Question 1

DIAC fundamentals — conduction direction and control:
Consider the DIAC (diode for alternating current). The statement claims: “The DIAC conducts in only one direction when gated on.” Evaluate this claim about its conduction characteristics and tri…

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Introduction / Context: The DIAC (diode for alternating current) is widely used as a symmetrical trigger device in AC phase-control circuits, commonly paired with TRIACs for lamp dimmers and speed controllers. This question probes whether a DIAC conducts in only one direction and whether it needs a gate like an SCR or TRIAC. Given Data / Assumptions: Device under discussion: DIAC. Operating context: AC applications with alternating polarity. Idealized understanding of its I–V characteristics, ignoring minor asymmetry between polarities. Concept / Approach: A DIAC is a bidirectional, two-terminal trigger device. It exhibits a breakover voltage of roughly the same magnitude in both polarities. Below breakover, only leakage flows; once the applied voltage exceeds its threshold, the device switches to a low-impedance state and conducts. Because it is two-terminal and symmetrical, it does not have a gate terminal. Therefore, it does not conduct “in only one direction,” nor is it “gated on.” Step-by-Step Solution: Identify terminals: only two (no gate), unlike TRIAC/SCR.Recall I–V behavior: symmetrical breakover for positive and negative polarities.Conclude conduction is bidirectional after breakover.Therefore, the one-direction, gated-on claim is incorrect. Verification / Alternative check: Standard DIAC datasheets plot nearly symmetrical V–I curves in the first and third quadrants, confirming conduction in both polarities after threshold is exceeded. Application notes show DIACs triggering TRIAC gates during both half-cycles of mains. Why Other Options Are Wrong: Correct: Not correct—contradicted by the device’s bidirectional nature. Applies only in DC circuits: DIACs are intended for AC; DC operation is atypical and misses the point. True for negative half-cycles only: Incorrect; both polarities conduct after breakover. Depends on the gate resistor value: DIACs have no gate; a gate resistor applies to TRIACs. Common Pitfalls: Confusing DIACs with diodes or TRIACs; assuming the presence of a gate; forgetting that DIACs are symmetric trigger devices, not rectifiers. Final Answer: Incorrect

Question 2

Opto devices — emission vs detection:
“A photodiode is a device that produces light.” Judge this statement by distinguishing between light-emitting and light-detecting semiconductor components.

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Introduction / Context: Optoelectronic components either emit or detect light. Mixing up their roles leads to incorrect circuit choices. This item checks whether a photodiode is primarily a source of light or a sensor of light. Given Data / Assumptions: Photodiode: a PN junction optimized for photon detection. Comparison device: LED (light-emitting diode) that emits photons under forward bias. Typical use: photodiode under reverse bias for fast, linear photoresponse. Concept / Approach: A photodiode converts incident photons into electrical current (photocurrent). Under reverse bias, the depletion region widens, enabling rapid collection of photo-generated carriers, producing a current roughly proportional to light intensity. By contrast, LEDs convert electrical current into photons via radiative recombination under forward bias. Hence, saying a photodiode “produces light” confuses the roles; it detects light rather than emits it. Step-by-Step Solution: Identify device function: photodiode → detection, LED → emission.Recall biasing: photodiodes often reverse-biased for sensitivity and speed.Note output variable: photocurrent proportional to irradiance.Conclude the statement is incorrect. Verification / Alternative check: Optocouplers demonstrate the roles: an LED on the input side emits light; a photodiode or phototransistor on the output side detects it. Datasheets specify responsivity (A/W) for photodiodes, not luminous flux output. Why Other Options Are Wrong: Correct: False—emission is the LED’s role. Valid only under reverse bias: Reverse bias enables detection; it does not cause emission. True for avalanche photodiodes: APDs still detect light with internal gain; they do not emit. Depends on wavelength: Photodiodes detect across a spectral range; they still do not emit like LEDs. Common Pitfalls: Assuming any “diode” can both emit and detect; mixing up photodiodes and LEDs because both are PN junctions. Final Answer: Incorrect

Question 3

DIAC definition — symmetry and switching mode:
Evaluate the statement: “A DIAC is a bidirectional diode that has a symmetrical switching mode.”

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Introduction / Context: Understanding the role of a DIAC in AC control circuits is fundamental. DIACs are often used to provide a sharp, symmetrical trigger for TRIAC gates, improving phase-control balance between positive and negative half-cycles. Given Data / Assumptions: Two-terminal device with similar breakover in both polarities. Used primarily as a trigger element, not a power switch. Idealized symmetry acknowledged; small tolerances may exist. Concept / Approach: A DIAC behaves like a bidirectional trigger diode. It remains in a high-impedance state until the applied voltage magnitude reaches its breakover threshold (approximately equal in positive and negative directions). At breakover, it snaps to a low-impedance state, discharging capacitors in RC phase-control networks to deliver sharp, symmetrical pulses to a TRIAC gate. This symmetrical switching is key to minimizing waveform distortion and DC offset in controlled loads. Step-by-Step Solution: Identify device: two-terminal, no gate.Recall V–I curve: similar breakover voltage for +/− polarities → bidirectional action.Note use case: symmetrical triggering of TRIAC across both half-cycles.Conclude the statement is correct. Verification / Alternative check: Datasheets show near-symmetric breakover voltages (e.g., ±30 V to ±35 V) and typical negative-resistance region upon conduction, supporting the symmetrical switching description. Why Other Options Are Wrong: Incorrect: Contradicts standard DIAC behavior. Applies only above 400 V: Breakover is tens of volts, not hundreds. True for TRIACs, not DIACs: TRIACs are power switches; DIACs are trigger devices, but symmetry belongs to DIACs as well. Depends on gate current: No gate terminal exists on a DIAC. Common Pitfalls: Confusing DIACs with standard diodes or thinking a DIAC requires a gate; expecting large power handling (it is a trigger component). Final Answer: Correct

Question 4

TRIAC triggering — bidirectional conduction control:
“The gate of a TRIAC can turn it on so that current flows in either direction.” Determine whether this description accurately reflects TRIAC operation.

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Introduction / Context: TRIACs are bidirectional thyristors used widely for controlling AC power to lamps, heaters, and small motors. The core idea is that a gate pulse can trigger conduction in both polarities, simplifying hardware compared to two antiparallel SCRs. Given Data / Assumptions: TRIAC conduction can be triggered in all four gate quadrants, with varying sensitivity. Once latched, the TRIAC conducts until current falls below the holding current, typically at the AC zero crossing. Load and supply are AC; device ratings are respected. Concept / Approach: The TRIAC can be triggered to conduct on either the positive or negative half-cycle. After the gate current initiates conduction, the device latches and continues to conduct for that half-cycle. This property provides bidirectional control with a single device and explains its dominance in single-phase phase-control applications. While a DIAC often improves trigger symmetry, the TRIAC itself is inherently bidirectional once properly gated. Step-by-Step Solution: Apply gate pulse of suitable polarity/magnitude at the desired phase angle.TRIAC latches and conducts for the remainder of the half-cycle in the present polarity.On the next half-cycle, another trigger allows conduction in the opposite polarity as well.Thus, gate control enables bidirectional conduction over AC cycles. Verification / Alternative check: Phase-angle control waveforms show conduction windows in both half-cycles after each trigger event; appliance dimmers confirm bi-directional current through the load. Why Other Options Are Wrong: Incorrect: Ignores the defining bidirectional nature of TRIACs. Valid only with a DIAC: A DIAC is not strictly required, though it improves symmetrical triggering. True only for positive cycles: Not true; both half-cycles are controllable. Depends only on heat-sink size: Thermal limits constrain power, not the bidirectional principle. Common Pitfalls: Assuming TRIACs behave like single SCRs; overlooking holding and latching current requirements; neglecting triggering quadrant sensitivities. Final Answer: Correct

Question 5

Thermistors — correct physical effect:
“A thermistor is a semiconductor device that acts like a piezoresistive resistor.” Decide if this identification of the underlying effect is accurate.

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Introduction / Context: Thermistors and piezoresistive elements are both resistive sensors but respond to different stimuli. Mixing these concepts results in selecting the wrong sensor for temperature or force/pressure measurement tasks. Given Data / Assumptions: Thermistor: temperature-sensitive semiconductor resistor (NTC or PTC). Piezoresistive element: resistance changes due to mechanical strain (as in strain gauges or MEMS bridges). Small-signal measurement context with proper biasing to limit self-heating. Concept / Approach: A thermistor’s resistance varies primarily with temperature due to semiconductor carrier concentration changes; it does not rely on mechanical stress. Piezoresistive devices, by contrast, change resistance when stressed (pressure, acceleration, strain). Therefore, stating that a thermistor “acts like a piezoresistive resistor” is incorrect; the correct term is “thermoresistive” or “temperature-dependent resistor.” Step-by-Step Solution: Identify sensing principle: thermistor → temperature dependence of resistance.Contrast with piezoresistivity: resistance change with mechanical strain.Note typical use: thermistors in temperature sensing/compensation networks.Conclude the statement is inaccurate. Verification / Alternative check: NTC thermistors exhibit a negative temperature coefficient; datasheets specify R–T curves and Beta values, not gauge factor. Piezoresistive sensors specify sensitivity to strain/pressure and gauge factors, confirming the different physics. Why Other Options Are Wrong: Correct: False; thermistors are temperature sensors, not strain sensors. Valid only for NTC types: Both NTC and PTC respond to temperature, not stress. True under mechanical stress: Any incidental stress sensitivity is a secondary effect, not the sensing principle. Depends on self-heating: Self-heating alters apparent temperature but does not change the sensor’s fundamental effect. Common Pitfalls: Confusing “piezoresistive” with “thermoresistive”; overlooking self-heating errors and required bias limits in precision temperature measurements. Final Answer: Incorrect

Question 6

SCR basics — layers and switching behavior:
“SCRs are four-layer devices that are either fully conducting or not conducting.” Determine whether this statement correctly characterizes a silicon-controlled rectifier.

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Introduction / Context: Silicon-controlled rectifiers (SCRs) are cornerstone devices in power electronics. Recognizing their internal structure and bistable conduction behavior is key to understanding controlled rectification and DC choppers. Given Data / Assumptions: SCR structure: four-layer PNPN device with three terminals (anode, cathode, gate). Latching behavior: once turned on by gate current and adequate anode current, it remains on until current falls below the holding current. Idealized behavior between off (blocking) and on (conducting) states. Concept / Approach: An SCR behaves like a latching switch. In the off state, it blocks forward voltage with very small leakage. A gate pulse that raises anode current above latching current turns it on; it then remains on without further gate drive as long as the current stays above the holding threshold. This binary-like operation justifies the phrase “either fully conducting or not conducting” within normal operating regions. Step-by-Step Solution: Confirm PNPN four-layer structure → thyristor family.Gate triggers conduction; device latches when current exceeds latching current.Conduction persists until current drops below holding current (e.g., at AC zero crossing or via forced commutation).Thus, the statement is correct for intended operation. Verification / Alternative check: V–I characteristics show distinct blocking and conduction regions; application notes define latching and holding currents that delimit the switching behavior. Why Other Options Are Wrong: Incorrect: Contradicts the canonical SCR model. True only at low temperature / only DC loads / depends on gate resistance: These factors affect thresholds and triggering ease, not the fundamental bistable nature. Common Pitfalls: Assuming gate drive is needed to keep the SCR on; confusing SCRs with transistors that modulate conduction level via continuous base/gate drive. Final Answer: Correct

Question 7

SCR terminals — correct naming versus BJT:
“A silicon-controlled rectifier (SCR) has 3 terminals labeled base, collector, and emitter.” Decide if these terminal names are correct for an SCR.

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Introduction / Context: Different semiconductor devices use different terminal names. Confusing an SCR with a bipolar junction transistor (BJT) can cause wiring errors and device damage. This question ensures you know the proper SCR terminal nomenclature. Given Data / Assumptions: Device: SCR (a thyristor). Standard SCR terminals: anode (A), cathode (K), gate (G). BJT terminals: base (B), collector (C), emitter (E). Concept / Approach: SCRs are four-layer PNPN devices, not transistors. Their control electrode is the gate, and the main current path is between anode and cathode. Referring to SCR terminals as base, collector, and emitter is incorrect; those names apply to BJTs. Correct naming is critical because pinouts and driver circuits differ completely between SCRs and BJTs. Step-by-Step Solution: Identify the device family: thyristor (SCR), not BJT.Recall correct terminal names: anode, cathode, gate.Compare with BJT terminal names: base, collector, emitter.Conclude the given statement is incorrect. Verification / Alternative check: Manufacturers’ datasheets for SCRs label pins as A, K, and G. Application circuits show gate triggering between gate and cathode, not “base–emitter.” Why Other Options Are Wrong: Correct / valid only for certain SCRs / depends on package: No SCR uses BJT terminal naming; package does not alter terminology. True when used as a switch: Function does not change terminal names. Common Pitfalls: Assuming all three-terminal power devices share transistor naming; miswiring gate/cathode as if they were base/emitter. Final Answer: Incorrect

Question 8

Turning off an SCR — gate removal vs commutation:
“To turn an SCR off, simply remove the gate voltage.” Evaluate this statement in light of SCR latching behavior and holding current.

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Introduction / Context: Unlike transistors, SCRs latch on once triggered. Many beginners assume removing the gate signal will turn an SCR off. This question clarifies the correct turn-off mechanisms and prevents control-circuit mistakes. Given Data / Assumptions: SCR is already conducting (latched). Gate current has ceased after triggering. Load and source support continuous current unless interrupted. Concept / Approach: An SCR remains on after triggering as long as its anode current exceeds the holding current. Removing gate drive does not force turn-off. To turn it off, the anode current must be reduced below the holding current—achieved naturally at AC zero crossings (natural commutation) or via forced commutation in DC circuits using auxiliary switches, capacitors, or inductors. Negative gate pulses may aid in special devices but are generally not relied upon for turn-off in standard SCRs. Step-by-Step Solution: Trigger SCR → device latches when current > latching current.Remove gate signal → SCR continues conducting if current > holding current.Reduce anode current below holding current (via line zero crossing or forced commutation) → device turns off.Therefore, gate removal alone does not turn off an SCR. Verification / Alternative check: Textbook waveforms show the gate pulse only at the beginning of conduction; the device remains on without continuous gate drive and turns off at current zero or by commutation circuits. Why Other Options Are Wrong: Correct: Incorrect; contradicts latching behavior. Valid only with negative gate bias: Not standard; most designs do not depend on reverse gating to turn off. True for TRIACs but not SCRs: TRIACs also latch; they turn off at current zero. Depends exclusively on ambient temperature: Temperature shifts thresholds slightly but not the basic mechanism. Common Pitfalls: Designing SCR controls like transistor drivers; forgetting holding/latching current concepts and commutation requirements. Final Answer: Incorrect

Question 9

Optical isolation — purpose of optically coupled interrupter/isolator modules:
“The optically coupled isolator interrupter module is used to transfer switching information between two electrically isolated points.” Evaluate this statement.

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Introduction / Context: Optical isolators (optocouplers) and interrupter modules use light to communicate signals across galvanic isolation. They are essential for breaking ground loops, protecting low-voltage logic from high-voltage transients, and sensing position or speed without electrical contact. Given Data / Assumptions: Isolator contains a light emitter (LED) and a light sensor (phototransistor, photodiode, or phototriac). Input and output sides are electrically isolated but optically coupled. Interrupter modules may be slot-type sensors detecting the presence/absence of an opaque vane. Concept / Approach: Optical coupling transfers information via photons instead of electrons across a direct conductor, providing high isolation voltage. Driving the LED encodes a switching state; the detector reproduces this state on the isolated side. Interrupter modules convert mechanical motion (blocking/unblocking the optical path) into logic-level transitions, which is still “switching information” conveyed from a different domain without electrical continuity. Step-by-Step Solution: Recognize functional blocks: LED emitter and photodetector separated by insulation or an optical gap.Input current → LED on → light reaches detector → output changes state.No conductive path → high common-mode and surge immunity; grounds are isolated.Thus, switching information is conveyed across isolation. Verification / Alternative check: Datasheets specify isolation voltages (e.g., 2.5 kV) and CTR (current transfer ratio), confirming the intent to pass control signals across isolation barriers in power supplies, motor drives, and digital interfaces. Why Other Options Are Wrong: Incorrect: Contrary to the fundamental purpose of optocouplers. Valid only at DC below 12 V: Optocouplers work with logic-level AC or DC, limited by device bandwidth, not a fixed 12 V cap. Depends on LED wavelength only: Wavelength matters for sensitivity, but isolation function does not hinge solely on it. True only with magnetic shielding: Magnetic fields are not the primary concern; optical isolation is immune to conduction paths. Common Pitfalls: Confusing isolators with transformers or magnetic sensors; overlooking CTR, propagation delay, and creepage/clearance requirements in high-voltage designs. Final Answer: Correct

Question 10

Hall effect sensors — what they detect:
“A semiconductor using the Hall effect will detect magnetic fields.” Assess the correctness of this statement based on the Hall phenomenon.

Accepted Answer

Introduction / Context: Hall effect sensors are ubiquitous in automotive, industrial, and consumer applications for measuring position, speed, and current. This question asks whether a semiconductor employing the Hall effect detects magnetic fields—capturing the essence of how these sensors work. Given Data / Assumptions: A thin semiconductor element carries a bias current. A magnetic field perpendicular to the current flow is present. Steady-state operation with appropriate signal conditioning (e.g., amplifiers, temperature compensation). Concept / Approach: The Hall effect states that when a current-carrying conductor or semiconductor is placed in a magnetic field (with a component perpendicular to the current), charge carriers experience a Lorentz force, accumulating on one side and producing a transverse voltage (Hall voltage). This voltage is proportional to magnetic flux density (within the sensor’s range), enabling detection and measurement of magnetic fields. Integrated Hall sensors include amplifiers, references, and comparators to provide analog or digital outputs. Step-by-Step Solution: Bias the Hall plate with a known current I.Apply magnetic flux density B perpendicular to the plate.A transverse Hall voltage V_H develops, roughly proportional to I * B / (q * n * t), where n and t relate to carrier density and thickness.Condition V_H to yield an output indicative of magnetic field presence/magnitude. Verification / Alternative check: Current sensors place Hall elements near busbars to measure magnetic fields from load current; rotary position sensors use magnets and Hall ICs to generate angle-dependent outputs—both practical validations of magnetic field detection. Why Other Options Are Wrong: Incorrect: Directly contradicts the Hall principle. Valid only at cryogenic temperatures: Modern Hall ICs operate over wide industrial ranges. True only for ferromagnetic fields: Any magnetic field component normal to the plate is detectable; ferromagnetism is not required. Depends solely on carrier mobility: Mobility influences sensitivity, but detection does not depend solely on it. Common Pitfalls: Confusing magnetoresistive and Hall sensors; overlooking the need for proper orientation (field perpendicular to current for maximum response). Final Answer: Correct

Thyristors and Tranducers Questions