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Subelement ZLF
Power Supplies
Section ZLF21
Power Supplies
A mains operated DC power supply
  • converts DC from the mains into AC of the same voltage
  • Correct Answer
    converts energy from the mains into DC for operating electronic equipment
  • is a diode-capacitor device for measuring mains power
  • is a diode-choked device for measuring inductance power
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The following unit in a DC power supply performs a rectifying operation
  • an electrolytic capacitor
  • a fuse
  • a crowbar
  • Correct Answer
    a full-wave diode bridge
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The following unit in a DC power supply performs a smoothing operation
  • Correct Answer
    an electrolytic capacitor
  • a fuse
  • a crowbar
  • a full-wave diode bridge
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The following could power a solid-state 10 watt VHF transceiver
  • Correct Answer
    a 12 volt car battery
  • 6 penlite cells in series
  • a 12 volt, 500 mA plug-pack
  • a 6 volt 10 Amp-hour Gel cell.
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A fullwave DC power supply operates from the New Zealand AC mains. The ripple frequency is
  • 25 Hz
  • 50 Hz
  • 70 Hz
  • Correct Answer
    100 Hz
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The capacitor value best suited for smoothing the output of a 12 volt 1 amp DC power supply is
  • 100 pF
  • 10 nF
  • 100 nF
  • Correct Answer
    10,000 uF
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The following should always be included as a standard protection device in any power supply
  • a saturating transformer
  • Correct Answer
    a fuse in the mains lead
  • a zener diode bridge limiter
  • a fuse in the filter capacitor negative lead

There should always be a fuse in the phase or active AC mains lead for protection if a fault develops in the equipment. The fuse should be of the correct rating for the task. Keep some spare fuses handy!

Last edited by cmscouler. Register to edit

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A halfwave DC power supply operates from the New Zealand AC mains. The ripple frequency will be
  • 25 Hz
  • Correct Answer
    50 Hz
  • 70 Hz
  • 100 Hz
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The output voltage of a DC power supply decreases when current is drawn from it because
  • drawing output current causes the input mains voltage to decrease
  • drawing output current causes the input mains frequency to decrease
  • Correct Answer
    all power supplies have some internal resistance
  • some power is reflected back into the mains.
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Electrolytic capacitors are used in power supplies because
  • they are tuned to operate at 50 Hz
  • they have very low losses compared to other types
  • they radiate less RF noise than other types
  • Correct Answer
    they can be obtained in larger values than other types
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