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ITI/NCVT Mechanic Consumer Electronics (AITT Theory) study materials
ITI/NCVT Mechanic Consumer Electronics (AITT Theory) study resources

ITI/NCVT Mechanic Consumer Electronics (AITT Theory) practice questions

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Question 1Safety, Environment and Workshop Practices

A technician must discharge a large electrolytic filter capacitor on a switch-mode power supply board before handling it. Which practice is the correct workshop procedure?

  1. Touch the terminals with a wet cloth to bleed the charge to earth
  2. Connect a resistor of suitable wattage across the terminals and verify near-zero voltage with a meter Correct answer.
  3. Short the capacitor terminals directly with a screwdriver until no spark appears
  4. Leave the board unplugged overnight, since capacitors always self-discharge fully

Explanation

A bleeder resistor limits the discharge current so the capacitor and PCB tracks are not damaged by a violent surge, and the meter reading proves the charge is actually gone. Direct shorting can weld metal, damage the capacitor and injure the hand holding the tool. Self-discharge through leakage is unpredictable and cannot be relied on for safety.

Question 2Safety, Environment and Workshop Practices

While servicing a CRT television, a technician follows the high-voltage safety rule for the anode connection. What is the correct sequence?

  1. Remove the anode cap first, then discharge the exposed CRT button to the chassis
  2. Discharge the anode only after removing the CRT from its mounting brackets
  3. Wear rubber gloves and pull the anode cap off with insulated pliers without any discharge step
  4. Discharge the anode cap to the chassis earth with a proper probe before removing the cap, with the set unplugged Correct answer.

Explanation

The CRT and its anode lead store a lethal charge, so the set must be unplugged and the anode discharged through a rated probe to chassis earth before the cap is handled. Doing the discharge first removes the stored energy from the circuit path the technician is about to touch.

Question 3Safety, Environment and Workshop Practices

A soldering iron is left idle on the bench between joints during a repair. Which practice best prevents both tip oxidation and burns?

  1. Leave the iron at maximum temperature so joints are always fast
  2. Keep the iron in a proper stand, wipe and re-tin the tip, and use the lowest temperature that gives a good joint Correct answer.
  3. Rest the iron on a heat-resistant mat with the tip pointing toward the technician
  4. Switch the iron off and on for every joint to save energy

Explanation

A stand keeps the hot tip away from hands and cables, wiping removes oxide, re-tinning protects the tip from corrosion, and the lowest working temperature reduces oxidation and fume generation. Idle time at excessive temperature is the main cause of tip burnout.

Question 4Safety, Environment and Workshop Practices

In a workshop, a technician notices that the earth pin of a mains plug on a bench test lead has been broken off. What is the correct action?

  1. Use the lead only with double-insulated equipment
  2. Continue using the lead but stand on an insulating mat
  3. Tape over the earth pin so it fits the socket firmly
  4. Withdraw the lead from service immediately and replace or repair the plug before further use Correct answer.

Explanation

The protective earth conductor is the fault-current path that lets the protective device clear a live-to-chassis fault; without it, the metal case of Class I equipment can become live. A damaged plug must be taken out of service and correctly rewired or replaced.

Question 5Safety, Environment and Workshop Practices

Which fire extinguisher is appropriate for a small electrical fire in an energized consumer electronics test bench?

  1. Water-type extinguisher
  2. Carbon dioxide extinguisher Correct answer.
  3. Foam extinguisher
  4. Wet chemical extinguisher

Explanation

Carbon dioxide is non-conductive, leaves no residue and displaces oxygen, so it suits fires in energized electrical equipment. Water, foam and wet chemical agents conduct electricity and can cause shock to the user as well as damage the equipment.

Question 6Safety, Environment and Workshop Practices

A technician is about to desolder a through-hole component from a densely populated PCB. Which combination of controls best reflects standard ESD and fume precautions?

  1. Anti-static gloves only, with the iron tip left floating and the room door closed
  2. Wrist strap bonded to earth through a resistor, grounded iron tip, and local fume extraction near the work Correct answer.
  3. Rubber-soled shoes and a cotton apron, with the bench fan blowing fumes toward the face
  4. A grounded metal bench top with bare arms resting on it, and no extraction

Explanation

A resistive earthed wrist strap bleeds static charge safely, a grounded iron tip prevents a potential difference across the device, and local extraction removes flux fumes before they reach the breathing zone. Together these address both device damage and operator health.

Question 7Safety, Environment and Workshop Practices

Which statement correctly describes the purpose of a residual current circuit breaker (RCCB) on a workshop distribution board?

  1. It protects motors against sustained overload by thermal tripping
  2. It stabilizes the mains voltage supplied to sensitive test instruments
  3. It trips when the difference between live and neutral currents indicates leakage to earth Correct answer.
  4. It limits the total load current to the rating of the connected appliances

Explanation

An RCCB compares the current in the live and neutral conductors; any imbalance flows to earth and trips the device, giving protection against earth-leakage shock. It does not replace a fuse or MCB for overload and short-circuit protection.

Question 8Safety, Environment and Workshop Practices

A lithium-ion battery pack from a cordless tool is found swollen and warm on the charging bench. What is the safest immediate action?

  1. Puncture the casing to release internal pressure and then recharge it
  2. Isolate it on a non-combustible surface away from combustible material and follow the site disposal procedure Correct answer.
  3. Place it in a sealed plastic bag and store it in a wooden cupboard
  4. Continue charging at a lower current to bring the cells back into balance

Explanation

A swollen, warm lithium-ion pack indicates internal gas generation and possible thermal runaway, so it must be isolated on a non-combustible surface and handled under the site's battery disposal procedure. Charging or puncturing such a pack can trigger fire or explosion.

Question 9Safety, Environment and Workshop Practices

When handling a bare printed circuit board during assembly, a technician should hold it in which way to minimize contamination and damage?

  1. Firmly across the component side to spread the grip load
  2. By the edges or the grounded frame, avoiding contact with gold fingers and pads Correct answer.
  3. By any convenient area, provided a wrist strap is worn
  4. By the connector gold fingers so the board hangs vertically

Explanation

Skin oils and salts contaminate pads and gold fingers, causing poor solderability, leakage and corrosion, while flexing the board can crack joints and ceramic parts. Holding the edges or a grounded frame keeps the functional surfaces clean.

Question 10Safety, Environment and Workshop Practices

A bench multimeter is used to measure the mains voltage at a wall socket. Which precaution is most important for the instrument and the technician?

  1. Confirm the meter and probes are rated for the CAT and voltage, select the correct AC range, and keep fingers behind the probe barriers Correct answer.
  2. Set the meter to resistance and rely on auto-ranging to select the function
  3. Select the lowest DC voltage range to get maximum resolution
  4. Hold both probe tips with bare fingers to steady them during the reading

Explanation

Mains measurement requires an instrument and probes with an adequate safety category and voltage rating, correct function selection, and finger barriers to prevent contact with live metal. These measures protect against arc flash and shock if the probe slips or the range is wrong.

Question 11Safety, Environment and Workshop Practices

Which practice is correct for storing and using isopropyl alcohol and flux cleaners in an electronics workshop?

  1. Store them in open trays so vapours escape and do not build up
  2. Keep them in unlabelled squeeze bottles near the soldering iron for convenience
  3. Keep them in labelled, closed containers away from ignition sources, with ventilation and suitable gloves Correct answer.
  4. Dilute them with water in an open beaker before every cleaning operation

Explanation

Isopropyl alcohol and many flux cleaners are flammable and can irritate skin and eyes, so labelled closed containers, separation from ignition sources such as irons, ventilation and appropriate gloves are the standard controls. Labelling also prevents mistaken use of the wrong liquid.

Question 12Safety, Environment and Workshop Practices

During a repair, a technician must replace a fuse in a television power supply. What is the correct replacement rule?

  1. Fit a slow-blow fuse of any rating because the inrush current is high
  2. Fit a fuse with a higher current rating so it does not blow again
  3. Wrap the blown fuse in foil and refit it to test the circuit
  4. Fit a fuse of the same current rating and the same type (fast-blow or time-lag) as specified Correct answer.

Explanation

The fuse is selected to protect the wiring and components, so both the current rating and the time-current characteristic must match the manufacturer's specification. A higher-rated or bridged fuse allows fault currents that can start a fire before the protective device operates.

Question 13Safety, Environment and Workshop Practices

Which of the following is the correct way to lift and carry a heavy CRT television set on the workshop floor?

  1. Grip the set by the plastic bezel at the top and lean backward while lifting
  2. Bend the knees, keep the back straight, hold the load close to the body and get help if it is too heavy Correct answer.
  3. Lift the set quickly with a twisting motion to clear the bench in one movement
  4. Bend at the waist, keep legs straight and hold the set away from the body

Explanation

Bending the knees and keeping the back straight uses the leg muscles, while holding the load close reduces the moment on the spine. Twisting or holding the load away from the body greatly increases the risk of back injury, and heavy CRTs often need a two-person lift.

Question 14Safety, Environment and Workshop Practices

An oscilloscope probe is used to observe a signal on a mains-referenced switch-mode power supply primary side. Which arrangement is correct?

  1. Connect the probe earth clip to any convenient primary-side point and float the oscilloscope chassis
  2. Clip the probe earth lead to the switching transistor collector for a stable reference
  3. Remove the oscilloscope's protective earth pin so the chassis cannot create a short
  4. Use an isolation transformer on the circuit under test and keep the probe earth clip on a point that is truly at earth potential Correct answer.

Explanation

The probe earth clip is bonded to the oscilloscope chassis and therefore to protective earth, so clipping it to a live primary node creates a short through the earth path. Powering the circuit under test through an isolation transformer, or using a properly rated differential probe, keeps the reference point safe.

Question 15Safety, Environment and Workshop Practices

A workshop's electrical installation uses an ELCB/RCCB of 30 mA sensitivity for socket outlets. A technician receives a mild tingle from a metal-cased instrument while it is running, but the device has not tripped. What is the most likely explanation?

  1. The RCCB is unnecessary because the instrument is Class II and has no metal case
  2. The tingle proves the RCCB is faulty and must be bypassed for the test to continue
  3. The leakage current is below the trip threshold but the earth path is degraded, so the case is at a potential above earth Correct answer.
  4. The supply voltage is too low for the RCCB to sense any leakage current

Explanation

A tingle indicates the metalwork is not held firmly at earth potential, usually because of high earth-path resistance or low-level insulation leakage that is still under the 30 mA trip threshold. The instrument must be taken out of service and the earth continuity and insulation checked.

Question 16Safety, Environment and Workshop Practices

A technician must dispose of a failed surface-mount assembly containing a small lithium coin cell and lead-based solder joints. Which disposal approach is correct?

  1. Store it in a covered e-waste container, with the coin cell removed or the assembly treated as battery-containing e-waste, and hand it to an authorized recycler Correct answer.
  2. Burn the board in the open to recover the metals, then discard the ash
  3. Break the board and put the pieces in general waste after removing the cell
  4. Dissolve the solder in acid in the workshop sink and pour the solution down the drain

Explanation

Electronic assemblies with batteries and lead-containing solder are hazardous and must go to an authorized e-waste recycler, with the battery segregated where the site procedure requires it. Breaking, burning or dissolving the assembly releases lead and lithium into the environment and exposes the technician to harm.

Question 17Basic Electricity and Electronic Components

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

  1. 1 kΩ with ±5 % tolerance Correct answer.
  2. 100 Ω with ±10 % tolerance
  3. 1 kΩ with ±20 % tolerance
  4. 10 kΩ with ±5 % tolerance

Explanation

The first two bands give the significant digits brown = 1 and black = 0, so the base value is 10. The third band red means multiply by 100, giving 10 × 100 = 1000 Ω, i.e. 1 kΩ. The fourth band gold indicates a tolerance of ±5 %.

Question 18Basic Electricity and Electronic Components

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

  1. The energy stored in a magnetic field around a conductor
  2. The ability to store electric charge per unit of applied voltage Correct answer.
  3. The rate at which heat is dissipated in a resistor
  4. The opposition offered to alternating current by a coil

Explanation

One farad is defined as the capacitance that stores one coulomb of charge when one volt is applied across it, so the farad measures charge stored per volt. Inductive reactance, power dissipation and magnetic-field energy belong to other component properties.

Question 19Basic Electricity and Electronic Components

A carbon resistor carries a current of 20 mA and the voltage across it is 10 V. What power does the resistor dissipate?

  1. 2 W
  2. 0.2 W Correct answer.
  3. 200 W
  4. 5 W

Explanation

Power equals voltage multiplied by current, so P = 10 V × 0.020 A = 0.2 W. Using the current in milliamperes without converting to amperes is the common source of error here.

Question 20Basic Electricity and Electronic Components

Which instrument connection is correct when measuring the current through a resistor in a live circuit?

  1. The voltmeter is connected in series and the ammeter in parallel
  2. The ammeter is connected across the supply terminals only
  3. The ammeter is connected in series with the resistor Correct answer.
  4. The ammeter is connected in parallel with the resistor

Explanation

An ammeter must carry the current being measured, so it is inserted in series with the component and has a very low internal resistance. Connecting it in parallel would divert current through the meter and could damage it.

Question 21Basic Electricity and Electronic Components

In a purely capacitive circuit supplied with sinusoidal AC, how does the current relate to the applied voltage?

  1. The current lags the voltage by 90 degrees
  2. The current leads the voltage by 90 degrees Correct answer.
  3. The current lags the voltage by 45 degrees
  4. The current is exactly in phase with the voltage

Explanation

In a capacitor the current depends on the rate of change of voltage, so it reaches its peak a quarter cycle before the voltage does. This gives a 90-degree leading phase angle in a purely capacitive circuit.

Question 22Basic Electricity and Electronic Components

What is the effect of connecting a 100 Ω resistor and a 100 Ω resistor in parallel?

  1. The equivalent resistance becomes 50 Ω Correct answer.
  2. The equivalent resistance becomes 25 Ω
  3. The equivalent resistance becomes 200 Ω
  4. The equivalent resistance stays at 100 Ω

Explanation

For two equal resistors in parallel the equivalent resistance is half of one value, because 1/R = 1/100 + 1/100 = 2/100, giving R = 50 Ω. Adding resistors in parallel always lowers the total resistance below the smallest branch value.

Question 23Basic Electricity and Electronic Components

A silicon diode is forward biased in a simple circuit. Which statement describes its behaviour?

  1. It conducts heavily in reverse until breakdown occurs
  2. It conducts equally in both directions at all times
  3. It conducts once the forward voltage exceeds about 0.7 V Correct answer.
  4. It behaves as an open circuit for all applied voltages

Explanation

A silicon PN junction needs roughly 0.7 V of forward bias before the depletion barrier is overcome and significant conduction begins. Below that knee voltage the current remains very small.

Question 24Basic Electricity and Electronic Components

A half-wave rectifier with a smoothing capacitor is used in a small power supply. What determines the amount of ripple remaining on the DC output?

  1. The frequency of the AC mains only
  2. The reverse breakdown voltage of the rectifier diode
  3. The load current and the value of the smoothing capacitor Correct answer.
  4. The colour code tolerance of the series resistor

Explanation

Ripple is caused by the capacitor discharging into the load between successive charging pulses. A larger capacitor or a lighter load current reduces the discharge and therefore the ripple amplitude.

Question 25Basic Electricity and Electronic Components

In the colour code for a capacitor marked 104, what is the capacitance?

  1. 0.01 µF
  2. 104 pF
  3. 0.1 µF Correct answer.
  4. 10 µF

Explanation

In the three-digit code the first two digits are significant figures and the third is the number of zeros, giving 10 followed by four zeros, i.e. 100000 pF. Dividing by one million converts picofarads to microfarads, giving 0.1 µF.

Question 26Basic Electricity and Electronic Components

Which statement correctly describes the function of the base region in an NPN bipolar transistor?

  1. It acts as a permanent reverse-biased junction in normal operation
  2. It forms the main heat path to the metal case
  3. It is thin and lightly doped so that emitter charge carriers can be controlled by base current Correct answer.
  4. It is thick and heavily doped to block all carrier flow

Explanation

The base is made thin and lightly doped so that most carriers injected from the emitter pass through to the collector. The small base current then controls a much larger collector current, giving current amplification.

Question 27Basic Electricity and Electronic Components

A 12 V supply is connected across a 6 Ω resistor. What current flows, and what is the voltage drop across a further 4 Ω resistor in series in the same circuit?

  1. 0.5 A and 2 V
  2. 1.2 A and 7.2 V
  3. 1.2 A and 4.8 V Correct answer.
  4. 2 A and 4 V

Explanation

The total series resistance is 6 + 4 = 10 Ω, so the current is 12 V ÷ 10 Ω = 1.2 A. The drop across the 4 Ω resistor is then 1.2 A × 4 Ω = 4.8 V.

Question 28Basic Electricity and Electronic Components

What is the purpose of a zener diode operated in reverse breakdown in a power supply?

  1. To hold a nearly constant voltage across the load Correct answer.
  2. To amplify the audio signal
  3. To convert alternating current into pulses
  4. To store energy for the load during mains failure

Explanation

A zener diode in reverse breakdown maintains an almost constant voltage across its terminals over a range of reverse currents. This property makes it suitable as a simple shunt voltage regulator or reference.

Question 29Basic Electricity and Electronic Components

A transformer used in a consumer audio power supply has 400 turns on the primary and 40 turns on the secondary, with 230 V applied to the primary. What secondary voltage is expected?

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

Explanation

The turns ratio is 400:40, i.e. 10:1, and for an ideal transformer the voltage ratio equals the turns ratio. Dividing 230 V by 10 gives 23 V on the secondary.

Question 30Basic Electricity and Electronic Components

Which statement about an intrinsic semiconductor at room temperature is correct?

  1. Its resistance falls as the temperature is lowered
  2. Only free electrons exist and no holes are present
  3. The material behaves as a perfect insulator at all temperatures
  4. Holes and electrons are produced in equal numbers by thermal generation Correct answer.

Explanation

In a pure semiconductor, thermal energy breaks covalent bonds and creates an electron and a hole together, so the two carrier concentrations are equal. Conductivity therefore rises with temperature, unlike a metal.

Question 31Basic Electricity and Electronic Components

A resistor has bands of orange, orange, brown and silver. Which reading is correct?

  1. 330 Ω with ±10 % tolerance Correct answer.
  2. 33 Ω with ±10 % tolerance
  3. 3.3 kΩ with ±10 % tolerance
  4. 330 Ω with ±5 % tolerance

Explanation

Orange is 3 and orange is 3, so the significant digits are 33; brown multiplies by 10, giving 330 Ω. The silver fourth band denotes a tolerance of ±10 %.

Question 32Basic Electricity and Electronic Components

A coil of 0.5 H carries a steadily increasing current of 200 A per second. What is the self-induced EMF?

  1. 0.1 V
  2. 100 V Correct answer.
  3. 0.4 V
  4. 400 V

Explanation

The induced EMF equals inductance multiplied by the rate of change of current, so E = 0.5 H × 200 A/s = 100 V. The EMF opposes the change in current according to Lenz's law.

Question 33Basic Electricity and Electronic Components

A common-emitter amplifier stage is being analysed. Which phase relationship exists between the base input signal and the collector output signal at mid-band?

  1. The output lags the input by 45 degrees
  2. The output leads the input by 90 degrees
  3. The output is inverted, i.e. 180 degrees out of phase with the input Correct answer.
  4. The output is in phase with the input

Explanation

In a common-emitter stage, an increase in base current raises collector current and therefore increases the drop across the collector load resistor, lowering the collector voltage. This gives a 180-degree phase inversion between input and output.

Question 34Basic Electricity and Electronic Components

A capacitor of 100 µF is connected across a 10 V DC supply and fully charged. How much charge is stored, and how much energy is held?

  1. 1 mC and 5 mJ Correct answer.
  2. 0.1 mC and 0.5 mJ
  3. 1 mC and 10 mJ
  4. 10 mC and 50 mJ

Explanation

Charge is capacitance times voltage, Q = 100 × 10⁻⁶ F × 10 V = 1 × 10⁻³ C, i.e. 1 mC. Energy is ½CV², giving 0.5 × 100 × 10⁻⁶ × 100 = 5 × 10⁻³ J, i.e. 5 mJ.

Question 35Basic Electricity and Electronic Components

In a series RLC circuit at resonance, which condition holds?

  1. Inductive reactance equals capacitive reactance and the impedance is purely resistive Correct answer.
  2. Inductive reactance is twice the capacitive reactance and the current is minimum
  3. The total impedance reaches its maximum value
  4. The capacitive reactance is zero and the circuit behaves as a short circuit

Explanation

Resonance occurs at the frequency where XL = XC, so the two reactive voltages cancel and only resistance remains. The impedance is then at its minimum, equal to R, and the current is at its maximum.

Question 36Basic Electricity and Electronic Components

A transistor in a switching circuit is driven hard into saturation. Which condition best describes the collector-emitter voltage in this state?

  1. It equals the full supply voltage
  2. It falls to a small value, typically a few tenths of a volt Correct answer.
  3. It oscillates around half the supply voltage
  4. It is clamped at exactly 0.7 V

Explanation

In saturation both junctions are forward biased and the transistor behaves almost like a closed switch. The collector-emitter voltage then drops to a small saturation value, usually a few tenths of a volt, so very little power is dissipated.

Question 37Semiconductor Devices and Power Supplies

In a consumer electronics repair bench, a silicon rectifier diode is forward biased. Which value is closest to the typical junction voltage drop of this diode?

  1. 5.0 V
  2. 1.5 V
  3. 0.2 V
  4. 0.7 V Correct answer.

Explanation

A forward-biased silicon pn junction conducts at roughly 0.6 to 0.7 V, which is the barrier potential of silicon. This drop is what a technician measures across a healthy silicon rectifier during a diode test.

Question 38Semiconductor Devices and Power Supplies

A zener diode is to be used as a simple shunt voltage regulator in a small power supply. Which operating condition must the circuit maintain for regulation to work?

  1. Reverse bias beyond the breakdown voltage with current limited by a series resistor Correct answer.
  2. Forward bias with the anode tied to the positive rail
  3. Reverse bias well below the knee voltage of the diode
  4. Zero bias so that the depletion region stays widest

Explanation

A zener regulates only in reverse breakdown, where a small change in current produces almost no change in voltage. A series resistor is essential to limit the breakdown current and dissipate the excess power.

Question 39Semiconductor Devices and Power Supplies

Which semiconductor device is controlled by voltage at an insulated gate and is widely used as the main switching element in modern switch-mode power supplies?

  1. BJT
  2. SCR
  3. Diode
  4. Power MOSFET Correct answer.

Explanation

The power MOSFET is a voltage-controlled device whose insulated gate draws almost no steady gate current, which suits high-frequency switching. Its low on-resistance keeps conduction losses small in SMPS designs.

Question 40Semiconductor Devices and Power Supplies

A half-wave rectifier with a capacitor-input filter feeds a light load. Which statement describes the output ripple?

  1. Ripple frequency is four times the supply frequency
  2. Ripple frequency is half the supply frequency
  3. Ripple frequency equals the supply frequency Correct answer.
  4. Ripple frequency is twice the supply frequency

Explanation

In a half-wave rectifier only one half-cycle is passed, so the capacitor recharges once per input cycle. The ripple therefore repeats at the same frequency as the AC mains.

Question 41Semiconductor Devices and Power Supplies

In a bipolar junction transistor acting as an amplifier, the emitter junction is forward biased and the collector junction is reverse biased. In which region is the transistor operating?

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

Explanation

The active region requires the emitter-base junction forward biased and the collector-base junction reverse biased, giving useful current gain. This is the normal bias condition for linear amplification.

Question 42Semiconductor Devices and Power Supplies

A transistor in a power supply is mounted on a mica insulating washer with a metal screw. Why is this mounting arrangement used?

  1. To increase the current gain of the transistor
  2. To add capacitance between the collector and the heatsink
  3. To reduce the switching frequency of the supply
  4. To electrically isolate the device tab from the grounded heatsink while keeping good thermal contact Correct answer.

Explanation

The mica washer conducts heat but blocks DC, so a device whose tab is at collector potential can share a grounded heatsink safely. Thermal compound on both faces keeps the thermal resistance low.

Question 43Semiconductor Devices and Power Supplies

A bridge rectifier is connected to a 12 V RMS transformer secondary feeding a large filter capacitor. What approximate DC voltage appears across the capacitor at no load, ignoring diode drops?

  1. 24 V
  2. 8.5 V
  3. 17 V Correct answer.
  4. 12 V

Explanation

The capacitor charges to the peak value of the secondary voltage, which is 12 V multiplied by 1.414, about 17 V. The RMS value is not the value stored on the capacitor.

Question 44Semiconductor Devices and Power Supplies

A silicon diode is connected in series with a 470 ohm resistor across a 10 V DC supply. Using a 0.7 V diode drop, what current flows?

  1. 21.3 mA
  2. 4.7 mA
  3. 19.8 mA Correct answer.
  4. 10 mA

Explanation

The resistor sees 10 V minus 0.7 V, or 9.3 V, so the current is 9.3 divided by 470, about 19.8 mA. Ignoring the diode drop would overstate the current.

Question 45Semiconductor Devices and Power Supplies

A 5.1 V zener with a series resistor of 220 ohm is fed from a 12 V supply. The load draws negligible current. What power is dissipated in the zener, approximately?

  1. 600 mW
  2. 26 mW
  3. 55 mW
  4. 160 mW Correct answer.

Explanation

The resistor drops 12 V minus 5.1 V, or 6.9 V, giving about 31 mA through the zener. Multiplying 5.1 V by 31 mA gives roughly 160 mW, so a 400 mW or 1 W zener is chosen.

Question 46Semiconductor Devices and Power Supplies

A full-wave bridge rectifier with a capacitor filter feeds a load drawing 500 mA. The filter capacitor is 2200 microfarad and the ripple is to be estimated. Which expression gives the approximate peak-to-peak ripple voltage?

  1. I divided by (f C)
  2. I divided by (2 f C) Correct answer.
  3. I multiplied by (2 f C)
  4. I divided by (2 C)

Explanation

For a full-wave rectifier the capacitor discharges for half of each cycle, so the ripple is load current divided by 2fC. The factor 2 appears because the recharge happens twice per mains cycle.

Question 47Semiconductor Devices and Power Supplies

A series-pass linear regulator uses a pass transistor with a 15 V input and a 5 V output at 1 A. What is the power dissipated in the pass element?

  1. 3 W
  2. 10 W Correct answer.
  3. 5 W
  4. 15 W

Explanation

The pass element drops the difference between input and output, 10 V, while carrying 1 A, so it dissipates 10 W. This loss is why linear regulators need substantial heatsinking.

Question 48Semiconductor Devices and Power Supplies

A flyback converter stores energy in a coupled inductor. During which interval is energy transferred to the output?

  1. When both the switch and the secondary diode conduct together
  2. When the primary switch is off and the secondary diode conducts Correct answer.
  3. When the primary switch is on and the secondary diode is forward biased
  4. When the primary switch is on and primary current ramps up

Explanation

In a flyback the primary winding stores energy while the switch is on, and the dot convention reverse biases the secondary diode. When the switch turns off, the secondary diode conducts and delivers the stored energy to the output.

Question 49Semiconductor Devices and Power Supplies

An SCR is used to control a lamp from a DC supply. Once the SCR has latched on, what is required to turn it off?

  1. Apply a negative pulse to the gate
  2. Increase the gate current further
  3. Reverse the gate-cathode voltage permanently
  4. Reduce the anode current below the holding current Correct answer.

Explanation

An SCR is a latching device; the gate loses control after conduction begins. It turns off only when anode current falls below the holding current, which in DC circuits needs a separate commutation circuit.

Question 50Semiconductor Devices and Power Supplies

A transistor is to be used as a switch to drive a relay coil. Which bias condition keeps the transistor in saturation with minimum dissipation?

  1. Base current just at the edge of conduction
  2. Base current well above the value needed for the collector current divided by hFE Correct answer.
  3. Collector-base junction reverse biased with emitter open
  4. Base left open so that only leakage flows

Explanation

Overdriving the base ensures the transistor stays hard in saturation, where the collector-emitter voltage is only a fraction of a volt. Power dissipation is then the small saturation voltage times the collector current.