a PV cell directly as its power source. Given Data / Assumptions: PV cell provides DC power proportional to irradiance and area. Applications vary widely in power needs—from microwatts to kilowatts. Focus is on the primary, direct use of PV output as an energy source. Concept / Approach: PV is most useful where line power is impractical: remote telemetry, outdoor sensors, off-grid lighting, and satellites. A “remote power source” explicitly captures this use case. While some calculators are solar, the TI-92 specifically used batteries, and other listed items typically rely on conventional supplies or are not energy sources themselves. Step-by-Step Solution: Identify characteristic: PV supplies DC without grid connection.Map to applications: remote or off-grid devices benefit most.Compare options: only “remote power source” universally and directly fits PV usage.Conclude that the best answer is the remote (off-grid) power source. Verification / Alternative check: Field deployments (weather stations, IoT sensor nodes, rural lighting) commonly use PV plus batteries for energy autonomy, validating the selection. Why Other Options Are Wrong: Automobile horn: Requires high instantaneous current from vehicle battery. TI 92 calculator: That model is battery-powered; not a standard PV application. Magnetic field detector: A sensor function, not an energy source description. Crystal microphone preamp: Typically powered by batteries or phantom power, not PV specifically. Common Pitfalls: Assuming all calculators are solar; many are not. Confusing sensors with power sources. Final Answer: a remote power source

Question 5

Within thyristor families, what is a DIAC best characterized as?

Accepted Answer

Introduction / Context: DIACs are commonly used as trigger devices in AC control circuits, especially for firing TRIACs in lamp dimmers and motor speed controllers. Recognizing their directional behavior is essential for correct application in alternating-current waveforms. Given Data / Assumptions: Device type: DIAC (diode for AC). Two terminals, no gate. Used symmetrically on AC lines. Concept / Approach: A DIAC conducts only after its breakover voltage is reached; importantly, this breakover and conduction are symmetrical in both polarities. That makes it a bidirectional trigger device. It is not gated and does not behave like a unidirectional diode or a transistor. Step-by-Step Solution: Consider polarity: DIAC’s V–I curve is symmetric about the origin.Below breakover (both polarities), current is negligible.At breakover, the DIAC snaps into conduction and then resets when current falls.This behavior in both directions identifies it as bidirectional. Verification / Alternative check: Practical lamp dimmers place a DIAC in series with a TRIAC gate to produce symmetrical triggering on positive and negative half-cycles, reducing waveform distortion—clear evidence of bidirectional action. Why Other Options Are Wrong: Transistor: Has three terminals and different physics. Unidirectional: Would conduct in one polarity only (like a rectifier diode or zener in breakdown direction). Three-layer device: DIAC structures vary; the key characteristic is bidirectionality. A gated triode device: Describes a TRIAC (with a gate), not a DIAC. Common Pitfalls: Confusing DIAC (2-terminal) with TRIAC (3-terminal). Assuming all trigger diodes are unidirectional zener-like devices. Final Answer: bidirectional device

Question 6

When checking a known-good SCR or TRIAC with an ohmmeter (no gate drive applied), what resistance behavior should be observed between the main terminals?

Accepted Answer

Introduction / Context: Basic ohmmeter checks can reveal gross faults in thyristors. An SCR or a TRIAC that is not triggered should block current. Recognizing the expected high resistance reading prevents misdiagnosis and helps identify shorted devices. Given Data / Assumptions: No gate current is applied during the test. Meter polarity is reversed to check both directions. Device is not damaged and has no internal leakage beyond normal levels. Concept / Approach: Without gate drive and below breakover voltage, an SCR (anode–cathode path) and a TRIAC (MT1–MT2 path) should be non-conductive. Therefore the ohmmeter—especially a DMM with low test voltage—will show a very high resistance (open) in either polarity for a healthy device. Step-by-Step Solution: Connect ohmmeter leads across main terminals (A–K for SCR, MT1–MT2 for TRIAC).Measure resistance with one polarity: observe very high resistance.Reverse the leads: again observe very high resistance.Conclusion: High resistance in both directions indicates a good, untriggered device. Verification / Alternative check: Applying a small gate current will cause the SCR/TRIAC to conduct (low resistance). Removing gate drive and reducing current below the holding current returns the device to high-resistance state, confirming correct operation. Why Other Options Are Wrong: b) and c): Suggest diode-like conduction in one direction; an SCR/TRIAC needs triggering. d): Low resistance both ways indicates a shorted device. e): No fixed midrange resistance is expected for an untriggered thyristor. Common Pitfalls: Using an old analog ohmmeter with high test voltage can accidentally trigger the device—use caution. For TRIACs, mistaken terminal identification can lead to confusing readings. Final Answer: show high resistance in both directions

Question 7

Component identification in displays: Which option correctly describes a seven-segment display module commonly used to show decimal digits (0–9)?

Accepted Answer

Introduction / Context: Seven-segment displays are ubiquitous in instruments, clocks, and embedded products for showing digits. Identifying them correctly among other display types helps when selecting parts, designing drivers, and reading schematics. Given Data / Assumptions: We are distinguishing between common display modules. Seven-segment modules typically are LEDs (though LCD versions exist) with clear segment geometry. Options include other display technologies for contrast. Concept / Approach: A seven-segment display contains seven LED segments (labeled a–g) arranged to form a figure-8 outline. By lighting specific segments, digits 0–9 are formed; an eighth LED is often present as a decimal point (dp). Modules are available as common-anode or common-cathode, determining the driving polarity. Step-by-Step Solution: Identify geometry: seven bars forming a digit “8”.Confirm function: combinations of segments render numerals 0–9.Check electrical style: common-anode or common-cathode pins plus segment pins.Therefore, the correct description is the figure-8 segmented LED module. Verification / Alternative check: Datasheets show pinouts and segment maps; driver ICs like 74HC595 with transistor arrays or MAX7219 are commonly used, matching the described module. Why Other Options Are Wrong: LED bar-graph: Linear indicator, not digit-forming segments. 5×7 dot-matrix: Pixel matrix for arbitrary glyphs. 16×2 LCD: Character module with internal controller, not seven segments. VFD tube: Different technology and appearance. Common Pitfalls: Confusing common-anode vs common-cathode when choosing drivers. Assuming the decimal point is always present; some variants omit it. Final Answer: An LED module with seven individually addressable segments arranged in a figure-8 pattern (often with a decimal point)

Question 8

What is an SCR (silicon-controlled rectifier) in terms of structure and terminals?

Accepted Answer

Introduction / Context: The SCR is the foundational member of the thyristor family used in controlled rectifiers, AC regulators, and motor drives. Understanding its internal structure and external terminals is basic to applying it correctly. Given Data / Assumptions: SCR is a four-layer (PNPN) device. Three accessible terminals: anode (A), cathode (K), and gate (G). Gate provides turn-ON control; turn-OFF requires current reduction. Concept / Approach: The PNPN structure can be modeled as a pair of coupled transistors (PNP and NPN). A small gate current initiates regenerative feedback, latching the device into conduction from anode to cathode. Hence, the concise and correct description is “PNPN thyristor with 3 terminals.” Step-by-Step Solution: Identify layers: P–N–P–N forms the thyristor core.Identify terminals: Anode on outer P, cathode on outer N, gate into inner P layer region.Gate trigger → conduction → device latches until current < holding current.Therefore, SCR = PNPN, three terminals (A, K, G). Verification / Alternative check: Equivalent two-transistor model shows positive feedback loop gain > 1 after gate injection, validating the latch behavior and the structural description. Why Other Options Are Wrong: 4 terminals: No such standard SCR terminal set. PNP or NPN thyristor: Thyristors are four-layer, not simple three-layer BJTs. Zener device: Two-terminal breakdown diode, unrelated to SCR operation. Common Pitfalls: Expecting the gate to turn the SCR OFF; it cannot. Confusing SCRs with TRIACs (which are bidirectional and have MT1, MT2, G). Final Answer: a PNPN thyristor with 3 terminals

Question 9

Component identification in thyristor families: Which description best matches a DIAC?

Accepted Answer

Introduction / Context: DIACs are small, inexpensive trigger devices that provide symmetrical turn-on for AC control circuits. Being able to describe one accurately prevents confusion with SCRs, TRIACs, UJTs, and zener diodes. Given Data / Assumptions: Two-terminal device with no gate lead. Breakover occurs at approximately equal magnitudes for positive and negative polarities. Commonly placed in series with a TRIAC gate. Concept / Approach: The defining trait of a DIAC is bidirectional breakover. It remains off until |V| reaches V_BO, then conducts sharply, providing a clean, symmetrical trigger pulse to a TRIAC in either half-cycle. This symmetry reduces waveform distortion in phase-control applications. Step-by-Step Solution: Identify terminals: Only two—no gate.Identify conduction: Symmetric in both polarities at breakover.Application: Series with TRIAC gate to produce consistent firing angle both halves.Therefore, it is a two-terminal bidirectional trigger diode. Verification / Alternative check: Manufacturer curves show nearly mirrored V–I characteristics in quadrants I and III, confirming bidirectional behavior and trigger functionality. Why Other Options Are Wrong: SCR: Has a gate and is unidirectional. UJT: Three-terminal negative-resistance device, used in triggering but different physics. Zener diode: Unidirectional breakdown for voltage reference. Four-terminal bilateral switch: Not a standard DIAC description. Common Pitfalls: Assuming DIACs can control power flow directly; they primarily trigger other devices. Confusing DIAC symbols with TRIAC symbols; TRIACs have three terminals. Final Answer: A two-terminal bidirectional trigger diode used to fire TRIACs; breaks over in either polarity

Question 10

For a TRIAC used in AC phase control, which statement about gate triggering polarities is correct?

Accepted Answer

Introduction / Context: TRIACs are bilateral thyristors that conduct in both directions and can be triggered in multiple quadrants. Understanding permitted gate polarities is crucial for reliable dimmer and motor-control designs. Given Data / Assumptions: TRIAC terminals: MT1, MT2, and Gate (G). Device supports multiple triggering quadrants depending on the polarities of MT2 and gate relative to MT1. Typical circuits use DIACs or direct resistive/capacitive networks for triggering. Concept / Approach: TRIACs can be triggered with either positive or negative gate current relative to MT1, enabling conduction in both half-cycles. Trigger current thresholds differ slightly by quadrant, but both polarities are valid and commonly used. Step-by-Step Solution: Recognize bilateral conduction: TRIAC conducts for either MT2 polarity after triggering.Gate triggering: Provide sufficient I_G in the appropriate quadrant (I–IV) as per datasheet.Result: A TRIAC may be triggered by positive or negative gate drive.Therefore, option stating “either positive or negative” is correct. Verification / Alternative check: Triggering curves in datasheets show different I_GT for quadrants I–IV; all are feasible with proper design, confirming bidirectional gate triggering capability. Why Other Options Are Wrong: a) and b): Restrict to one polarity; not generally true. c): Contradicts the basic TRIAC operation which depends on gate triggering. e): Limiting to one quadrant is incorrect and overly restrictive. Common Pitfalls: Ignoring quadrant-dependent gate current requirements, leading to unreliable firing. Failing to provide proper dv/dt snubbing, causing unintended triggering. Final Answer: can be triggered by either a positive or a negative gate voltage

Question 11

Lead configuration of a unijunction transistor (UJT): which set correctly describes its terminals?

Accepted Answer

Introduction / Context: The UJT is a three-terminal device used for triggering and relaxation oscillators. Correctly identifying its terminal names and functions is essential for wiring timing circuits and firing thyristors. Given Data / Assumptions: Device: UJT (unijunction transistor). Terminals: E (emitter), B1, and B2. Applications: sawtooth generators, SCR gate drivers. Concept / Approach: A UJT has a lightly doped bar of semiconductor with two base contacts (B1, B2) forming a resistive path. A single emitter junction injects carriers into the bar, exhibiting negative resistance over part of its characteristic, enabling relaxation oscillation and triggering functions. Step-by-Step Solution: Identify terminals: E, B1, and B2 (three leads total).Understand structure: emitter junction + resistive base channel between B1 and B2.Recognize usage: charging a capacitor until the UJT fires and discharges through B1.Therefore, the correct description is “one emitter lead and two base leads.” Verification / Alternative check: Datasheets and pinouts (e.g., 2N2646) show E, B1, B2 pins consistently, confirming the lead configuration. Why Other Options Are Wrong: a) and b): Each lists only part of the terminals, not the full set. c) Two emitters are not present in a UJT. e) Collectors are transistor terms; UJTs use emitter and bases. Common Pitfalls: Miswiring B1/B2—polarity matters for oscillator direction and thresholds. Assuming UJT is a standard BJT; its behavior and terminals differ significantly. Final Answer: one emitter lead and two base leads

Question 12

In instrumentation and control, what is the primary function of a transducer?

Accepted Answer

Introduction / Context: A transducer is a foundational concept in measurement and control systems. It forms the bridge between physical phenomena and electrical signals (or vice versa). Understanding what a transducer does enables correct selection and interfacing in engineering designs. Given Data / Assumptions: The term is used broadly across sensors and actuators. Energy domains may include mechanical, thermal, chemical, optical, and electrical. Focus is on the core function, not the specific direction (input or output). Concept / Approach: A transducer converts energy from one form to another. Sensors (e.g., thermocouples, strain gauges, photodiodes) convert non-electrical quantities into electrical signals. Actuators (e.g., loudspeakers, piezo buzzers, motors) convert electrical energy into mechanical or acoustic energy. Transmission, prevention of current, or pure amplification are not definitional; conversion is. Step-by-Step Solution: Define: transducer = energy converter between domains.Examples (sensor): light → electricity (photodiode), temperature gradient → voltage (thermocouple).Examples (actuator): electricity → motion (motor), electricity → sound (speaker).Hence, the essential function is energy conversion. Verification / Alternative check: Standards and textbooks define transducers by their input/output energy domains and sensitivity/transfer characteristics, always centering on conversion, which confirms the selected definition. Why Other Options Are Wrong: Transmit electrical energy: That describes conductors or transmission lines, not transducers. Produce mechanical energy: Only a subset (actuators) does this. Prevent current flow: A function of insulators or switches, not transducers. Amplify voltage: Amplification can occur within signal conditioning, but transduction is about changing energy form. Common Pitfalls: Equating “sensor” with “transducer” and forgetting actuators are also transducers. Assuming a transducer always outputs an electrical signal; some output mechanical or optical energy. Final Answer: convert energy

Question 13

SCR terminology — In thyristor (SCR) operation, what is the smallest anode–cathode current that will still sustain conduction once the device has been triggered on (i.e., the minimum current below which the SCR turns off)?

Accepted Answer

Introduction: The silicon-controlled rectifier (SCR) is a latching device used widely in controlled rectifiers, motor drives, and power controllers. To apply it correctly, you must understand the specific current thresholds that define turn-on and turn-off behavior. One such key parameter is the holding current, which determines whether the SCR remains in conduction or reverts to its blocking state. Given Data / Assumptions: Singly triggered SCR operating in its forward-conducting quadrant. Device has already been turned on via a gate pulse or by exceeding breakover. We consider steady conduction vs. turn-off after the trigger event. Concept / Approach: After triggering, an SCR continues to conduct as long as anode–cathode current exceeds a device-specific threshold called the holding current (I_H). If the current falls below I_H, internal carrier recombination reduces conductivity and the device switches back to its high-impedance, forward-blocking state. This is distinct from the latching current (I_L), which is the minimum anode current immediately after turn-on needed to “latch” conduction once the gate pulse ends; typically, I_L > I_H. Step-by-Step Solution: Identify required definition: minimum anode–cathode current to sustain conduction.Match to SCR parameters: this is the holding current I_H.Differentiate from latching current: I_L applies right after triggering; I_H applies during steady conduction.Conclude that the correct term is “holding current”. Verification / Alternative check: Examine SCR datasheets: both I_L (latching) and I_H (holding) are specified. In practical AC circuits, the SCR commutates off naturally near current zero because load current dips below I_H, confirming the operational meaning of holding current. Why Other Options Are Wrong: maximum forward current: A safe operating limit, not a sustaining threshold. maximum forward gate current: Limits gate drive but does not define conduction sustaining. reverse gate leakage current: A small undesired current with no bearing on conduction sustain. latching current: Required immediately after turn-on, not the minimum to maintain conduction thereafter. Common Pitfalls: Confusing I_L and I_H; remember I_L ≥ I_H and applies only at the instant after triggering. Assuming the gate must be held on; once latched, gate current is not needed. Final Answer: holding current

Question 14

PUT identification — The programmable unijunction transistor (PUT) is actually a member of which device family, despite its name suggesting similarity to the UJT?

Accepted Answer

Introduction: The programmable unijunction transistor (PUT) is a classic device used in relaxation oscillators, timing circuits, and triggering applications. Its name can be misleading: although it performs functions reminiscent of a unijunction transistor, its internal structure and behavior align more closely with the thyristor family. Given Data / Assumptions: Device named “programmable unijunction transistor”. Used for threshold triggering with a programmable firing point. We seek the actual device class, not just functional similarity. Concept / Approach: A PUT is essentially a four-layer PNPN thyristor configured and biased to emulate the UJT’s negative-resistance characteristic. The key difference is that a PUT’s firing (trigger) point is set by external resistors feeding its “gate,” allowing programmable threshold behavior. Structurally and operationally (breakover, latching), it belongs to the SCR/thyristor family rather than the FET or BJT families. Step-by-Step Solution: Identify layers: PUT behaves as a PNPN device with a controllable gate.Recognize behavior: exhibits breakover and conduction similar to small-signal SCRs.Program trigger: external divider sets the equivalent anode–gate threshold.Conclude: PUT is a type of SCR (thyristor). Verification / Alternative check: Application notes and datasheets categorize PUTs alongside thyristors, often modeling them as gated PNPN devices that latch when triggered—behavior distinct from UJTs (which are three-terminal, two-junction devices) and entirely different from FETs or BJTs. Why Other Options Are Wrong: UJT thyristor: UJT is not a four-layer thyristor; it has a different structure and curve. FET device: Field-effect operation is unrelated to PUT physics. TRIAC: A bidirectional thyristor; PUT is unidirectional small-signal. BJT: Three-layer current-controlled device, not PNPN with latching. Common Pitfalls: Equating functional similarity (sawtooth generators) with structural identity. Forgetting that “programmable” refers to setting the trigger level via external bias. Final Answer: SCR

Question 15

Piezoelectricity — What physical cause produces the piezoelectric effect used in sensors, oscillators, and actuators?

Accepted Answer

Introduction: The piezoelectric effect underpins devices from quartz oscillators to ultrasonic transducers and force sensors. Understanding its origin helps in selecting materials, predicting polarity, and designing conditioning circuits that translate mechanical signals into electrical outputs (and vice versa). Given Data / Assumptions: Crystalline materials such as quartz, PZT, or PVDF polymer can be piezoelectric. We are interested in the primary cause that generates a voltage or charge. Quasi-static and dynamic (AC) pressures are both relevant. Concept / Approach: The direct piezoelectric effect states that applying mechanical stress (pressure, tension, shear) to certain non-centrosymmetric crystals creates an electric polarization, resulting in measurable charge and voltage across specific faces. The converse effect also exists: an applied electric field causes mechanical strain, enabling actuators and precise motion control. Step-by-Step Solution: Apply stress to a suitably oriented crystal with electrodes on its faces.The crystal’s unit cells shift, creating a net dipole moment and surface charge.Measure the resulting voltage/charge with a high-impedance interface (charge amps).Reverse the process (converse effect) to obtain controlled deformation from an applied voltage. Verification / Alternative check: Device datasheets show sensitivity in pC/N or mV/N, directly relating mechanical force to electrical response. Oscillators exploit the mechanical resonance of quartz, where minute stresses translate into precise frequency stability via electromechanical coupling. Why Other Options Are Wrong: heat or dissimilar metals: Describes the thermocouple (Seebeck) effect, not piezoelectricity. water running on iron: Not a recognized electromechanical transduction principle. a magnetic field: Leads to magnetostriction or electromagnetic induction, not piezoelectricity. exposure to visible light: Relates to the photovoltaic or photoelectric effects. Common Pitfalls: Confusing piezoelectricity with pyroelectricity (temperature-induced polarization) or triboelectric effects (contact charge transfer). Loading the sensor with low impedance and losing signal due to charge bleed-off. Final Answer: pressure on a crystal

Question 16

Injection laser diode (ILD) usage — Which application commonly employs injection laser diodes as optical sources due to their high intensity, coherence, and modulation capability?

Accepted Answer

Introduction: Injection laser diodes (ILDs), often just called laser diodes, provide coherent, narrowly diverging light with fast modulation, making them ideal in optical communications. Recognizing where they outperform LEDs or incandescent sources clarifies system design choices for bandwidth, reach, and coupling efficiency. Given Data / Assumptions: ILD emits coherent, high-brightness light. Can be modulated at high speeds (MHz to multi-GHz). Couples efficiently into optical fibers with appropriate optics. Concept / Approach: Fiber optic links require light sources that can inject large optical power into tiny fiber cores and switch rapidly for digital signaling. ILDs meet these criteria. By contrast, LCDs are light modulators, not emitters; 10BASE-T uses copper twisted pairs; flashlights are incoherent, wide-beam, and unmodulated; radio antennas operate at RF, not optics. Step-by-Step Solution: Identify performance needs: high optical power density, fast modulation, small spot size.Match to device: ILD exhibits stimulated emission and narrow spectral linewidth.Evaluate applications: fiber transmission benefits most from these characteristics.Conclude: fiber optic transmission line is the correct choice. Verification / Alternative check: Standards for long-haul and high-speed links (e.g., SONET/SDH, DWDM) specify laser-based transmitters precisely for their coherence and bandwidth advantages over LEDs. Why Other Options Are Wrong: 10BASE-T Ethernet: Electrical transmission over copper; no laser source required. LCD: Relies on a backlight with liquid crystal modulation; not a laser application. a good flashlight: Uses incoherent broad-spectrum sources; beam quality differs radically. an AM radio antenna: Operates with RF electromagnetic waves, not light. Common Pitfalls: Confusing LEDs (spontaneous emission) with laser diodes (stimulated emission); LEDs are slower and less coherent. Ignoring coupling optics and safety considerations for laser diodes. Final Answer: a fiber optic transmission line

Question 17

Silicon-controlled rectifier (SCR) structure and terminals: evaluate the claim—“An SCR is a three-layer device consisting of a gate, an anode, and a cathode.”

Accepted Answer

Introduction / Context: SCRs are foundational thyristor devices used in power control, rectification, and switching. Accurately recalling their internal structure and external terminals is essential for proper application. This question checks whether you can distinguish between the number of semiconductor layers and the number of terminals in an SCR. Given Data / Assumptions: SCR refers to a standard silicon-controlled rectifier. Terminals: gate (G), anode (A), cathode (K). Focus is on physical semiconductor layer count vs. external terminal count. Concept / Approach: An SCR is a four-layer PNPN semiconductor device with three junctions. It has three terminals: anode, cathode, and gate. The phrasing in the claim confuses layers with terminals. Saying it is a “three-layer device consisting of a gate, anode, and cathode” is incorrect because the layers are PNPN (four layers), and the listed items (gate, anode, cathode) are terminals, not layers. The gate terminal controls turn-on by injecting current into the appropriate junction region, allowing the device to latch in conduction until current falls below the holding current. Step-by-Step Solution: 1) Recall SCR structure: PNPN (four layers) and three junctions. 2) Recall terminals: anode, cathode, and gate (three terminals). 3) Compare the claim: it asserts “three-layer” and lists terminals rather than layers. 4) Conclude the claim is incorrect on both counts: wrong layer count and wrong description. Verification / Alternative check: Textbook cross-sections depict the SCR as a series of P-N-P-N layers. Its equivalent model can be represented by a PNP and an NPN transistor interconnected, reinforcing the presence of four layers and explaining latching behavior. Why Other Options Are Wrong: “Correct”: contradicts standard definitions. Conditional variants referencing low current, germanium devices, or unused gates do not change the internal layer count; they are irrelevant distractions. Common Pitfalls: Confusing the number of terminals with the number of layers; thinking that a terminal count equals layer count; overlooking the PNPN depiction. Final Answer: Incorrect

Question 18

Definition of a transducer: evaluate the statement—“A transducer does not convert energy from one form to another.”

Accepted Answer

Introduction / Context: Transducers are everywhere in measurement and control systems, bridging the physical world and electronics. A correct conceptual understanding is fundamental when selecting sensors and actuators. This question tests the basic definition of a transducer and whether it indeed performs energy conversion between domains (mechanical, thermal, optical, electrical, etc.). Given Data / Assumptions: We use the broad engineering definition common in instrumentation texts. Examples include microphones, thermocouples, strain gauges, speakers, and motor drives. Both passive and active devices are considered. Concept / Approach: A transducer is any device that converts energy from one form to another. Sensors convert physical stimuli into electrical signals (e.g., a thermocouple converts heat differences into voltage). Actuators convert electrical energy into other forms (e.g., a loudspeaker converts electrical signals into sound pressure waves). The conversion can be direct (thermoelectric, piezoelectric) or via an intermediate field (capacitive/inductive). Therefore, a statement that a transducer does not convert energy is contrary to its very definition. Step-by-Step Solution: 1) State the accepted definition: transducer = energy conversion device. 2) Provide sensor example: microphone converts acoustic pressure to electrical voltage/current. 3) Provide actuator example: piezo buzzer converts electrical input into mechanical vibration and sound. 4) Conclude that the claim denying conversion is incorrect. Verification / Alternative check: Instrumentation standards and textbooks consistently define transducers as energy converters. Even “passive” devices like resistive temperature detectors convert thermal energy variation into an electrical resistance change that is interpreted electrically, satisfying the conversion principle. Why Other Options Are Wrong: “Correct”: negates the definition. “Correct only for passive sensors/active sensors”: both still involve energy-domain transduction. “Applies only in digital systems”: the digital/analog distinction is orthogonal to energy conversion. Common Pitfalls: Confusing signal amplification (which may or may not convert energy domains) with transduction; assuming energy conversion requires power flow comparable to macroscopic actuators—many sensors convert with very small energies. Final Answer: Incorrect

Question 19

TRIAC triggering polarity: assess the claim—“A TRIAC can be triggered by either a positive or a negative gate trigger input.”

Accepted Answer

Introduction / Context: TRIACs (triode for alternating current) are bidirectional thyristor devices commonly used for AC power control in dimmers, motor speed controls, and heater regulators. Their key advantage over SCRs is bidirectional conduction and flexible triggering in multiple quadrants of the V-I plane. This question probes whether a TRIAC can respond to both positive and negative gate triggers. Given Data / Assumptions: Standard TRIAC with main terminals MT1 and MT2 and a gate terminal G. Operation on AC mains or AC waveforms. Manufacturer-recommended gate drive currents are achievable in either polarity. Concept / Approach: A TRIAC can be triggered in four quadrants depending on the polarity of MT2 relative to MT1 and the gate current direction. Practical devices are often most sensitive in Quadrant I and III, but they can be triggered by either positive or negative gate pulses, enabling conduction in both positive and negative half-cycles. This is why a single TRIAC can replace back-to-back SCRs for many AC control applications. Step-by-Step Solution: 1) Identify that TRIAC conduction is symmetrical for AC: it can conduct when MT2 is positive or negative relative to MT1 once triggered. 2) Recognize that the gate can be driven with either polarity to initiate conduction, depending on the quadrant. 3) Therefore, a TRIAC accepts both positive and negative gate triggers (with different sensitivities). 4) Conclude the statement is correct. Verification / Alternative check: Device datasheets provide gate trigger current specifications for each quadrant (I–IV), confirming that both positive and negative gate currents can trigger conduction. The exact currents differ by quadrant but the bipolar triggering capability is fundamental. Why Other Options Are Wrong: Incorrect: contradicts TRIAC fundamentals. Only Quadrant I or only with DC loads/MT2 positive: these are artificial restrictions not inherent to TRIAC operation, though sensitivity varies by quadrant. Common Pitfalls: Assuming a TRIAC behaves like a unidirectional SCR; forgetting that snubber networks and dv/dt limits can affect reliable triggering, but not the allowable gate polarity. Final Answer: Correct

Question 20

TRIAC conduction over AC cycle: evaluate the statement—“A TRIAC will control AC voltage during the positive half-cycle only.”

Accepted Answer

Introduction / Context: A TRIAC is designed for bidirectional control of AC power. Unlike an SCR, which conducts only when forward biased and triggered, a TRIAC conducts in both polarities once triggered. This makes it ideal for phase-angle control across the entire AC cycle. The question challenges whether TRIAC control is limited to the positive half-cycle only. Given Data / Assumptions: Standard TRIAC used with appropriate gate triggering circuitry. AC mains waveform with alternating positive and negative half-cycles. Basic phase-control or burst-firing applications. Concept / Approach: Because the TRIAC is symmetrical, it can be triggered into conduction during either half-cycle, and once triggered above the latching current, it will remain on until the current falls below the holding current (usually at the next current zero crossing). Gate drive circuits deliver a pulse at the desired firing angle in each half-cycle, thereby controlling the portion of the waveform conducted and hence the RMS voltage delivered to the load across both halves of the cycle. Step-by-Step Solution: 1) Recognize TRIAC symmetry: conduction is possible with MT2 positive or negative relative to MT1. 2) Apply phase control: trigger at angle α in the positive half-cycle and again at angle α in the negative half-cycle. 3) Since both halves can be controlled, RMS voltage and power are modulated over the full cycle. 4) Conclude that limiting control to the positive half-cycle only is incorrect. Verification / Alternative check: Oscilloscope traces of lamp dimmers show chopped sine waves in both half-cycles. Most application notes for TRIAC dimming explicitly present symmetrical firing for positive and negative halves. Why Other Options Are Wrong: “Correct”: contradicts TRIAC operation. Conditional statements about series diodes, low firing angles, or resistive loads do not change the fundamental bidirectional control capability. Common Pitfalls: Confusing TRIACs with SCRs; overlooking that inductive loads may require snubbers or special triggering but still allow bidirectional phase control; assuming firing only once per cycle. Final Answer: Incorrect

Thyristors and Tranducers Questions