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Punjab PSEB Matric NSQF Electronics study materials
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Punjab PSEB Matric NSQF Electronics practice questions

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Question 1NSQF vocational trade theory (Electronics)

In NSQF Electronics trade theory, which colour code is used for the earth (protective) conductor in a standard three-core flexible mains cable?

  1. Green with yellow stripe Correct answer.
  2. Brown
  3. Black
  4. Blue

Explanation

The internationally harmonised mains colour code reserves green-with-yellow-stripe for the protective earth conductor, which is connected to the metal chassis and to the earth pin of the plug. Brown is the live conductor and blue is the neutral conductor in this scheme. Black is not used as an earth colour in harmonised flexible cords.

Question 2NSQF vocational trade theory (Electronics)

A resistor is marked with bands brown, black, red and gold. What is its nominal value and tolerance?

  1. 1 kΩ ±10%
  2. 1 kΩ ±5% Correct answer.
  3. 100 Ω ±5%
  4. 10 kΩ ±10%

Explanation

Brown = 1, black = 0, red = multiplier ×100, giving 10 × 100 = 1000 Ω or 1 kΩ. The gold fourth band means ±5% tolerance. A silver band would indicate ±10%.

Question 3NSQF vocational trade theory (Electronics)

Which property makes a junction field-effect transistor a voltage-controlled device rather than a current-controlled one?

  1. The source current is fed back to the gate through a resistor
  2. The gate current is amplified to give the drain current
  3. The drain current depends only on the ambient temperature
  4. The drain current is set by the reverse-biased gate-source voltage Correct answer.

Explanation

In a JFET the gate-channel PN junction is always reverse biased, so gate current is extremely small and the depletion width, hence channel width and drain current, is controlled by the gate-source voltage. This is why the transfer characteristic is plotted as drain current against VGS. Bipolar transistors, by contrast, are controlled by base current.

Question 4NSQF vocational trade theory (Electronics)

A 12 V supply feeds three resistors of 2 Ω, 3 Ω and 6 Ω connected in parallel. What current is drawn from the supply?

  1. 2 A
  2. 12 A Correct answer.
  3. 1 A
  4. 6 A

Explanation

The parallel equivalent is found from 1/R = 1/2 + 1/3 + 1/6 = 3/6 + 2/6 + 1/6 = 6/6, so R = 1 Ω. Using Ohm's law, I = V/R = 12/1 = 12 A. In parallel circuits the smallest branch resistance dominates the total.

Question 5NSQF vocational trade theory (Electronics)

What is the main function of the bridge rectifier section in a regulated DC power supply?

  1. To step the mains voltage down to a safe level
  2. To hold the output constant against load changes
  3. To smooth the ripple to a steady DC level
  4. To convert alternating current into pulsating direct current Correct answer.

Explanation

The four diodes of the bridge conduct in pairs on alternate half cycles so that the output is unidirectional, that is, full-wave rectified but still pulsating. Smoothing is done by the filter capacitor and regulation by the regulator stage. Voltage reduction is the job of the transformer.

Question 6NSQF vocational trade theory (Electronics)

A 10 kΩ resistor carries 20 mA. What power is dissipated in it?

  1. 0.2 W
  2. 200 W
  3. 2 W
  4. 4 W Correct answer.

Explanation

Power can be found from P = I²R = (0.02)² × 10 000 = 0.0004 × 10 000 = 4 W. Alternatively the voltage is V = IR = 200 V and P = VI = 200 × 0.02 = 4 W. The resistor must therefore be rated above 4 W.

Question 7NSQF vocational trade theory (Electronics)

Which logic gate produces a HIGH output only when all of its inputs are HIGH?

  1. OR gate
  2. NAND gate
  3. AND gate Correct answer.
  4. XOR gate

Explanation

The AND gate output is the logical product of its inputs, so it is 1 only for the all-ones input combination. The OR gate gives 1 when any input is 1, the NAND gate is its inverse, and XOR gives 1 only for an odd number of ones.

Question 8NSQF vocational trade theory (Electronics)

In a seven-segment display driven by a decoder, which segment lines must be active to display the digit 1 on a common-cathode display?

  1. e and f
  2. b and c Correct answer.
  3. c and g
  4. a and d

Explanation

The digit 1 is formed by the two right-hand vertical segments, labelled b and c in the standard segment naming. Segments a, d, e, f and g remain off. On a common-cathode display an active segment line is driven HIGH.

Question 9NSQF vocational trade theory (Electronics)

What is the purpose of tinning the tip of a soldering iron before making a joint?

  1. To insulate the bit from the mains supply
  2. To clean flux residues from the printed circuit board
  3. To raise the melting point of the solder
  4. To improve heat transfer from the bit to the joint Correct answer.

Explanation

A thin film of molten solder on the bit bridges the microscopic gap between bit and joint, so heat flows quickly and the joint is made before components are damaged. Tinning also protects the bit from oxidation. It does not change the solder's melting point or clean the board.

Question 10NSQF vocational trade theory (Electronics)

Which instrument setting should be selected to measure the forward voltage drop of a silicon diode?

  1. AC volts, with the diode reverse biased
  2. DC volts, with the diode forward biased through a series resistor Correct answer.
  3. Resistance range, with the diode reverse biased
  4. DC current, connected directly across the diode

Explanation

A forward-biased silicon diode drops about 0.6 to 0.7 V, which must be measured with a DC voltmeter while a series resistor limits the current. Connecting a current meter directly across the diode would short it and likely damage the meter. The resistance range applies its own small voltage and does not give a calibrated drop.

Question 11NSQF vocational trade theory (Electronics)

An NPN transistor in common-emitter configuration has a base current of 50 µA and a collector current of 5 mA. What is its DC current gain?

  1. 0.01
  2. 10
  3. 100 Correct answer.
  4. 1000

Explanation

The DC current gain hFE is the ratio of collector current to base current, so 5 mA / 50 µA = 5000 µA / 50 µA = 100. This value is typical of a small-signal transistor. Gain is dimensionless and is always quoted for a specified collector current.

Question 12NSQF vocational trade theory (Electronics)

Why must the mains plug be removed before replacing a blown fuse in a domestic appliance?

  1. To discharge the appliance's smoothing capacitors only
  2. To prevent the fuse rating from changing
  3. To allow the fuse to cool down before removal
  4. To ensure the circuit is isolated from the supply before it is touched Correct answer.

Explanation

Isolation removes the hazard of electric shock and of arcing while the fuse holder is opened, which is the primary safety reason. A blown fuse may be hot, but cooling is not the safety principle. Capacitors can hold charge even after isolation, so they are a separate precaution.

Question 13NSQF vocational trade theory (Electronics)

A capacitor of 100 µF is charged to 50 V. How much energy does it store?

  1. 0.125 J Correct answer.
  2. 0.25 J
  3. 12.5 J
  4. 1.25 J

Explanation

Energy stored is W = ½CV² = 0.5 × 100 × 10⁻⁶ × 50² = 0.5 × 10⁻⁴ × 2500 = 0.125 J. The square law means doubling the voltage quadruples the stored energy. Forgetting the factor of one half gives the common wrong answer.

Question 14NSQF vocational trade theory (Electronics)

Which statement correctly describes the behaviour of an ideal operational amplifier used as a comparator?

  1. Its output varies linearly with input difference for all input levels
  2. Its output follows the inverting input exactly
  3. Its output saturates near one supply rail whenever the non-inverting input is more positive than the inverting input Correct answer.
  4. Its output is always zero when the input difference is large

Explanation

With no feedback the very high open-loop gain means even a millivolt difference drives the output to saturation, close to the positive rail when V+ exceeds V− and close to the negative rail otherwise. This switching action is what makes the circuit a comparator. Linear operation requires negative feedback.

Question 15NSQF vocational trade theory (Electronics)

In a two-stroke cycle, what is the function of the transfer port in a small petrol engine used with a generator set?

  1. To admit the fresh fuel-air mixture from the crankcase into the cylinder Correct answer.
  2. To release burnt gases to the exhaust
  3. To lubricate the crankshaft bearing directly
  4. To admit air only during the compression stroke

Explanation

During the downward stroke the descending piston pressurises the crankcase and the transfer port opens, letting the fresh mixture flow into the cylinder while burnt gas leaves through the exhaust port. This scavenging replaces the separate induction and exhaust strokes of a four-stroke engine. The exhaust port, not the transfer port, releases burnt gas.

Question 16NSQF vocational trade theory (Electronics)

Which type of optical fibre has a core diameter of about 9 µm and carries a single propagation mode?

  1. Single-mode step-index fibre Correct answer.
  2. Multimode graded-index fibre
  3. Plastic optical fibre
  4. Multimode step-index fibre

Explanation

A core of roughly 8 to 10 µm is close to the wavelength of the light, so only one mode propagates and modal dispersion is almost eliminated. This makes single-mode fibre the standard for long-distance links. Multimode fibres have cores of 50 to 62.5 µm or larger.

Question 17NSQF vocational trade theory (Electronics)

What is the primary purpose of a heat sink fitted to a power transistor?

  1. To provide electrical isolation from the chassis
  2. To conduct heat away from the device and keep the junction temperature within limits Correct answer.
  3. To filter ripple from the supply rail
  4. To increase the current gain of the transistor

Explanation

A power transistor dissipates heat in its junction, and the heat sink increases the surface area available for convection so that the junction stays below its maximum rated temperature. Thermal resistance from junction to ambient determines the size needed. The heat sink does not alter gain or filtering.

Question 18NSQF vocational trade theory (Electronics)

A transformer has 500 turns on the primary and 50 turns on the secondary, with 230 V applied to the primary. What is the secondary voltage?

  1. 2.3 V
  2. 23 V Correct answer.
  3. 230 V
  4. 2300 V

Explanation

The turns ratio is 500:50, or 10:1, and a step-down transformer divides the voltage by that ratio, so 230 / 10 = 23 V. The secondary current rises in the same proportion, so power is conserved apart from losses. The relationship is Vs/Vp = Ns/Np.

Question 19NSQF vocational trade theory (Electronics)

Which memory type is volatile and must be refreshed periodically because its cells store charge on small capacitors?

  1. Flash memory
  2. Dynamic RAM Correct answer.
  3. Electrically erasable PROM
  4. Mask-programmed ROM

Explanation

Each DRAM cell is a transistor plus a capacitor, and the charge leaks away, so the contents must be refreshed thousands of times per second. It is volatile, losing data when power is removed. ROM, flash and EEPROM retain their contents without power.

Question 20NSQF vocational trade theory (Electronics)

A signal of 1 kHz is sampled at 8 kHz. Which statement about the sampled signal is correct?

  1. It can be reconstructed without aliasing because the rate exceeds twice the signal frequency Correct answer.
  2. It will be attenuated to half its amplitude by the sampling process
  3. It cannot be reconstructed because the sampling rate is an exact multiple of the signal
  4. It will alias because the sampling rate is below the signal frequency

Explanation

The Nyquist criterion requires a sampling rate greater than twice the highest signal frequency. Here 8 kHz is well above the 2 kHz minimum for a 1 kHz signal, so no aliasing occurs and the original can be recovered by a reconstruction filter. Aliasing arises only when the rate falls below twice the signal frequency.

Question 21NSQF vocational trade theory (Electronics)

Which effect makes the resistance of a typical carbon resistor increase when its temperature rises?

  1. Increased lattice vibration scattering the charge carriers Correct answer.
  2. Reduction of the resistor's physical length
  3. Increased number of free electrons in the material
  4. Increase of the applied voltage across it

Explanation

In a conductor, higher temperature increases the amplitude of lattice vibrations, so electrons are scattered more often and resistivity rises. This gives carbon and most metals a positive temperature coefficient. Voltage and physical dimensions are not temperature effects on resistivity.

Question 22NSQF vocational trade theory (Electronics)

What does the term 'duty cycle' mean for a pulse-width-modulated signal?

  1. The number of pulses produced in one second
  2. The fraction of each period for which the signal is HIGH, expressed as a percentage Correct answer.
  3. The peak amplitude of the pulse above ground
  4. The time taken for the pulse to fall from peak to zero

Explanation

Duty cycle is the on-time divided by the period, usually quoted as a percentage, so a 1 ms pulse repeating every 4 ms gives 25%. It controls the average power delivered in switching circuits such as lamp dimmers and motor drives. Pulse count per second is the frequency, not the duty cycle.

Question 23NSQF vocational trade theory (Electronics)

A moving-coil ammeter is used to measure current in a circuit. Why must it be connected in series and never across the supply?

  1. It requires an external shunt to operate at all
  2. Its coil has very low resistance, so a parallel connection would draw a destructive current Correct answer.
  3. Its coil has very high resistance, so a parallel connection would read zero
  4. It measures voltage directly and would bypass the load

Explanation

An ammeter is designed to drop as little voltage as possible, so its internal resistance is very small. Connected across a supply it would present almost a short circuit and a very large current would flow, damaging the meter and the circuit. A voltmeter, with high resistance, is connected in parallel.

Question 24NSQF vocational trade theory (Electronics)

Which statement about a Zener diode operating in its breakdown region is correct?

  1. It conducts heavily in the forward direction only
  2. It stops conducting entirely once breakdown begins
  3. It maintains a nearly constant voltage across itself while the current varies within rated limits Correct answer.
  4. It converts the current into a proportional voltage like a resistor

Explanation

In reverse breakdown the Zener characteristic is nearly vertical, so the voltage stays close to the nominal Zener value over a wide range of current. This is why it is used with a series resistor as a simple shunt regulator. The series resistor limits the current so the device is not destroyed.

Question 25NSQF vocational trade theory (Electronics)

Which precaution is essential when handling CMOS integrated circuits during assembly?

  1. Handle the pins directly with dry fingers
  2. Ground the workbench and wrist strap to dissipate static charge Correct answer.
  3. Apply full supply voltage before inserting the IC
  4. Store the ICs in ordinary polythene bags

Explanation

The very thin gate oxide of a CMOS device can be punctured by the static charge built up on a person, so the workbench, tools and operator must be bonded to a common earth through a high-value resistor. Antistatic tubes or conductive foam are used for storage. Ordinary polythene generates and holds static charge.

Question 26NSQF vocational trade theory (Electronics)

In an NSQF electronics workshop, a resistor is marked with the colour bands brown, black, red and gold. What is its nominal value and tolerance?

  1. 10 kΩ with ±5% tolerance
  2. 1 kΩ with ±10% tolerance
  3. 100 Ω with ±10% tolerance
  4. 1 kΩ with ±5% tolerance Correct answer.

Explanation

Brown = 1, black = 0 and red = multiplier of 100, giving 10 × 100 = 1000 Ω, i.e. 1 kΩ. The fourth band gold indicates a tolerance of ±5%. Reading the multiplier band as 10 instead of 100 is the usual source of error.

Question 27NSQF vocational trade theory (Electronics)

Which property of a capacitor is described by the unit farad?

  1. The rate at which it dissipates heat at rated voltage
  2. The charge stored per volt of potential difference across it Correct answer.
  3. The resistance offered to alternating current at 50 Hz
  4. The energy stored per ampere of current through it

Explanation

Capacitance is defined as C = Q/V, so one farad equals one coulomb stored per volt applied. It is a measure of charge-storage ability, not of energy per ampere, heat loss or AC resistance. Reactance and power loss are separate quantities with their own units.

Question 28NSQF vocational trade theory (Electronics)

A copper conductor of uniform cross-section is replaced by one of the same length but twice the cross-sectional area. How does its resistance change at the same temperature?

  1. It becomes half of the original value Correct answer.
  2. It becomes four times the original value
  3. It becomes twice the original value
  4. It stays exactly the same

Explanation

Resistance is given by R = ρL/A, so it is inversely proportional to cross-sectional area. Doubling A while keeping length and resistivity fixed halves the resistance. Thicker conductors therefore carry current with less voltage drop.

Question 29NSQF vocational trade theory (Electronics)

What is the main purpose of using a flux while hand-soldering a copper wire joint?

  1. To insulate the joint electrically after cooling
  2. To remove oxide films so that molten solder wets the metal surface Correct answer.
  3. To cool the component leads during soldering
  4. To raise the melting point of the solder for a stronger joint

Explanation

Flux chemically dissolves light oxide layers and lowers surface tension so solder alloys with the clean metal. Without it the solder tends to bead and form a poor, high-resistance joint. Flux does not change the solder melting point or insulate the joint.

Question 30NSQF vocational trade theory (Electronics)

A 12 V battery is connected across a 6 Ω resistor. What current flows through the resistor?

  1. 18 A
  2. 0.5 A
  3. 2 A Correct answer.
  4. 72 A

Explanation

Ohm's law gives I = V/R = 12/6 = 2 A. The current rises with voltage and falls with resistance. The other figures come from multiplying or inverting the quantities incorrectly.

Question 31NSQF vocational trade theory (Electronics)

Which safety practice is correct when using a soldering iron at a workbench?

  1. Lay the hot iron on the bench mat between joints
  2. Leave the iron switched on unattended to save warm-up time
  3. Return the iron to its stand immediately after each joint is made Correct answer.
  4. Test whether the iron is hot by touching the bit briefly

Explanation

A hot soldering iron must always rest in its proper stand so that the bit cannot contact skin, cables or flammable items. Leaving it on the bench or unattended creates burn and fire hazards. Temperature should never be judged by touch.

Question 32NSQF vocational trade theory (Electronics)

Which type of capacitor is normally connected with correct polarity because reversing it can damage the component?

  1. Mica capacitor
  2. Air-spaced variable capacitor
  3. Aluminium electrolytic capacitor Correct answer.
  4. Ceramic disc capacitor

Explanation

Electrolytic capacitors contain a liquid electrolyte and a thin anodic oxide layer, so they are polarised and must be fitted with the marked terminal at the more positive potential. Reversal breaks down the oxide and can cause gassing or rupture. Ceramic, mica and air capacitors are non-polarised.

Question 33NSQF vocational trade theory (Electronics)

Which meter function should be selected to check whether a copper track on a printed circuit board is continuous?

  1. Capacitance mode
  2. Resistance (continuity) mode Correct answer.
  3. DC voltage mode
  4. AC current mode

Explanation

Continuity or resistance mode applies the meter's own small test voltage and shows whether a low-resistance path exists along the track. A good track reads near zero ohms or sounds the beeper. Voltage, current and capacitance ranges do not test a conductive path in this way.

Question 34NSQF vocational trade theory (Electronics)

A transformer secondary supplies 24 V AC to a bridge rectifier. What is the approximate peak voltage available at the rectifier output before smoothing?

  1. About 34 V Correct answer.
  2. About 24 V
  3. About 17 V
  4. About 48 V

Explanation

The quoted 24 V is an RMS value, and a bridge rectifier charges the output to the peak of the waveform, Vpk = 1.414 × 24 ≈ 34 V, less two diode drops. Smoothing then holds this near-peak value under light load. The RMS figure is not the peak, and 17 V would be the half-wave average.

Question 35NSQF vocational trade theory (Electronics)

An emitter follower (common-collector) stage is used as a buffer between a high-impedance source and a low-impedance load. Which statement explains this choice?

  1. It inverts the signal and provides current gain of exactly one
  2. It has high input impedance and low output impedance with voltage gain close to unity Correct answer.
  3. It converts the AC input directly into a DC level without any current gain
  4. It has low input impedance and high output impedance with large voltage gain

Explanation

In a common-collector stage the input is taken at the base and the output at the emitter, so the signal sees a high input impedance and a low output impedance. Voltage gain is slightly below one but current gain is high, which is ideal for driving heavy loads without loading the source.

Question 36NSQF vocational trade theory (Electronics)

Which statement about a relay used to switch a 230 V lamp from a low-voltage control circuit is correct?

  1. The contacts carry the coil current and the coil carries the lamp current
  2. The relay works only with AC control signals and never with DC
  3. The coil circuit and the contact circuit remain electrically isolated from each other Correct answer.
  4. The relay coil must be wired in series with the lamp to operate it

Explanation

A relay provides galvanic separation: the low-voltage coil operates a magnetic armature that closes a separate contact set in the mains circuit. This lets a small, safe signal control a dangerous load. The coil is not in series with the load, and relays are available for both AC and DC coils.

Question 37NSQF vocational trade theory (Electronics)

A 555 timer is wired as an astable multivibrator with timing resistors and a capacitor. What output does this configuration produce?

  1. A single pulse of fixed width after each trigger input
  2. A continuous rectangular wave whose frequency depends on the RC timing network Correct answer.
  3. A steady DC level that follows the supply voltage
  4. A sine wave whose amplitude depends on the capacitor value

Explanation

In astable mode the 555 repeatedly charges and discharges the timing capacitor between one-third and two-thirds of the supply, producing a free-running rectangular wave. Frequency and duty cycle are set by the external resistors and capacitor. Monostable mode, by contrast, gives one pulse per trigger.

Question 38NSQF vocational trade theory (Electronics)

Which type of cable would be most suitable for connecting a microphone to an amplifier over several metres in an environment with electrical noise?

  1. Single-core unscreened wire
  2. Twisted-pair screened cable Correct answer.
  3. Solid copper mains cable
  4. Ribbon cable with no screen

Explanation

A screened twisted pair carries the small microphone signal while the earthed screen intercepts interfering electric fields, and twisting reduces magnetic pickup. This preserves signal quality over a long run. Unscreened or mains-grade conductors would pick up hum and noise.

Question 39NSQF vocational trade theory (Electronics)

In a domestic wiring circuit protected by a miniature circuit breaker, what is the function of the earth wire?

  1. To provide a low-resistance path that lets fault current trip the protective device Correct answer.
  2. To reduce the voltage supplied to appliances
  3. To balance the current in the live conductor during normal operation
  4. To carry the normal load current back to the supply transformer

Explanation

The earth conductor bonds exposed metalwork to the installation earth so that a fault creates a large current, which quickly operates the MCB or fuse. This limits the time that dangerous touch voltage appears on the casing. It carries no current in normal service.

Question 40NSQF vocational trade theory (Electronics)

Which statement correctly describes the difference between a microprocessor and a microcontroller in an embedded electronics product?

  1. A microcontroller can only perform analogue processing, whereas a microprocessor handles only digital data
  2. A microcontroller integrates CPU, memory and peripherals on one chip, whereas a microprocessor needs external support chips Correct answer.
  3. A microprocessor always contains flash program memory, whereas a microcontroller never does
  4. A microprocessor is limited to 8-bit data, whereas a microcontroller is always 32-bit

Explanation

A microcontroller is a complete computer on a chip with CPU, RAM, program memory and peripherals such as timers and ADCs, which suits compact embedded control. A microprocessor is mainly the CPU and relies on external memory and interface chips. Data width and memory type vary in both families.

Question 41NSQF vocational trade theory (Electronics)

A step-down transformer is used in a power supply unit. Which statement about its action is correct?

  1. It reduces the secondary voltage while the secondary current rises for the same power level Correct answer.
  2. It reduces both secondary voltage and secondary current
  3. It increases the secondary voltage while the current also rises
  4. It changes the frequency of the AC supply at the same time

Explanation

For an ideal transformer the power is essentially constant, so a lower secondary voltage is accompanied by a proportionally higher secondary current. The turns ratio sets the voltage ratio, and the current ratio is its inverse. A transformer does not alter the supply frequency.

Question 42NSQF vocational trade theory (Electronics)

In a sinusoidal waveform displayed on an oscilloscope, the peak value is 10 V. What is the RMS value?

  1. About 10 V
  2. About 20 V
  3. About 7.07 V Correct answer.
  4. About 6.37 V

Explanation

For a sine wave, Vrms = Vpeak / √2 = 10 / 1.414 ≈ 7.07 V. The RMS value is the equivalent heating value of the waveform. The average value of a full sine cycle is zero, and peak-to-peak would be 20 V.

Question 43NSQF vocational trade theory (Electronics)

A student needs a stable 5 V reference for a comparator circuit and has only a 9 V supply. Which device is the appropriate choice?

  1. A germanium signal diode in forward bias
  2. A 5 kΩ potentiometer across the 9 V supply
  3. A 5 V electrolytic capacitor
  4. A Zener diode of 5.1 V with a series resistor Correct answer.

Explanation

A Zener diode biased into reverse breakdown holds a nearly constant voltage across itself, and a series resistor limits the current. A 5.1 V type is the nearest standard value to 5 V. Forward-biased diodes give about 0.3 V or 0.7 V, and a capacitor or potentiometer gives no stable reference.

Question 44NSQF vocational trade theory (Electronics)

Which statement accurately distinguishes an intrinsic semiconductor from an extrinsic one?

  1. Extrinsic material contains no free charge carriers at room temperature
  2. Intrinsic material has a fixed resistance that never changes with temperature
  3. Intrinsic material is chemically pure, while extrinsic material has controlled impurity doping Correct answer.
  4. Intrinsic material conducts mainly by holes, while extrinsic material conducts only by electrons

Explanation

An intrinsic semiconductor such as pure silicon has equal numbers of thermally generated electrons and holes. Extrinsic material is doped with donor or acceptor impurities to raise conductivity and make it n-type or p-type. Doping does not remove carriers, and intrinsic resistance still falls as temperature rises.

Question 45NSQF vocational trade theory (Electronics)

A half-wave rectifier feeds a capacitor-input filter. Which change would reduce the ripple voltage at the output?

  1. Increasing the value of the smoothing capacitor Correct answer.
  2. Increasing the AC input frequency only by lowering it
  3. Removing the capacitor and leaving the load directly connected
  4. Reducing the load resistance so that more current is drawn

Explanation

A larger reservoir capacitor discharges less between successive charging peaks, so the ripple amplitude falls. Heavier load current or removal of the capacitor increases ripple. Lowering the supply frequency lengthens the discharge interval and also worsens ripple.

Question 46NSQF vocational trade theory (Electronics)

A digital circuit must detect when an odd number of three input lines are HIGH. Which combinational block performs this function directly?

  1. A three-input NOR gate
  2. A three-input AND gate
  3. A set-reset latch
  4. A three-input XOR gate arrangement Correct answer.

Explanation

The XOR function gives a HIGH output when an odd number of inputs are HIGH, so cascading XOR gates over three inputs produces the required parity output. AND gives HIGH only when all inputs are HIGH, and NOR only when all are LOW. A latch stores state rather than computing parity.

Question 47NSQF vocational trade theory (Electronics)

A moving-coil meter with a full-scale deflection of 1 mA and a coil resistance of 100 Ω is to read 10 mA full scale. What shunt resistance is required?

  1. About 1 kΩ
  2. About 0.9 Ω
  3. About 100 Ω
  4. About 11.1 Ω Correct answer.

Explanation

The shunt must carry 9 mA while 1 mA passes through the meter, so the voltage across both is 1 mA × 100 Ω = 0.1 V. Then Rshunt = 0.1 V / 9 mA ≈ 11.1 Ω. This parallel divider extends the current range without exceeding full-scale deflection.

Question 48NSQF vocational trade theory (Electronics)

An inverting amplifier uses an operational amplifier with an input resistor of 10 kΩ and a feedback resistor of 100 kΩ. A 0.2 V signal is applied to the input resistor. What is the output voltage?

  1. −0.2 V
  2. +2.0 V
  3. −2.0 V Correct answer.
  4. −20 V

Explanation

The closed-loop gain is −Rf/Rin = −100 kΩ/10 kΩ = −10, so the output is −10 × 0.2 V = −2.0 V. The negative sign shows the 180° phase inversion of the inverting configuration. The magnitude depends only on the resistor ratio for an ideal op-amp.

Question 49NSQF vocational trade theory (Electronics)

A separately excited DC motor is running at steady load when its field current is suddenly reduced. What happens to its speed?

  1. The speed falls because the magnetic flux has decreased
  2. The speed stays constant because the armature voltage is unchanged
  3. The motor stops immediately because the field has been weakened
  4. The speed rises because the back EMF must fall to allow a larger armature current Correct answer.

Explanation

Speed is approximately proportional to back EMF divided by flux. Reducing field current weakens the flux, so at the same terminal voltage the armature current and hence speed increase until a new balance is reached. Uncontrolled field loss can therefore cause dangerous overspeeding.

Question 50NSQF vocational trade theory (Electronics)

A 4-bit binary counter is clocked continuously from 0000. What is its output after 13 clock pulses?

  1. 1001
  2. 1110
  3. 1011
  4. 1101 Correct answer.

Explanation

The counter output is simply the binary value of the number of pulses received, so 13 in binary is 1101 (8 + 4 + 1). The counter returns to 0000 after 16 pulses. Each listed alternative corresponds to a different decimal count.