Amateur Extra E7D: Power supplies and regulators
These are the 15 official questions in group E7D of the 2024–2028 Amateur Extra question pool, with the correct answer marked and a plain-English explanation of why it's right. The real exam asks one question from this group.
- E7D01Question 1
How does a linear electronic voltage regulator work?
- AIt has a ramp voltage as its output
- BIt eliminates the need for a pass transistor
- CThe control element duty cycle is proportional to the line or load conditions
- DThe conduction of a control element is varied to maintain a constant output voltage Correct
Why: A linear regulator varies a control element's conduction to hold the output steady.
- E7D02Question 2
How does a switchmode voltage regulator work?
- ABy alternating the output between positive and negative voltages
- BBy varying the duty cycle of pulses input to a filter Correct
- CBy varying the conductivity of a pass element
- DBy switching between two Zener diode reference voltages
Why: A switchmode regulator varies the duty cycle of pulses into a filter.
- E7D03Question 3
What device is used as a stable voltage reference?
- AA Zener diode Correct
- BA digital-to-analog converter
- CAn SCR
- DAn analog-to-digital converter
Why: A Zener diode is a stable voltage reference.
- E7D04Question 4
Which of the following describes a three-terminal voltage regulator?
- AA series current source
- BA series regulator Correct
- CA shunt regulator
- DA shunt current source
Why: A three-terminal regulator is a series regulator.
- E7D05Question 5
Which of the following types of linear voltage regulator operates by loading the unregulated voltage source?
- AA constant current source
- BA series regulator
- CA shunt current source
- DA shunt regulator Correct
Why: A shunt regulator works by loading the source.
- E7D06Question 6
What is the purpose of Q1 in the circuit shown in Figure E7-2?
- AIt provides negative feedback to improve regulation
- BIt provides a constant load for the voltage source
- CIt controls the current to keep the output voltage constant Correct
- DIt provides regulation by switching or “chopping” the input DC voltage
Why: Q1 controls the current to keep the output voltage constant.
- E7D07Question 7
What is the purpose of C2 in the circuit shown in Figure E7-2?
- AIt bypasses rectifier output ripple around D1 Correct
- BIt is a brute force filter for the output
- CTo prevent self-oscillation
- DTo provide fixed DC bias for Q1
Why: C2 bypasses rectifier ripple around the Zener D1.
- E7D08Question 8
What type of circuit is shown in Figure E7-2?
- ASwitching voltage regulator
- BCommon emitter amplifier
- CLinear voltage regulator Correct
- DCommon base amplifier
Why: Figure E7-2 is a linear voltage regulator.
- E7D09Question 9
How is battery operating time calculated?
- AAverage current divided by capacity in amp-hours
- BAverage current divided by internal resistance
- CCapacity in amp-hours divided by average current Correct
- DInternal resistance divided by average current
Why: Battery run time = amp-hours ÷ average current.
- E7D10Question 10
Why is a switching type power supply less expensive and lighter than an equivalent linear power supply?
- AThe inverter design does not require an output filter circuit
- BThe control circuitry uses less current, therefore smaller heat sinks are required
- CThe high frequency inverter design uses much smaller transformers and filter components for an equivalent power output Correct
- DIt recovers power from the unused portion of the AC cycle, thus using fewer components
Why: High-frequency operation allows much smaller transformers and filters.
- E7D11Question 11
What is the purpose of an inverter connected to a solar panel output?
- AReduce AC ripple on the output
- BMaintain voltage with varying illumination levels
- CPrevent discharge when panel is not illuminated
- DConvert the panel’s output from DC to AC Correct
Why: An inverter converts the panel's DC to AC.
- E7D12Question 12
What is the dropout voltage of a linear voltage regulator?
- AMinimum input voltage for rated power dissipation
- BMaximum output voltage drop when the input voltage is varied over its specified range
- CMinimum input-to-output voltage required to maintain regulation Correct
- DMaximum that the output voltage may decrease at rated load
Why: Dropout is the minimum input-to-output difference needed to stay in regulation.
- E7D13Question 13
Which of the following calculates power dissipated by a series linear voltage regulator?
- AInput voltage multiplied by input current
- BInput voltage divided by output current
- CVoltage difference from input to output multiplied by output current Correct
- DOutput voltage multiplied by output current
Why: Dissipation = (input − output voltage) × output current.
- E7D14Question 14
What is the purpose of connecting equal-value resistors across power supply filter capacitors connected in series?
- AEqualize the voltage across each capacitor
- BDischarge the capacitors when voltage is removed
- CProvide a minimum load on the supply
- DAll these choices are correct Correct
Why: The resistors equalize the voltages and discharge the capacitors. All the choices are correct.
- E7D15Question 15
What is the purpose of a step-start circuit in a high-voltage power supply?
- ATo provide a dual-voltage output for reduced power applications
- BTo compensate for variations of the incoming line voltage
- CTo prevent arcing across the input power switch or relay contacts
- DTo allow the filter capacitors to charge gradually Correct
Why: A step-start lets the filter capacitors charge gradually.