Question 1

Power supplies — Regulation function: Providing a nearly constant output despite changes in AC input or load resistance is the function of which block?

Accepted Answer

Introduction / Context: Modern power supplies are built from functional blocks: transformer, rectifier, filter, and regulator. Distinguishing their roles is crucial for design and troubleshooting. Regulation specifically means holding the DC output stable as input or load varies within limits. Given Data / Assumptions: AC mains or AC secondary feeding a supply. Rectification and filtering produce a DC rail with residual ripple. A regulator stage adjusts to input/load changes to keep output constant. Concept / Approach: The regulator senses output and corrects for deviations using feedback or reference-based control (linear LDOs, series regulators, or switching regulators such as buck/boost). The transformer sets voltage level and isolation; the rectifier converts AC to pulsating DC; the filter smooths ripple; only the regulator enforces a constant output voltage or current target. Step-by-Step Solution: Identify blocks: transformer → rectifier → filter → regulator.State functions: isolation/step, AC-to-DC, smoothing, and stabilization respectively.Select the block providing constancy: the regulator.Examples: 7805 LDO, buck converters, and active feedback controllers. Verification / Alternative check: Measure load regulation and line regulation: with a regulator, output change remains small over a range of line and load; without one, DC varies significantly with input and current draw. Why Other Options Are Wrong: transformer: Sets AC level/isolation; does not regulate DC output. filter: Reduces ripple but cannot hold a setpoint under load or line changes. rectifier: Determines polarity and conversion to DC; not regulation. surge suppressor: Protects against spikes, not steady regulation. Common Pitfalls: Assuming a large capacitor alone guarantees steady output; load changes still move the DC level without regulation. Ignoring thermal and dropout limits in linear regulators and duty-cycle limits in switching regulators. Final Answer: regulator

Question 2

Atomic structure for electronics: Electrons that reside in the outermost shell (orbit) of an atom are called:

Accepted Answer

Introduction / Context: Semiconductor physics and materials science hinge on how electrons are arranged in atoms and solids. The electrons in the outermost shell determine bonding, doping behavior, and electrical properties—hence the importance of correctly identifying valence electrons. Given Data / Assumptions: Bohr-shell language used loosely to describe outer electron occupancy. Distinction between bound electrons in valence shells and mobile carriers in conduction bands. Neutral atoms before ionization or bonding considered. Concept / Approach: Valence electrons occupy the highest-energy filled shell and are responsible for chemical bonding and the transition to conduction when promoted. In solids, band theory describes these as electrons near the top of the valence band which can move to the conduction band when provided sufficient energy or doping creates states near the Fermi level. Step-by-Step Solution: Identify the outer shell: electrons there are valence electrons.In semiconductors (e.g., Si), four valence electrons participate in covalent bonds.When energy is provided, some may transition to the conduction band, becoming mobile carriers.Doping modifies availability of carriers (donors/acceptors) by changing band occupancy. Verification / Alternative check: Chemical periodic trends (valence count) predict bonding patterns and correlate with measured electrical properties: monovalent metals conduct readily due to a single valence electron that easily becomes free. Why Other Options Are Wrong: free electrons: These are already delocalized conduction carriers, not necessarily bound in the outer shell. negative ions: Atoms with net negative charge after gaining electrons; not a term for outer electrons themselves. conduction band electrons: Carriers after promotion; distinct from bound valence electrons. donor cores: Not a standard term; donors provide extra electrons when ionized, but the electrons themselves are carriers, not “cores.” Common Pitfalls: Equating valence electrons with always-free carriers; in intrinsic semiconductors, they are bound until excited. Confusing atomic shell language with band diagrams; they map conceptually but are not identical models. Final Answer: valence electrons

Question 3

Rectifier orientation and output polarity: The polarity of the DC output from a rectifier primarily depends on which factor?

Accepted Answer

Introduction / Context: Rectifiers convert AC to DC. While ripple content and average value depend on topology and filtering, the actual polarity of the rectified output is dictated by diode orientation. Correct understanding prevents reverse-polarity damage in power supplies. Given Data / Assumptions: Standard rectifier circuits (half-wave, full-wave center-tap, bridge). Ideal diodes for polarity reasoning. Load connected with conventional positive and negative rails. Concept / Approach: Diodes conduct in one direction. The way they are installed relative to the transformer or AC source determines which half-cycles are passed to the positive rail and which to the negative return. Topology (half or full wave) affects ripple frequency, not which node is positive. Capacitor polarity must match the output polarity that the diodes establish, not define it. Step-by-Step Solution: Observe diode anode/cathode orientation in the schematic.Trace current paths during each half-cycle; the node where current enters the load becomes the positive output.Changing a diode’s orientation reverses which terminal becomes positive.Bridge rectifier example: the plus and minus outputs are defined by the fixed diode arrangement. Verification / Alternative check: Swap the diode orientation in a simulator: the output polarity flips accordingly even though the input AC and frequency remain unchanged, confirming the dependency on installation. Why Other Options Are Wrong: cycles of input: AC cycles determine time behavior, not DC polarity. capacitor polarity: The capacitor must follow the established polarity; it does not set it. half or full wave: Determines ripple frequency and average value, not the sign of output. transformer kVA rating: Concerns capacity, not polarity. Common Pitfalls: Miswiring electrolytic capacitors because polarity is misunderstood; always reference diode orientation first. Assuming full-wave automatically means positive rail at a certain node; verify the bridge orientation. Final Answer: diode installation

Question 4

Power supply functional blocks — output identification: In a typical block diagram of a linear power supply, which block produces a pulsating DC output (before smoothing)?

Accepted Answer

Introduction / Context: Understanding the sequence of conversion in power supplies helps diagnose faults and design robust systems. The rectifier stage is responsible for converting AC into a unidirectional, pulsating DC waveform that is later smoothed by filters and stabilized by regulators. Given Data / Assumptions: Standard stages: transformer → rectifier → filter → regulator. AC source provides sinusoidal input to the transformer or directly to the rectifier. Ideal behavior assumed for conceptual clarity. Concept / Approach: The rectifier, using diodes, passes current only during certain portions of each AC cycle. As a result, its output is unidirectional but time-varying—a pulsating DC. The filter reduces ripple by energy storage in capacitors/inductors, and the regulator maintains a steady DC level under varying load/line conditions. Step-by-Step Solution: Input AC is applied to the rectifier.Diodes conduct on one (half-wave) or both halves (full-wave/bridge), producing unidirectional pulses.The resulting waveform is DC in polarity but still contains substantial ripple.A downstream filter then smooths this pulsating DC into a more constant level prior to regulation. Verification / Alternative check: Scope measurements after the rectifier show a waveform always above (or below) zero but with peaks at line frequency (half-wave) or twice line frequency (full-wave), confirming pulsating DC. Why Other Options Are Wrong: transformer: Provides AC at a different voltage and isolation; output is still AC. filter: Produces smoothed DC, not the initial pulsating DC. regulator: Outputs stabilized DC with minimal variation. surge limiter: Protects against inrush/spikes; not a conversion stage. Common Pitfalls: Confusing the rectifier output with fully smoothed DC; the capacitor's role is essential for ripple reduction. Overlooking ripple frequency differences between half-wave and full-wave rectification. Final Answer: rectifier

Question 5

Diode symbol orientation (PN junction and rectifiers): In a standard diode schematic, the triangular/arrow-like indicator is drawn pointing toward which terminal or material type?

Accepted Answer

Introduction / Context: Reading diode symbols correctly is essential for assembling and debugging power supplies, rectifiers, clamping networks, and signal paths. A diode conducts conventional current from its anode (p-type) to its cathode (n-type) when forward-biased. Many learners confuse which side is which on the schematic, leading to reversed parts and nonfunctional circuits. Given Data / Assumptions: We consider the conventional semiconductor diode symbol used in electronics. Anode corresponds to p-type material; cathode corresponds to n-type material. Conventional current flows from anode to cathode when forward-biased. Concept / Approach: The diode symbol depicts a triangle/arrow-like shape or a pointed side going toward a vertical bar. A helpful memory aid is ”arrow/point to bar.” The bar represents the cathode. Physically, the cathode lead on many packaged diodes is marked with a stripe. Thus, the pointed side of the schematic symbol is oriented toward the cathode side, aligning with current flow direction (anode to cathode) when forward-biased. Step-by-Step Solution: Identify the bar in the diode symbol: it denotes the cathode terminal.Note that the pointed side (the arrow-like tip) faces the bar.Therefore, the pointer is directed toward the cathode lead (n-type side). Verification / Alternative check: On physical parts, the striped end marks the cathode. When you place the diode so that the schematic symbol’s bar faces the striped end, polarity is correctly observed and the biasing behavior in circuits (e.g., rectifiers) matches expectation. Why Other Options Are Wrong: Trivalent-doped material/anode lead: These refer to the p-type side; the symbol points toward the opposite side. Positive axial lead: The diode's cathode need not be the ”positive” terminal; polarity depends on operating conditions. Common Pitfalls: Confusing electron flow with conventional current, and assuming the physical package ”dot” always marks the cathode (markings vary; look for the stripe). Always verify with a meter’s diode test if in doubt. Final Answer: cathode lead

Question 6

Band structure basics in solid-state physics: In which class of materials is the energy gap between the valence band and the conduction band the smallest?

Accepted Answer

Introduction / Context: Understanding band diagrams explains why some materials conduct easily, some conduct only under certain conditions, and others barely conduct at all. The relative spacing between the valence band and the conduction band (the ”band gap”) directly determines carrier availability and electrical behavior. Given Data / Assumptions: Idealized band theory picture for conductors (metals), semiconductors (Si, Ge), and insulators (glass, ceramics). Room-temperature behavior under small applied electric fields. ”Closest” means smallest separation or overlap between valence and conduction bands. Concept / Approach: In conductors, the valence band overlaps with the conduction band, or a partially filled band exists. That means effectively zero band gap, so electrons require negligible additional energy to participate in conduction. Semiconductors have a small but nonzero band gap (for silicon, about 1.1 eV at room temperature), and insulators have a large band gap (typically greater than 3 eV). Therefore, the ”closest” relationship is in conductors, where the bands meet or overlap. Step-by-Step Solution: Identify typical gaps: conductor ≈ 0 eV (overlap), semiconductor ≈ 0.5–2 eV, insulator >≈ 3 eV.Compare: smallest separation occurs in conductors.Select the material class with overlapping/touching bands: conductors. Verification / Alternative check: The resistivity orders of magnitude match: metals have the lowest resistivity due to free carriers from the overlapping bands; semiconductors require doping or thermal excitation; insulators remain highly resistive because excitation across the large gap is unlikely at room temperature. Why Other Options Are Wrong: Semiconductors: Small gap, but not the smallest; conduction still requires excitation. Insulators: Largest band gap among the three. Same for all: Contradicts well-established band theory and measured properties. Common Pitfalls: Confusing ”carrier concentration” with ”band gap”; doping affects the Fermi level and carrier density, but does not turn a semiconductor into a metal in terms of band overlap. Final Answer: conductors

Question 7

Integrated-circuit linear regulators: Under what condition does an IC regulator invoke thermal shutdown protection?

Accepted Answer

Introduction / Context: Most IC voltage regulators (linear or switching controllers) include protection features such as current limiting, safe operating area protection, and thermal shutdown. Knowing precisely what triggers thermal shutdown prevents misdiagnosis when a regulator shuts off under heavy load or insufficient heatsinking. Given Data / Assumptions: Standard monolithic regulator with on-die temperature sensor. Heatsink and ambient conditions can raise junction temperature during operation. Thermal shutdown is distinct from current limiting or foldback. Concept / Approach: Thermal shutdown is activated by an internal temperature threshold (junction temperature), not directly by power, current, or load resistance. While high dissipation or short circuits may indirectly raise temperature, the shutdown comparator monitors temperature and latches or cycles the device off once the threshold is exceeded, protecting the silicon from damage. Step-by-Step Solution: Identify the monitored variable: junction temperature (T_j), not current or voltage.Recognize indirect causes: high current and voltage drop increase P = V_drop * I, which heats the die.Conclude: the immediate trigger for thermal shutdown is excessive internal temperature. Verification / Alternative check: Device datasheets specify a thermal shutdown threshold (for example, around 150–175 °C) and a hysteresis value for auto-restart. Load and ambient tests confirm cycling when T_j crosses this threshold, regardless of the specific current value at that instant. Why Other Options Are Wrong: Power/current too high: Can contribute to heating, but shutdown trip is based on temperature, not directly on power or current. Load resistance increases: Usually reduces current and lowers die temperature, the opposite of causing shutdown. Common Pitfalls: Assuming a fixed current limit is the cause when a regulator turns off under thermal stress; in fact, many devices hit current limit first, then heat rises and thermal shutdown follows later. Final Answer: internal temperature is too high

Question 8

Semiconductor device identification: A diode designed so that changing the reverse-bias voltage varies its junction capacitance is called a ______.

Accepted Answer

Introduction / Context: Voltage-controlled capacitance is widely used in RF tuning, voltage-controlled oscillators (VCOs), and phase-locked loops (PLLs). The semiconductor device that provides a controllable junction capacitance is the varactor (also called varicap or tuning diode). Given Data / Assumptions: Reverse-biased diode junction is operated well below breakdown. Capacitance depends on depletion region width, which varies with reverse voltage. Small-signal RF operation around the bias point. Concept / Approach: In a varactor, the junction is engineered so the depletion region changes predictably with reverse bias, producing a capacitance C_j(V_R) that decreases as reverse voltage increases. This effect enables electronic tuning without mechanical variable capacitors. Other diode types (zener, tunnel) have different operating principles and intended uses (breakdown regulation and negative resistance respectively). Step-by-Step Solution: Recognize the control mechanism: reverse-bias voltage modulates depletion width.Relate depletion width increase to capacitance decrease: C ≈ εA / W.Identify the device optimized for this behavior: the varactor diode. Verification / Alternative check: Datasheets for varactors provide C–V curves and quality factor (Q) at RF; tuning ranges in VCOs directly correspond to the chosen varactor’s C_j(V_R) characteristics. Why Other Options Are Wrong: Tunnel diode: Used for high-speed switching/oscillation via quantum tunneling and negative resistance. Zener diode: Operated in reverse breakdown for voltage regulation, not for variable capacitance. Switching diode: General-purpose fast diode for logic-level switching; capacitance is not the primary feature. Common Pitfalls: Biasing a varactor forward by mistake (destroying the tuning function) or neglecting series resistance and Q in RF designs. Final Answer: varactor diode

Question 9

Rectifier principle: Because a diode mimics an open/closed switch depending on polarity, an alternating current (AC) input can be ______ by a diode network.

Accepted Answer

Introduction / Context: Diodes conduct current primarily in one direction. This directional behavior allows conversion of AC to pulsating DC, the first step in most power supplies before filtering and regulation. Recognizing ”rectification” distinguishes the role of diodes from regulators, attenuators, or general control elements. Given Data / Assumptions: Ideal diode forward-conducts and reverse-blocks (open/closed switch analogy). Input is sinusoidal or any alternating waveform centered around zero. Load and transformer details are not required to define the action. Concept / Approach: Half-wave and full-wave rectifiers pass only portions of the AC cycle to the load. A single diode creates half-wave rectification; two diodes with a center-tapped transformer or a four-diode bridge produce full-wave rectification. Regulation (steady DC level) requires additional components such as capacitors, inductors, or IC regulators beyond simple diodes. Step-by-Step Solution: Model each diode as a polarity-sensitive switch.Ensure only one polarity reaches the load per half-cycle (half-wave) or both polarities are steered as the same direction (full-wave).Conclude the action is rectification: AC → pulsating DC. Verification / Alternative check: Oscilloscope inspection of load voltage in a diode rectifier shows unidirectional pulses; adding a capacitor smooths these pulses toward DC, demonstrating the separation between rectification and filtering/regulation stages. Why Other Options Are Wrong: Regulated: Requires active control or reference; diodes alone do not maintain a constant output level (except zeners used differently). Controlled/attenuated: Generic terms; the diode's primary use in this context is rectification. Common Pitfalls: Expecting a ”smooth DC” directly from a rectifier without filters; the immediate output is pulsating and load-dependent. Final Answer: rectified

Question 10

Diode symbol polarity (PN junction): The arrow/point in the schematic depiction of a diode points toward which semiconductor region and terminal name?

Accepted Answer

Introduction / Context: Correctly identifying diode terminals from the schematic is critical when orienting the device on a PCB. The cathode end must align with the symbol's bar to guarantee forward conduction in the intended direction and proper rectifier behavior. Given Data / Assumptions: Anode corresponds to p-type; cathode corresponds to n-type. Conventional current flows from anode to cathode when forward-biased. Markings: the cathode lead is often marked with a painted stripe on axial diodes. Concept / Approach: The conventional diode symbol shows a triangle/arrow-like point directed toward a vertical bar. The bar denotes the cathode (n-type side). Thus, the pointed tip indicates the direction toward the cathode. Remember ”point to bar = cathode,” and ”current flows from anode (no bar) to cathode (bar)” when forward-biased. Step-by-Step Solution: Find the bar in the symbol; it represents the cathode.Note the pointed side faces the bar; therefore it faces the cathode.Relate material types: the cathode is the n-type region, so the point faces n-type. Verification / Alternative check: Use a DMM diode test: positive probe to anode and negative to cathode yields a forward drop reading (for silicon, about 0.6–0.7 V). Reversing the probes yields open circuit, matching the symbol orientation rule. Why Other Options Are Wrong: Anode/p-type answers: Opposite terminal; the point does not face p-type. Calling the n-type region the anode (Option A) is incorrect terminology. Common Pitfalls: Mixing up electron flow with conventional current or confusing LED light arrows (which depict emitted light) with the diode polarity indicator. Final Answer: the n-type material, which is called the cathode

Question 11

Full-wave rectifier output polarity: In a properly wired full-wave rectifier, the current through the load resistor flows ______ over time.

Accepted Answer

Introduction / Context: Full-wave rectification is designed to convert both halves of an AC waveform into unidirectional current through the load, improving efficiency and reducing filter requirements compared with half-wave rectification. Understanding load current direction is fundamental to predicting ripple and sizing capacitors. Given Data / Assumptions: Full-wave rectification via either a center-tapped two-diode circuit or a bridge rectifier. Ideal diodes for conceptual clarity. Resistive load. Concept / Approach: Each half-cycle of AC is routed so that the load always sees the same polarity. In a bridge, two diodes conduct on the positive half-cycle and the other two on the negative half-cycle, but current through the load keeps the same direction. In the center-tapped scheme, one diode conducts per half-cycle to maintain the same load polarity. Thus, the load current direction is constant over time (though its magnitude pulsates). Step-by-Step Solution: Track conduction paths for the positive half-cycle and note load current direction.Track conduction paths for the negative half-cycle; different diodes conduct, but load polarity is unchanged.Conclude that the load current flows in the same direction for both halves. Verification / Alternative check: Oscilloscope across the load shows a pulsating DC waveform that never reverses polarity. After adding a filter capacitor, ripple decreases, but polarity remains the same. Why Other Options Are Wrong: Opposite directions: That describes AC directly across the load (no rectifier). ”From the reverse-biased diode”: Reverse-biased diodes do not conduct. ”To the external load”: Vague and does not address directionality. Common Pitfalls: Assuming current direction alternates because the input is AC; the rectifier topology specifically prevents this at the load. Final Answer: in the same direction

Question 12

Transformer requirement for 180° phase difference to a rectifier: Which transformer configuration supplies two equal-magnitude secondary voltages that are 180° out of phase for full-wave rectification?

Accepted Answer

Introduction / Context: One classic full-wave rectifier topology uses a center-tapped transformer secondary and two diodes. The center tap provides a reference, and the two half-windings produce equal voltages that are 180° out of phase, steering both halves of the AC cycle in the same direction through the load. Given Data / Assumptions: Desire is to obtain two anti-phase secondary voltages for a two-diode full-wave rectifier. Transformer considered ideal for phase discussion. Load is connected from the diode junction to the center tap. Concept / Approach: A center-tapped secondary comprises two identical half-windings about a common midpoint. The voltages at the ends are equal in magnitude but opposite in instantaneous polarity with respect to the center tap. Each half-cycle forward-biases exactly one diode, producing full-wave rectification with only two diodes. Alternatives like ”step-down” or ”stepped-up” describe magnitude changes, not phase relationships; ”split winding primary” pertains to primary connections, not the secondary phase requirement for this rectifier style. Step-by-Step Solution: Identify the need: two secondary voltages 180° apart.Use a center-tapped secondary to obtain equal and opposite voltages around the midpoint.Connect two diodes so each conducts on alternate half-cycles, yielding full-wave output across the load. Verification / Alternative check: Phasor diagrams show V_s1 = +V/2 and V_s2 = −V/2 relative to the center tap, confirming the 180° phase difference. Lab measurements of each end versus the center confirm opposite polarity at any instant. Why Other Options Are Wrong: Step-down/stepped-up: Only specify turns ratio; no guarantee of phase-split outputs. Split winding primary: Concerns primary-side flexibility; does not directly yield the required secondary phase relationship. Common Pitfalls: Confusing the bridge rectifier (which needs no center tap) with the two-diode full-wave rectifier that requires one. Final Answer: center-tapped secondary

Question 13

Smoothing requirements after rectification: Which rectifier configuration delivers the least-ripple output for a given load and capacitor, thereby requiring the least additional filtering?

Accepted Answer

Introduction / Context: After converting AC to pulsating DC, designers usually add filters to reduce ripple. The ripple frequency of the rectified output determines how large a capacitor or inductor is needed. Full-wave and bridge rectifiers double the ripple frequency compared with half-wave rectifiers, reducing filter requirements. Given Data / Assumptions: Same load resistance and same filter capacitor across compared rectifiers. Input is a sine wave at mains frequency f. Ideal diodes for conceptual comparison. Concept / Approach: A half-wave rectifier passes only one half-cycle, producing ripple at frequency f. Both the center-tapped full-wave rectifier (two diodes) and the bridge rectifier (four diodes) produce pulses on both half-cycles, so ripple occurs at 2f. For the same load and capacitance, higher ripple frequency means less discharge time per cycle and therefore less ripple amplitude. Hence, full-wave and bridge rectifiers require less filtering than a half-wave rectifier. Step-by-Step Solution: Compare ripple frequencies: f (half-wave) vs 2f (full-wave/bridge).For identical RC, ripple voltage approximately scales inversely with frequency.Conclude: full-wave and bridge need the least filtering among the listed choices. Verification / Alternative check: Use the approximate capacitor-input filter relation: ΔV ≈ I_load / (f_ripple * C). Doubling f_ripple halves the ripple, demonstrating why full-wave/bridge are superior in this respect. Why Other Options Are Wrong: Half-wave: Lowest ripple frequency, largest ripple, most filtering required. Full-wave alone or bridge alone: Each is correct individually; the best answer groups them because both exhibit 2f ripple. Common Pitfalls: Confusing diode count with output quality; more diodes in a bridge do not worsen ripple—voltage drop changes but ripple frequency still doubles. Final Answer: A full-wave rectifier and a bridge rectifier

Question 14

Power-supply regulation calculation: Given a no-load output voltage Vnl = 20 V and a full-load output voltage Vfl = 19.8 V for a regulated DC supply, what is the percent voltage regulation (use % regulation = ((Vnl − Vfl) / Vfl) * 100)?

Accepted Answer

Introduction / Context: Voltage regulation quantifies how much a power supply’s output voltage changes from no-load to full-load conditions. A smaller regulation percentage indicates a “stiffer” supply that holds its output more constant as current demand increases. This metric is widely used when comparing linear and switching regulators, or when validating a design against a specification sheet. Given Data / Assumptions: No-load voltage, Vnl = 20 V. Full-load voltage, Vfl = 19.8 V. Use the common definition: % regulation = ((Vnl − Vfl) / Vfl) * 100. Steady-state, room-temperature conditions; meter loading is negligible. Concept / Approach: When a load is connected, internal resistance and control-loop limitations cause a slight output drop. Regulation expresses that drop as a percentage referenced to the loaded voltage. Using the definition provided, compute the difference between no-load and full-load, then divide by the full-load voltage and multiply by 100 to convert to percent. Step-by-Step Solution: Compute the drop: ΔV = Vnl − Vfl = 20 − 19.8 = 0.2 V.Form the ratio to the full-load voltage: ΔV / Vfl = 0.2 / 19.8.Evaluate: 0.2 / 19.8 ≈ 0.010101…Convert to percent: 0.010101… * 100 ≈ 1.01%, which rounds to 1%. Verification / Alternative check: Some texts reference Vnl instead of Vfl in the denominator. Here the question explicitly specifies division by Vfl, removing ambiguity. If one mistakenly used Vnl, the result would be (0.2/20)*100 = 1.0%, which is essentially the same to two significant figures, reinforcing the answer of about 1%. Why Other Options Are Wrong: 0%: would require Vnl = Vfl (no drop), which is not the case..1%: ten times too small; ignores the 0.2 V change relative to ~20 V.5%: five times too large given only a 0.2 V change. Common Pitfalls: Confusing which voltage to use in the denominator, mixing up units (mV vs V), or rounding too aggressively before the percent conversion. Always compute the difference first, then divide by the specified reference voltage, then multiply by 100. Final Answer: 1%

Question 15

Device identification in electronics: What is a varistor commonly defined as in circuit protection and voltage-clamping applications?

Accepted Answer

Introduction / Context: A varistor is widely used to protect circuits from voltage transients such as surges and spikes. Recognizing its basic definition helps technicians select appropriate surge protection (e.g., MOVs across mains lines, relay coils, or DC rails) and understand how clamping behavior differs from that of zener diodes and transient-voltage-suppression diodes. Given Data / Assumptions: We are classifying a varistor as a component type. General engineering usage (e.g., metal-oxide varistors, MOVs). Focus is on basic definition rather than construction details. Concept / Approach: A varistor (variable resistor) exhibits a nonlinear, voltage-dependent resistance. At normal line or signal voltages, its resistance is very high and current is negligible. When a transient pushes the voltage above its knee, the device’s resistance drops dramatically, clamping the voltage by conducting surge current and thereby protecting sensitive electronics downstream. Step-by-Step Solution (behavioral view): At nominal voltage: R_varistor is very large → negligible current.At surge voltage: R_varistor collapses → current diverts through the varistor.Voltage at the protected node is held near the clamping level.After the surge: voltage returns to nominal → varistor returns to high resistance. Verification / Alternative check: Datasheets show V–I curves with a pronounced knee: current rises steeply past the clamping region. This is characteristic of voltage-dependent resistance, not a diode’s unidirectional conduction curve. Why Other Options Are Wrong: Voltage-dependent diode: describes zener/TVS diodes, which conduct in a defined direction and mechanism.Current-dependent resistor/diode: does not match the primary varistor control variable, which is voltage. Common Pitfalls: Confusing varistors with zeners; both clamp voltage, but varistors are bidirectional (symmetrical V–I), while zeners are diodes with polarity and specific breakdown behaviors. Final Answer: a voltage-dependent resistor

Question 16

PN junction behavior under reverse bias: The voltage at which a reverse-biased diode begins to conduct heavily due to avalanche or Zener effects is called the:

Accepted Answer

Introduction / Context: Reverse-biased diodes normally pass only tiny leakage currents. Above a certain negative voltage, however, strong electric fields enable avalanche or Zener tunneling, causing dramatic current increase. Correctly naming this threshold is essential when specifying protection components or reference regulators. Given Data / Assumptions: Standard PN diode under reverse bias. We are identifying the voltage where reverse current rises sharply. Mechanism could be avalanche (higher voltage) or Zener tunneling (lower voltage), depending on doping. Concept / Approach: The term “breakdown voltage” denotes the reverse voltage at which the junction conducts substantially. Zener diodes are designed to operate in this region safely, providing a quasi-constant voltage for regulation. Ordinary signal diodes are not intended for sustained breakdown operation, as it can cause overheating and damage. Step-by-Step Solution: Increase reverse bias on a PN junction.At the breakdown threshold, high field enables carrier multiplication or tunneling.Current rises steeply; the associated voltage is the breakdown voltage.Zener devices exploit this region in a controlled way. Verification / Alternative check: Examine I–V curves: reverse current remains near-zero until reaching the knee where breakdown occurs and current increases rapidly with slight voltage change. Why Other Options Are Wrong: Barrier potential: small forward threshold (~0.7 V Si).Forward voltage: pertains to forward conduction.Biasing voltage: generic term; not the breakdown threshold. Common Pitfalls: Assuming breakdown always destroys the device. While true for non-Zener diodes, Zeners are engineered to operate there within power limits. Final Answer: breakdown voltage

Question 17

Silicon diode conduction check: If the external polarity matches the diode orientation and the measured potential difference across a silicon diode exceeds about 0.7 V, the diode is considered:

Accepted Answer

Introduction / Context: Technicians often assess diode orientation and bias using quick multimeter checks. For silicon PN diodes, forward conduction typically begins around 0.6–0.7 V at room temperature. Recognizing this threshold allows rapid fault isolation in rectifiers, clamp circuits, and logic protection paths. Given Data / Assumptions: Silicon diode (not Schottky or germanium). Polarity applied in the forward direction. Observed potential difference (PD) > 0.7 V. Concept / Approach: Forward bias reduces the depletion region, allowing majority carriers to cross the junction. The diode’s I–V curve shows an exponential rise in current beyond approximately 0.7 V (silicon), with exact value depending on temperature and current level. Therefore, a PD above ~0.7 V with matching polarity indicates forward bias and conduction. Step-by-Step Solution: Check polarity: anode at higher potential than cathode.Measure PD: if around or above ~0.7 V, carriers are injected; current flows.Conclude operational state: diode is forward biased.Expect some temperature dependence: hotter junction → slightly lower forward voltage. Verification / Alternative check: Most DMMs have a diode-test mode; a good silicon junction typically reads 0.6–0.7 V in forward direction, and OL (open) in reverse for low test currents. Why Other Options Are Wrong: Not working / open switch: contradicts evidence of forward conduction.Reverse biased: would show near-zero current and high resistance at low voltage. Common Pitfalls: Confusing Schottky diodes (forward drop ~0.2–0.4 V) and germanium (~0.3 V) with silicon. Always consider device type and temperature. Final Answer: forward biased

Question 18

Silicon diode I–V nuance: According to the more complete (complex) model of a silicon diode, how does the effective barrier/forward potential behave as forward current increases?

Accepted Answer

Introduction / Context: The simplistic “0.7 V” rule for silicon diodes is useful, but real diodes follow an exponential I–V relationship. Understanding that forward voltage rises with current (and falls with temperature) is crucial for accurate power dissipation estimates and thermal design in rectifiers and clamps. Given Data / Assumptions: Silicon PN junction diode. Room-temperature operation unless otherwise noted. Complex model includes series resistance and exponential conduction. Concept / Approach: The Shockley diode equation shows current increases exponentially with junction voltage. Practically, as forward current rises, both the junction voltage and series ohmic drop increase, leading to a higher observed forward voltage. Therefore, the “barrier” (effective forward drop) increases somewhat with current, rather than remaining fixed. Step-by-Step Solution: Low current region: forward voltage around 0.6–0.7 V.Higher current: additional drop appears due to series resistance and the diode’s exponential law.Net effect: measured forward voltage increases with forward current.Thermal note: rising temperature shifts the curve, reducing forward voltage for a given current. Verification / Alternative check: Datasheets include forward I–V curves at multiple temperatures, clearly showing V_F rising with I_F at a given temperature. Why Other Options Are Wrong: 0 V: a short circuit, not a diode.Fixed 0.7 V: a rough approximation only.Decreases with current: contradicts I–V behavior at constant temperature. Common Pitfalls: Using a constant 0.7 V in power calculations at high currents underestimates heat dissipation. Always consult I–V curves for accurate design. Final Answer: the barrier potential increases slightly with an increase in current

Question 19

Field testing a diode with an ohmmeter: What readings should a good PN diode show when measured with a multimeter in resistance mode for forward-bias and reverse-bias orientations?

Accepted Answer

Introduction / Context: A quick bench check of a diode uses a multimeter in resistance or diode-test mode. A healthy silicon diode should conduct in one direction (forward bias) and block in the other (reverse bias). Understanding expected readings avoids confusion during troubleshooting of rectifiers and protection networks. Given Data / Assumptions: Standard PN junction diode. Analog or digital multimeter with sufficient internal test voltage. Ohmmeter polarity defines forward vs reverse orientation. Concept / Approach: In forward bias, the diode conducts, giving a relatively low resistance reading (or a forward-voltage readout in diode mode). In reverse bias, the diode blocks, so the meter indicates very high resistance or open circuit. Large deviations (short both ways or open both ways) imply a faulty device. Step-by-Step Solution: Connect meter so anode is positive relative to cathode → forward bias.Observe low resistance (or ~0.6–0.7 V in diode mode).Reverse leads → reverse bias: observe very high resistance (or OL).Compare to known-good behavior to confirm health. Verification / Alternative check: Use diode-test mode: a good silicon diode reads ~0.6–0.7 V forward and OL reverse. Schottky or germanium diodes show lower forward readings. Why Other Options Are Wrong: High both ways: suggests an open diode or meter issue.Low both ways: indicates a shorted diode.High forward and low reverse: opposite of correct behavior. Common Pitfalls: Confusing meter lead polarity; some meters source current from the red lead, others from the black (older analog meters). Always consult the meter manual. Final Answer: high resistance when reverse biased and low resistance when forward biased

Question 20

IC voltage regulator protection behavior: When an integrated-circuit voltage regulator experiences an overload condition, what is its typical protective response?

Accepted Answer

Introduction / Context: Modern IC regulators incorporate protection features such as current limiting, foldback limiting, and thermal shutdown to survive overloads and short circuits. Understanding these responses is important when diagnosing why a supply output suddenly drops or cycles under fault conditions. Given Data / Assumptions: Linear or switching IC regulator under overload/short-circuit. Internal protection mechanisms are active. Goal: identify the typical immediate response. Concept / Approach: Under severe overloads, many regulators first limit current; if power dissipation drives the die temperature high, thermal shutdown engages, turning the pass element off until the die cools. This behavior appears externally as a “shutdown” or hiccup-mode cycling rather than increasing output. Therefore, the safe, intended reaction to protect both the IC and the load is to shut down (or limit so aggressively that output effectively turns off) until conditions are safe again. Step-by-Step Solution: Fault causes output current to exceed limit → current limit engages.Junction temperature rises → thermal shutdown may trip.Regulator ceases conduction (off state) until temperature falls.Some designs then auto-restart, repeating if the fault persists. Verification / Alternative check: Datasheets for common regulators (e.g., 78xx linear, many buck controllers) specify current limit and thermal shutdown thresholds. Lab observation shows the output collapsing to near 0 V during persistent overloads. Why Other Options Are Wrong: Compensate for heat: vague and not the protective action; the IC does not “compensate” by raising voltage.Provide more voltage: would worsen the overload.Sample and adjust: describes normal regulation, not overload protection. Common Pitfalls: Assuming a steady output under short-circuit; instead, expect current limiting and thermal cycling or hiccup shutdown depending on the design. Final Answer: shut down

Diodes and Applications Questions