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Maharashtra SSC Electronics and Hardware Pre-Voc/NSQF study materials
Maharashtra SSC Electronics and Hardware Pre-Voc/NSQF study resources

Maharashtra SSC Electronics and Hardware Pre-Voc/NSQF practice questions

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Question 1Basic Electricity and Electronic Components

In a closed circuit containing a single cell and a bulb, which statement about conventional current is correct?

  1. It flows in both directions alternately at mains frequency
  2. It flows from the negative terminal to the positive terminal through the external circuit
  3. It flows only inside the cell and not through the bulb
  4. It flows from the positive terminal to the negative terminal through the external circuit Correct answer.

Explanation

Conventional current is defined as the direction in which positive charge would move, so outside the source it goes from the positive terminal through the load to the negative terminal. Electron flow inside the metal wire is opposite to this conventional direction, but the standard convention used in circuit diagrams is positive to negative.

Question 2Basic Electricity and Electronic Components

The SI unit of electrical resistance, and the instrument used to measure it directly, are:

  1. Watt and wattmeter
  2. Ohm and ohmmeter Correct answer.
  3. Volt and voltmeter
  4. Ampere and ammeter

Explanation

Resistance is measured in ohms, and an ohmmeter is the instrument designed to read resistance directly, usually with the component isolated from the supply. A voltmeter reads potential difference and an ammeter reads current, so neither gives resistance directly.

Question 3Basic Electricity and Electronic Components

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

  1. 10 ohm, 5 percent
  2. 1000 ohm, 10 percent
  3. 1000 ohm, 5 percent Correct answer.
  4. 102 ohm, 5 percent

Explanation

Brown is 1, black is 0, and red is a multiplier of 100, giving 10 x 100 = 1000 ohm. Gold in the fourth band indicates a tolerance of plus or minus 5 percent.

Question 4Basic Electricity and Electronic Components

Two 6 ohm resistors are connected in parallel. What is the equivalent resistance?

  1. 6 ohm
  2. 12 ohm
  3. 1.5 ohm
  4. 3 ohm Correct answer.

Explanation

For two equal resistors in parallel the equivalent is half of one value, since 1/R = 1/6 + 1/6 = 2/6, so R = 3 ohm. The equivalent of a parallel network is always smaller than the smallest branch resistance.

Question 5Basic Electricity and Electronic Components

Which property makes a semiconductor such as silicon useful in diodes?

  1. It always behaves as a perfect insulator below 100 degrees Celsius
  2. Its resistance falls to zero at room temperature
  3. Its conductivity lies between that of conductors and insulators and can be controlled by doping Correct answer.
  4. It conducts only when heated above its melting point

Explanation

Semiconductors have a resistivity between metals and insulators, and adding small amounts of impurity atoms (doping) changes their conductivity in a controlled way. This controllability is what allows p-type and n-type regions to form a junction with one-way conduction.

Question 6Basic Electricity and Electronic Components

A 12 V battery drives a current of 0.5 A through a lamp. What power does the lamp consume?

  1. 12.5 W
  2. 24 W
  3. 0.04 W
  4. 6 W Correct answer.

Explanation

Power equals voltage multiplied by current, so P = 12 x 0.5 = 6 W. The same result follows from P = I squared R once the lamp resistance of 24 ohm is found.

Question 7Basic Electricity and Electronic Components

Which material is the standard conductor used for the tracks of a printed circuit board?

  1. Silicon
  2. Copper Correct answer.
  3. Mica
  4. Nichrome

Explanation

Copper foil laminated on the board is etched to form the tracks because copper has very low resistivity and solders easily. Silicon is a semiconductor, nichrome is a resistance alloy, and mica is an insulator.

Question 8Basic Electricity and Electronic Components

What is the main purpose of a fuse in an appliance circuit?

  1. To step down the supply voltage to a safe value
  2. To convert alternating current into direct current
  3. To store charge and smooth the supply
  4. To open the circuit when current exceeds a safe value Correct answer.

Explanation

A fuse contains a thin element that melts and breaks the circuit when the current rises above its rating, protecting wiring and components from overheating. It does not regulate voltage, rectify current, or store energy.

Question 9Basic Electricity and Electronic Components

A capacitor marked 104 has what capacitance?

  1. 100 nanofarad Correct answer.
  2. 104 picofarad
  3. 0.104 microfarad
  4. 10 nanofarad

Explanation

In the three-digit code the first two digits are significant figures and the third is the number of zeros in picofarads, so 104 means 10 followed by four zeros, i.e. 100000 pF. Dividing by 1000 gives 100 nF, equivalent to 0.1 microfarad.

Question 10Basic Electricity and Electronic Components

How are the cells of a battery connected when a higher terminal voltage is required?

  1. In parallel, positive to positive
  2. In series, positive terminal to negative terminal Correct answer.
  3. Through a capacitor placed between the cells
  4. In series, positive terminal to positive terminal

Explanation

Series connection adds the individual cell voltages while the same current passes through each cell, so the terminal voltage increases. Parallel connection keeps the voltage equal to one cell and instead increases the available current capacity.

Question 11Basic Electricity and Electronic Components

In a forward-biased silicon diode, the typical barrier voltage that must be exceeded before appreciable conduction begins is about:

  1. 0.7 V Correct answer.
  2. 5 V
  3. 1.5 V
  4. 0.3 V

Explanation

A silicon p-n junction needs roughly 0.7 V of forward bias to overcome the barrier and conduct noticeably. Germanium junctions are lower, about 0.3 V, which is why the other low value is a distractor.

Question 12Basic Electricity and Electronic Components

A wire of length L and resistance R is replaced by another wire of the same material and area but twice the length. What is the new resistance?

  1. R
  2. 4R
  3. R/2
  4. 2R Correct answer.

Explanation

Resistance is directly proportional to length for a uniform conductor of fixed material and cross-section, since R = rho L / A. Doubling the length while keeping resistivity and area constant therefore doubles the resistance.

Question 13Basic Electricity and Electronic Components

A resistor has bands orange, orange, brown, silver. Its value lies within which range?

  1. 270 ohm to 330 ohm
  2. 320 ohm to 340 ohm
  3. 297 ohm to 363 ohm Correct answer.
  4. 33 ohm to 37 ohm

Explanation

Orange is 3, orange is 3, brown is a multiplier of 10, giving 330 ohm, and silver indicates plus or minus 10 percent tolerance. Ten percent of 330 is 33, so the acceptable range is 297 ohm to 363 ohm.

Question 14Basic Electricity and Electronic Components

What happens to the total resistance of a circuit when an extra resistor is added in parallel with an existing one?

  1. It becomes zero
  2. It stays exactly the same
  3. It increases by the value of the new resistor
  4. It decreases Correct answer.

Explanation

Each parallel branch provides an additional path for current, so the total current for a given voltage rises and the equivalent resistance falls. The new equivalent is always less than the smallest individual branch resistance.

Question 15Basic Electricity and Electronic Components

A moving-coil ammeter is connected in a circuit. How must it be placed and why?

  1. Across the load terminals, because it has very high resistance
  2. In parallel with the load, because it has very high resistance
  3. In series with the load, because it has very low resistance Correct answer.
  4. In parallel with the supply, because it has very low resistance

Explanation

An ammeter must carry the current being measured, so it is inserted in series with the circuit. Its resistance is deliberately made very low so that it drops almost no voltage and does not disturb the current it measures.

Question 16Basic Electricity and Electronic Components

Which statement correctly describes the difference between a cell and a capacitor as energy sources in a circuit?

  1. A cell converts chemical energy into electrical energy, while a capacitor stores energy in an electric field Correct answer.
  2. A cell stores charge in an electric field, while a capacitor converts chemical energy
  3. Both generate energy without any conversion process
  4. Both store energy in a magnetic field

Explanation

A cell is an electrochemical source that converts stored chemical energy into electrical energy and can supply current for a long time. A capacitor stores separated charge on its plates, holding energy in the electric field between them, and discharges quickly.

Question 17Basic Electricity and Electronic Components

An LED is to be operated from a 9 V battery and needs about 20 mA. If the LED drops 2 V, what series resistor is required?

  1. 100 ohm
  2. 450 ohm
  3. 110 ohm
  4. 350 ohm Correct answer.

Explanation

The resistor must drop the difference, 9 V minus 2 V = 7 V, at 20 mA. Using R = V/I gives 7 / 0.02 = 350 ohm, which is the standard calculation for an LED series resistor.

Question 18Basic Electricity and Electronic Components

A transformer is used in a power supply mainly because it:

  1. increases the frequency of the supply
  2. stores large amounts of charge for smoothing
  3. converts alternating current into direct current by itself
  4. changes the amplitude of alternating voltage by mutual induction Correct answer.

Explanation

A transformer transfers energy between two coils linked by a common magnetic flux, so the secondary voltage is set by the turns ratio. It works only with alternating current, because a changing flux is needed to induce an EMF.

Question 19Basic Electricity and Electronic Components

In a series circuit of three unequal resistors, which quantity is the same through every resistor?

  1. The voltage across each resistor
  2. The power dissipated in each resistor
  3. The resistance of each resistor
  4. The current Correct answer.

Explanation

A series circuit offers only one path, so the same current must pass through every element in turn. The applied voltage divides across the resistors in proportion to their values, and power differs because it depends on both current and resistance.

Question 20Basic Electricity and Electronic Components

Which type of capacitor must be connected with correct polarity, and why?

  1. Mica, because its plates are silvered
  2. Electrolytic, because reverse voltage can break down its oxide layer Correct answer.
  3. Ceramic disc, because it has a high dielectric constant
  4. Polyester film, because it is wound in a roll

Explanation

An aluminium electrolytic capacitor uses a thin anodic oxide as its dielectric, and this oxide is damaged if the polarity is reversed. Ceramic, mica, and film capacitors are non-polarised and may be connected either way.

Question 21Basic Electricity and Electronic Components

A 100 ohm resistor dissipates 4 W. What current flows through it and what voltage appears across it?

  1. 2 A and 200 V
  2. 0.04 A and 4 V
  3. 0.2 A and 20 V Correct answer.
  4. 0.4 A and 40 V

Explanation

From P = I squared R, I squared = 4/100 = 0.04, so I = 0.2 A. Then V = I x R = 0.2 x 100 = 20 V, which also satisfies P = V squared / R.

Question 22Basic Electricity and Electronic Components

A coil of wire is placed near a changing magnetic field. Which principle explains the voltage appearing across its ends?

  1. Joule heating
  2. Electrostatic attraction
  3. Electromagnetic induction Correct answer.
  4. Thermoelectric effect

Explanation

A changing magnetic flux linking a coil induces an EMF in it, a relationship described by Faraday's law of electromagnetic induction. This is the principle behind generators, transformers, and relays.

Question 23Basic Electricity and Electronic Components

A 10 kohm resistor and a 2.2 kohm resistor form a voltage divider across a 12 V supply. What voltage appears across the 2.2 kohm resistor, approximately?

  1. 2.16 V Correct answer.
  2. 12 V
  3. 1.97 V
  4. 9.8 V

Explanation

The output across the lower resistor is V x R2/(R1+R2) = 12 x 2200/12200, which is about 2.16 V. The larger resistor takes the larger share of the supply, so the answer must be well below half of 12 V.

Question 24Basic Electricity and Electronic Components

A zener diode of 6.2 V is used to regulate a supply that rises from 9 V to 15 V. The series resistor is 470 ohm and the load draws negligible current. What is the current through the zener at 15 V input?

  1. About 18.7 mA Correct answer.
  2. About 6.2 mA
  3. About 13.2 mA
  4. About 32 mA

Explanation

The series resistor drops the difference between input and the regulated 6.2 V, i.e. 15 minus 6.2 = 8.8 V. With negligible load current all of this drop drives current through the resistor, so I = 8.8/470, about 18.7 mA.

Question 25Basic Electricity and Electronic Components

A bridge rectifier fed from a transformer secondary produces a smoothed DC output. If one diode of the bridge becomes open-circuited, what is the most likely result?

  1. The output becomes zero and the fuse blows immediately
  2. The output becomes a pure sine wave of double amplitude
  3. The circuit behaves as a half-wave rectifier with lower ripple frequency and greater ripple Correct answer.
  4. The output voltage doubles because fewer diodes drop voltage

Explanation

With one diode open, only one half of each AC cycle can find a conducting path through the bridge, so rectification continues but as half-wave. The ripple frequency falls to the supply frequency and the ripple amplitude increases because the capacitor recharges half as often.

Question 26Basic Electricity and Electronic Components

In the standard resistor colour code, a resistor marked with bands green, blue, red, gold has what nominal value and tolerance?

  1. 56 Ω with ±5 % tolerance
  2. 5.6 Ω with ±10 % tolerance
  3. 5.6 kΩ with ±5 % tolerance Correct answer.
  4. 560 Ω with ±5 % tolerance

Explanation

Green stands for 5 and blue for 6, so the first two digits are 56. The third band gives the multiplier: red means ×100, giving 5600 Ω or 5.6 kΩ. Gold in the fourth band indicates a tolerance of ±5 %.

Question 27Basic Electricity and Electronic Components

Which property of a coil is mainly responsible for the back e.m.f. that opposes a sudden rise of current when the supply is switched on?

  1. Its self-inductance Correct answer.
  2. Its stray capacitance between turns
  3. Its ability to rectify alternating current
  4. Its low DC resistance

Explanation

Self-inductance is the property by which a changing current in a coil induces an e.m.f. in that same coil, and by Lenz's law this e.m.f. opposes the change producing it. So the current rises gradually instead of instantly. Resistance only limits the final steady current and does not create the opposing e.m.f.

Question 28Basic Electricity and Electronic Components

A 6 V supply feeds two resistors of 220 Ω and 330 Ω connected in series. What current flows in the circuit?

  1. 0.055 A
  2. 0.011 A Correct answer.
  3. 0.027 A
  4. 0.018 A

Explanation

In series the resistances add, so the total is 220 + 330 = 550 Ω. Ohm's law then gives I = V/R = 6/550 ≈ 0.0109 A, which rounds to 0.011 A. The same current passes through both resistors because there is only one path.

Question 29Basic Electricity and Electronic Components

An electrolytic capacitor in a power-supply filter is fitted with its leads reversed. What is the likely result?

  1. The capacitor works normally but with higher leakage
  2. The capacitor may overheat, vent electrolyte and fail Correct answer.
  3. The capacitor behaves as an inductor at mains frequency
  4. The capacitance falls to exactly half its marked value

Explanation

Electrolytic capacitors are polarised because the thin oxide dielectric is formed by the applied polarity. Reversing the voltage breaks down that oxide layer, allowing large current to flow, which heats the component and can make it vent or burst. Correct polarity marking must therefore be observed during assembly.

Question 30Basic Electricity and Electronic Components

A silicon transistor in a circuit has its emitter-base junction forward biased and its collector-base junction reverse biased. In which region is the transistor operating?

  1. Active region Correct answer.
  2. Saturation region
  3. Avalanche breakdown region
  4. Cut-off region

Explanation

The active (linear) region is defined by forward bias on the emitter-base junction and reverse bias on the collector-base junction. In this condition the collector current is controlled by the base current, which is what makes amplification possible. Cut-off needs both junctions reverse biased and saturation needs both forward biased.

Question 31Electronic Measurements and Test Instruments

In a moving-coil (PMMC) voltmeter, what does the pointer deflection primarily depend on?

  1. The rate of change of flux in the core
  2. The frequency of the applied AC signal
  3. The current flowing through the coil in a fixed magnetic field Correct answer.
  4. The capacitance of the coil winding

Explanation

A PMMC movement produces torque proportional to coil current, because the coil sits in a permanent radial field and the deflecting torque equals N·B·I·A. The spring provides the controlling torque so deflection reads current directly, and a series multiplier converts it into a voltmeter. Frequency and coil capacitance play no part in the steady deflection.

Question 32Electronic Measurements and Test Instruments

An ammeter is to be connected to measure the current through a resistor in a live circuit. Which connection is correct?

  1. In series with a high resistance placed across the supply
  2. In parallel with the resistor, so the meter shares the voltage
  3. Across the supply terminals only, with the resistor disconnected
  4. In series with the resistor, so the same current passes through both Correct answer.

Explanation

An ammeter must carry the current being measured, so it is inserted in series in the branch, and its own resistance must be low to avoid disturbing the circuit. Connecting it in parallel would divert current and give a wrong reading. The voltmeter, by contrast, goes in parallel with high resistance.

Question 33Electronic Measurements and Test Instruments

A basic moving-coil movement has a full-scale current of 1 mA and a resistance of 100 Ω. What series multiplier makes it a 10 V voltmeter?

  1. 99 Ω
  2. 9900 Ω Correct answer.
  3. 100 Ω
  4. 1000 Ω

Explanation

Full-scale voltage across the movement is 1 mA × 100 Ω = 0.1 V, so the multiplier must drop 10 − 0.1 = 9.9 V at 1 mA. That gives 9.9 V / 0.001 A = 9900 Ω. The total meter resistance then becomes 10 kΩ, giving 1 kΩ per volt sensitivity.

Question 34Electronic Measurements and Test Instruments

Why is a shunt resistance connected across a moving-coil movement when it is used as an ammeter?

  1. To convert the movement into a rectifier type
  2. To increase the meter's voltage sensitivity
  3. To compensate for temperature changes in the spring
  4. To divert most of the current around the movement and extend its range Correct answer.

Explanation

A shunt of low resistance carries the bulk of the line current, leaving only the movement's full-scale current through the coil, so the measurable range is extended. It also lowers the effective ammeter resistance, reducing loading error. The shunt does not improve voltage sensitivity or rectify AC.

Question 35Electronic Measurements and Test Instruments

A moving-coil movement of 50 Ω resistance and 1 mA full scale is shunted to read 10 mA full scale. What shunt value is required?

  1. 4.5 Ω
  2. 5.56 Ω Correct answer.
  3. 0.5 Ω
  4. 50 Ω

Explanation

The shunt carries 10 − 1 = 9 mA while the movement carries 1 mA, and both have the same voltage across them. So Rsh = (Im × Rm) / (I − Im) = (0.001 × 50) / 0.009 ≈ 5.56 Ω. This low shunt value keeps the ammeter's insertion resistance small.

Question 36Electronic Measurements and Test Instruments

What is the main reason a digital multimeter has largely replaced an analog multimeter for general measurements?

  1. It works without any internal power source
  2. It needs no test leads
  3. It presents the reading numerically, reducing parallax and interpolation error Correct answer.
  4. It can measure only DC quantities

Explanation

A DMM displays a direct numerical value, so the user does not estimate a pointer position against a scale, eliminating parallax and interpolation errors. It also typically has high input impedance, reducing circuit loading. It still uses test leads and needs a battery or mains supply.

Question 37Electronic Measurements and Test Instruments

On an analog multimeter, what is the effect of parallax error and how is it reduced?

  1. It arises from viewing the pointer at an angle and is reduced by a mirror scale Correct answer.
  2. It comes from a weak battery and is reduced by zeroing on the ohms range
  3. It arises from AC frequency and is reduced by a rectifier
  4. It comes from the meter's internal resistance and is reduced by a shunt

Explanation

Parallax occurs because the pointer is slightly above the scale, so an angled view gives a shifted reading. A mirror band behind the scale lets the user align the pointer with its reflection, removing the offset. Zeroing and shunts address other error sources, not viewing angle.

Question 38Electronic Measurements and Test Instruments

Which instrument works on the principle that a current-carrying conductor in a magnetic field experiences a force, but is also usable on AC because its field is produced by the current itself?

  1. Electrostatic voltmeter
  2. Thermocouple instrument
  3. Electrodynamometer (dynamometer) instrument Correct answer.
  4. Permanent-magnet moving-coil instrument

Explanation

In a dynamometer instrument both the fixed and moving coils carry current, so the deflecting torque is proportional to the product of the two currents. When the current reverses, both fields reverse, so the torque direction is unchanged and the instrument works on AC as well as DC. A PMMC movement needs a permanent field and works only on DC.

Question 39Electronic Measurements and Test Instruments

An electrodynamometer wattmeter is connected to measure power in a single-phase circuit. Which statement about its connections is correct?

  1. The current coil is in series with the load and the voltage coil in parallel across it Correct answer.
  2. The voltage coil is in series and the current coil in parallel
  3. Both coils are in series with the load
  4. Both coils are in parallel across the supply

Explanation

A wattmeter has a low-resistance current coil that carries the load current and a high-resistance voltage (pressure) coil connected across the load voltage. The deflection is proportional to the product of these, giving power. Swapping the roles would give wrong readings and could damage the voltage coil.

Question 40Electronic Measurements and Test Instruments

Why is an energy meter (kWh meter) different from a wattmeter in what it measures?

  1. The energy meter measures instantaneous power while the wattmeter measures average power
  2. The energy meter integrates power over time, so it registers energy consumed Correct answer.
  3. The energy meter measures only reactive power
  4. The energy meter measures current while the wattmeter measures voltage

Explanation

A wattmeter indicates the instantaneous power at the moment of reading, whereas an induction energy meter's disc rotates at a speed proportional to power and the revolution count accumulates over time. The register therefore records energy in kilowatt-hours. This integration is the essential difference.

Question 41Electronic Measurements and Test Instruments

An induction energy meter is found to run slow. Which adjustment is normally used to correct this?

  1. The pointer spring tension of the voltage coil
  2. The phase of the supply voltage
  3. The braking magnet position, which controls the braking torque and hence disc speed Correct answer.
  4. The number of turns on the current coil

Explanation

In an induction energy meter the disc speed for a given power is set by the braking torque from the permanent magnet. Moving the magnet closer increases braking and slows the disc, while moving it away speeds it up, so this adjustment corrects a slow or fast meter. Other adjustments handle friction and light-load compensation.

Question 42Electronic Measurements and Test Instruments

What is the purpose of a megger (insulation resistance tester) in electrical work?

  1. To measure earth loop impedance
  2. To measure high insulation resistance by applying a high test voltage Correct answer.
  3. To measure the frequency of the supply
  4. To measure very small contact resistances accurately

Explanation

A megger applies a high DC voltage, commonly 500 V or 1000 V, and measures the very high resistance of insulation, often in megohms. This reveals moisture, cracks, or breakdown that an ordinary ohmmeter could not detect. It is not intended for low-resistance or frequency measurements.

Question 43Electronic Measurements and Test Instruments

A technician tests a motor winding with a megger and reads a very low insulation resistance. What is the most reasonable conclusion?

  1. The winding insulation is sound and the motor may be energised
  2. The megger's battery is flat
  3. The motor is drawing too much current
  4. The insulation may be damp or damaged and the winding should be investigated before use Correct answer.

Explanation

Insulation resistance falls sharply when insulation absorbs moisture or is cracked, so a low reading is a warning of possible breakdown. The correct action is to dry and re-test or inspect the winding rather than energise the motor. A flat battery would typically give no reading at all, not a low one.

Question 44Electronic Measurements and Test Instruments

What does an oscilloscope display on its screen?

  1. A plot of signal voltage against time Correct answer.
  2. A plot of resistance against temperature
  3. A plot of frequency against amplitude
  4. A plot of power against current

Explanation

An oscilloscope plots the input voltage on the vertical (Y) axis against time on the horizontal (X) axis, so the waveform shape is visible. This lets the user measure amplitude, period, phase, and distortion. Other quantities are inferred from these two axes.

Question 45Electronic Measurements and Test Instruments

On a CRO, the time-base control is set to 2 ms/div and one complete cycle of a sine wave occupies 5 divisions. What is the frequency of the signal?

  1. 100 Hz Correct answer.
  2. 200 Hz
  3. 500 Hz
  4. 50 Hz

Explanation

The period is 5 divisions × 2 ms/div = 10 ms = 0.01 s. Frequency is the reciprocal, f = 1/T = 1/0.01 = 100 Hz. The time-base setting converts horizontal distance into time, and the period then gives frequency.

Question 46Electronic Measurements and Test Instruments

A CRO is set to 0.5 V/div on the vertical axis and the peak-to-peak height of a waveform is 8 divisions. What is the peak-to-peak voltage?

  1. 16 V
  2. 4 V Correct answer.
  3. 8 V
  4. 2 V

Explanation

Peak-to-peak voltage is the number of vertical divisions multiplied by the volts-per-division setting, so 8 × 0.5 V = 4 V. The peak voltage would then be half of this, 2 V. This uses the vertical scale in the same way the time base gives period.

Question 47Electronic Measurements and Test Instruments

A common reason a CRO trace appears as a stationary waveform rather than a moving blur is that the time base is set to a sub-multiple of the signal period. What happens if the time base frequency is not synchronised with the signal?

  1. The vertical scale changes automatically
  2. The trace brightness increases
  3. The trace becomes a straight horizontal line
  4. The waveform appears to drift across the screen Correct answer.

Explanation

The spot traces the waveform repeatedly; if each sweep starts at a different point of the cycle, successive traces are offset and the pattern drifts. Synchronisation (triggering) locks the sweep to the signal so the image stands still. Amplitude and brightness are set by other controls.

Question 48Electronic Measurements and Test Instruments

What is the function of the Lissajous figure method on an oscilloscope?

  1. To measure the DC offset of a signal
  2. To compare two frequencies, one applied to each channel Correct answer.
  3. To measure the insulation resistance of a cable
  4. To calibrate the time base only

Explanation

When two sinusoidal signals are applied to the X and Y inputs, the resulting pattern depends on their frequency ratio and phase. Counting the points of tangency on the horizontal and vertical axes gives the frequency ratio. It is a comparison technique, not a DC or resistance measurement.

Question 49Electronic Measurements and Test Instruments

A signal generator is set to a sine wave of 1 kHz. What does the frequency control actually determine?

  1. The peak amplitude of the output
  2. The output impedance of the generator
  3. The number of cycles produced per second Correct answer.
  4. The level of harmonic distortion

Explanation

Frequency is the number of complete cycles per second, expressed in hertz, so a 1 kHz setting produces 1000 cycles each second. Amplitude is set by the output level control, and output impedance is a fixed design value. Distortion is a quality measure, not the frequency setting.

Question 50Electronic Measurements and Test Instruments

A voltmeter with a sensitivity of 1 kΩ/V is used on a 10 V range to measure the voltage across a 100 kΩ resistor in series with another 100 kΩ resistor across a 10 V supply. What is the approximate loading error?

  1. No error, because the meter reads the true voltage
  2. The reading is about 9.5 V because the meter resistance is high
  3. The reading is exactly 10 V because the supply is fixed
  4. The reading is about 4.76 V instead of 5 V, a loading error Correct answer.

Explanation

Meter resistance is 1 kΩ/V × 10 V = 10 kΩ, which is placed in parallel with the 100 kΩ resistor, giving about 9.09 kΩ. The divider then produces about 9.09/(100 + 9.09) × 10 V ≈ 4.76 V, so the meter draws current and reads low. This is the classic loading error of low-sensitivity analog meters.