## Question File Number 21

• A mains operated DC power supply:
• converts DC from the mains into AC of the same voltage
• 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
• ==========================
• The following unit in a DC power supply performs a rectifying operation:
• an electrolytic capacitor
• a fuse
• a crowbar
• a full-wave diode bridge
• ==========================
• The following unit in a DC power supply performs a smoothing operation:
• an electrolytic capacitor
• a fuse
• a crowbar
• a full-wave diode bridge
• ==========================
• The following could power a solid-state 10 watt VHF transceiver:
• 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.
• ==========================
• A fullwave DC power supply operates from the New Zealand AC mains. The ripple frequency is:
• 25 Hz
• 50 Hz
• 70 Hz
• 100 Hz
• ==========================
• The capacitor value best suited for filtering the output of a 12 volt 1 amp DC power supply is:
• 100 pF
• 10 nF
• 100 nF
• 10,000 uF
• ==========================
• The following should always be included as a standard protection device in any power supply:
• a saturating transformer
• a fuse in the mains lead
• a zener diode bridge limiter
• a fuse in the filter capacitor negative lead
• ==========================
• A halfwave DC power supply operates from the New Zealand AC mains. The ripple frequency will be:
• 25 Hz
• 50 Hz
• 70 Hz
• 100 Hz
• ==========================
• 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
• all power supplies have some internal resistance
• some power is reflected back into the mains
• ==========================
• 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
• they can be obtained in larger values than other types
• ==========================

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Compiled Sun Nov 28 2010 at 8:42:55pm

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