% FILENAME = N21.TXT
% Power Supplies
% Release version 2, January 2000
% Q 6 corrected 6 Mar 2012
 
%Question: 1
#21.1 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 

% ans 2 
	
%Question: 2
#21.2 The following unit in a DC power supply performs a rectifying operation: 

an electrolytic capacitor 

a fuse 

a crowbar 

a full-wave diode bridge 

% ans 4 
	
%Question: 3
#21.3 The following unit in a DC power supply performs a smoothing operation: 

an electrolytic capacitor 

a fuse 

a crowbar 

a full-wave diode bridge 

% ans 1 
	
%Question: 4
#21.4 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. 

% ans 1 
	
%Question: 5
#21.5 A fullwave DC power supply operates from the New Zealand AC mains. The ripple 
frequency is: 

25 Hz 

50 Hz 

70 Hz 

100 Hz 

% ans 4
	
%Question: 6
#21.6 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 

10,000 uF 

% ans 4 
	
%Question: 7
#21.7 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 

% ans 2 
	
%Question: 8
#21.8 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 

% ans 2 
	
%Question: 9
#21.9 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. 

% ans 3 
	
%Question: 10
#21.10 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 

% ans 4