DC Power Supply - Chapter Notes, Semester, Electrical and Electronics Engineering - Electrical Engineering (EE) PDF Download

 Page 1


                            
                                           DC POWER SUPPLY 
A power supply is a device that supplies electric power  to an electrical load. The term is most 
commonly applied to electric power converters that convert one form of electrical energy to another, 
though it may also refer to devices that convert another form of energy (mechanical, chemical, solar) to 
electrical energy. A regulated power supply is one that controls the output voltage or current to a 
specific value; the controlled value is held nearly constant despite variations in either load current or the 
voltage supplied by the power supply's energy source. 
A compound DC motor connects the armature and fields windings in a shunt and a series combination to 
give it characteristics of both a shunt and a series DC motor. This motor is used when both a high 
starting torque and good speed regulation is needed. The motor can be connected in two arrangements: 
cumulatively or differentially. An AC powered unregulated power supply usually uses a transformer to 
convert the voltage from the wall outlet (mains) to a different, nowadays usually lower, voltage. If it is 
used to produce DC, a rectifier is used to convert alternating voltage to a pulsating direct voltage, 
followed by a filter, comprising one or more capacitors, resistors, and sometimes inductors, to filter out 
(smooth) most of the pulsation. A small remaining unwanted alternating voltage component at mains or 
twice mains power frequency (depending upon whether half- or full-wave rectification is used)—
ripple—is unavoidably superimposed on the direct output voltage. 
                       BLOCK DIAGRAM OF POWER SUPPLY 
 
 
                                 
 
                            
Page 2


                            
                                           DC POWER SUPPLY 
A power supply is a device that supplies electric power  to an electrical load. The term is most 
commonly applied to electric power converters that convert one form of electrical energy to another, 
though it may also refer to devices that convert another form of energy (mechanical, chemical, solar) to 
electrical energy. A regulated power supply is one that controls the output voltage or current to a 
specific value; the controlled value is held nearly constant despite variations in either load current or the 
voltage supplied by the power supply's energy source. 
A compound DC motor connects the armature and fields windings in a shunt and a series combination to 
give it characteristics of both a shunt and a series DC motor. This motor is used when both a high 
starting torque and good speed regulation is needed. The motor can be connected in two arrangements: 
cumulatively or differentially. An AC powered unregulated power supply usually uses a transformer to 
convert the voltage from the wall outlet (mains) to a different, nowadays usually lower, voltage. If it is 
used to produce DC, a rectifier is used to convert alternating voltage to a pulsating direct voltage, 
followed by a filter, comprising one or more capacitors, resistors, and sometimes inductors, to filter out 
(smooth) most of the pulsation. A small remaining unwanted alternating voltage component at mains or 
twice mains power frequency (depending upon whether half- or full-wave rectification is used)—
ripple—is unavoidably superimposed on the direct output voltage. 
                       BLOCK DIAGRAM OF POWER SUPPLY 
 
 
                                 
 
                            
                            
                                    TRANSFORMER 
A transformer is a static electrical device that transfers energy by inductive coupling between 
its winding circuits. A varying current in the primary winding creates a varying magnetic flux in 
the transformer's core and thus a varying magnetic flux through the secondary winding. This 
varying magnetic flux induces a varying electromotive force (EMF) or voltage in the secondary 
winding. 
Transformers range in size from thumbnail-sized used in microphones to units weighing 
hundreds of tons interconnecting the power grid. A wide range of transformer designs are used in 
electronic and electric power applications. Transformers are essential for the transmission, 
distribution, and utilization of electrical energy. 
 
                              CENTER T AP TRANSFORMER 
In electronics, a center tap is a contact made to a point halfway along a winding of a transformer or 
inductor, or along the element of a resistor or a potentiometer. Taps are sometimes used on inductors 
for the coupling of signals, and may not necessarily be at the half-way point, but rather, closer to one 
end. A common application of this is in the Hartley oscillator. Inductors with taps also permit the 
transformation of the amplitude of alternating current (AC) voltages for the purpose of power 
conversion, in which case, they are referred to as autotransformers, since there is only one winding. An 
example of an autotransformer is an automobile ignition coil. Potentiometer tapping provides one or 
more connections along the device's element, along with the usual connections at each of the two ends 
of the element, and the slider connection. Potentiometer taps allow for circuit functions that would 
otherwise not be available with the usual construction of just the two end connections and one slider 
connection. 
• In a rectifier, a center-tapped transformer and two diodes can form a full-wave rectifier that 
allows both half-cycles of the AC waveform to contribute to the direct current, making it 
smoother than a half-wave rectifier. This form of circuit saves on rectifier diodes compared to a 
diode bridge, but has poorer utilization of the transformer windings. Center-tapped two-diode 
rectifiers were a common feature of power supplies in vacuum tube equipment. Modern 
semiconductor diodes are low-cost and compact so usually a four-diode bridge is used (up to a 
few hundred watts total output) which produces the same quality of DC as the center-tapped 
configuration with a more compact and cheaper power transformer. Center-tapped 
configurations may still be used in high-current applications, such as large automotive battery 
chargers, where the extra transformer cost is offset by less costly rectifiers. Center-tapped 
transformers are also used for dual-voltage power supplies. When a center-tapped transformer 
is combined with a bridge (four diode) rectifier, it is possible to produce a positive and a 
negative voltage with respect to a ground at the tap. Dual voltage supplies are important for all 
sorts of electronics equipment.  
 
                            
Page 3


                            
                                           DC POWER SUPPLY 
A power supply is a device that supplies electric power  to an electrical load. The term is most 
commonly applied to electric power converters that convert one form of electrical energy to another, 
though it may also refer to devices that convert another form of energy (mechanical, chemical, solar) to 
electrical energy. A regulated power supply is one that controls the output voltage or current to a 
specific value; the controlled value is held nearly constant despite variations in either load current or the 
voltage supplied by the power supply's energy source. 
A compound DC motor connects the armature and fields windings in a shunt and a series combination to 
give it characteristics of both a shunt and a series DC motor. This motor is used when both a high 
starting torque and good speed regulation is needed. The motor can be connected in two arrangements: 
cumulatively or differentially. An AC powered unregulated power supply usually uses a transformer to 
convert the voltage from the wall outlet (mains) to a different, nowadays usually lower, voltage. If it is 
used to produce DC, a rectifier is used to convert alternating voltage to a pulsating direct voltage, 
followed by a filter, comprising one or more capacitors, resistors, and sometimes inductors, to filter out 
(smooth) most of the pulsation. A small remaining unwanted alternating voltage component at mains or 
twice mains power frequency (depending upon whether half- or full-wave rectification is used)—
ripple—is unavoidably superimposed on the direct output voltage. 
                       BLOCK DIAGRAM OF POWER SUPPLY 
 
 
                                 
 
                            
                            
                                    TRANSFORMER 
A transformer is a static electrical device that transfers energy by inductive coupling between 
its winding circuits. A varying current in the primary winding creates a varying magnetic flux in 
the transformer's core and thus a varying magnetic flux through the secondary winding. This 
varying magnetic flux induces a varying electromotive force (EMF) or voltage in the secondary 
winding. 
Transformers range in size from thumbnail-sized used in microphones to units weighing 
hundreds of tons interconnecting the power grid. A wide range of transformer designs are used in 
electronic and electric power applications. Transformers are essential for the transmission, 
distribution, and utilization of electrical energy. 
 
                              CENTER T AP TRANSFORMER 
In electronics, a center tap is a contact made to a point halfway along a winding of a transformer or 
inductor, or along the element of a resistor or a potentiometer. Taps are sometimes used on inductors 
for the coupling of signals, and may not necessarily be at the half-way point, but rather, closer to one 
end. A common application of this is in the Hartley oscillator. Inductors with taps also permit the 
transformation of the amplitude of alternating current (AC) voltages for the purpose of power 
conversion, in which case, they are referred to as autotransformers, since there is only one winding. An 
example of an autotransformer is an automobile ignition coil. Potentiometer tapping provides one or 
more connections along the device's element, along with the usual connections at each of the two ends 
of the element, and the slider connection. Potentiometer taps allow for circuit functions that would 
otherwise not be available with the usual construction of just the two end connections and one slider 
connection. 
• In a rectifier, a center-tapped transformer and two diodes can form a full-wave rectifier that 
allows both half-cycles of the AC waveform to contribute to the direct current, making it 
smoother than a half-wave rectifier. This form of circuit saves on rectifier diodes compared to a 
diode bridge, but has poorer utilization of the transformer windings. Center-tapped two-diode 
rectifiers were a common feature of power supplies in vacuum tube equipment. Modern 
semiconductor diodes are low-cost and compact so usually a four-diode bridge is used (up to a 
few hundred watts total output) which produces the same quality of DC as the center-tapped 
configuration with a more compact and cheaper power transformer. Center-tapped 
configurations may still be used in high-current applications, such as large automotive battery 
chargers, where the extra transformer cost is offset by less costly rectifiers. Center-tapped 
transformers are also used for dual-voltage power supplies. When a center-tapped transformer 
is combined with a bridge (four diode) rectifier, it is possible to produce a positive and a 
negative voltage with respect to a ground at the tap. Dual voltage supplies are important for all 
sorts of electronics equipment.  
 
                            
                            
 
 
                  A full-wave rectifier using two diodes and a center tap transformer. 
                             
                                              RECTIFIER 
A rectifier is an electrical device that converts alternating current (AC), which periodically 
reverses direction, to direct current (DC), which flows in only one direction. The process is 
known as rectification. Physically, rectifiers take a number of forms, including vacuum tube 
diodes, mercury-arc valves, copper and selenium oxide rectifiers, solid-state diodes, silicon-
controlled rectifiers and other silicon-based semiconductor switches. Historically, even 
synchronous electromechanical switches and motors have been used. Early radio receivers, 
called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead 
sulfide) to serve as a point-contact rectifier or "crystal detector". 
Rectifiers have many uses, but are often found serving as components of DC power supplies and 
high-voltage direct current power transmission systems. Rectification may serve in roles other 
than to generate direct current for use as a source of power. As noted, detectors of radio signals 
serve as rectifiers. In gas heating systems flame rectification is used to detect presence of flame. 
The simple process of rectification produces a type of DC characterized by pulsating voltages 
and currents (although still unidirectional). Depending upon the type of end-use, this type of DC 
current may then be further modified into the type of relatively constant voltage DC 
characteristically produced by such sources as batteries and solar cells. 
Full-wave rectification 
A full-wave rectifier converts the whole of the input waveform to one of constant polarity 
(positive or negative) at its output. Full-wave rectification converts both polarities of the input 
waveform to pulsating DC (direct current), and yields a higher average output voltage. Two 
diodes and a center tapped transformer, or four diodes in a bridge configuration and any AC 
source (including a transformer without center tap), are needed.
[3]
 Single semiconductor diodes, 
double diodes with common cathode or common anode, and four-diode bridges, are 
manufactured as single components. 
Twice as many turns are required on the transformer For single-phase AC, if the transformer is 
center-tapped, then two diodes back-to-back (cathode-to-cathode or anode-to-anode, depending 
                            
Page 4


                            
                                           DC POWER SUPPLY 
A power supply is a device that supplies electric power  to an electrical load. The term is most 
commonly applied to electric power converters that convert one form of electrical energy to another, 
though it may also refer to devices that convert another form of energy (mechanical, chemical, solar) to 
electrical energy. A regulated power supply is one that controls the output voltage or current to a 
specific value; the controlled value is held nearly constant despite variations in either load current or the 
voltage supplied by the power supply's energy source. 
A compound DC motor connects the armature and fields windings in a shunt and a series combination to 
give it characteristics of both a shunt and a series DC motor. This motor is used when both a high 
starting torque and good speed regulation is needed. The motor can be connected in two arrangements: 
cumulatively or differentially. An AC powered unregulated power supply usually uses a transformer to 
convert the voltage from the wall outlet (mains) to a different, nowadays usually lower, voltage. If it is 
used to produce DC, a rectifier is used to convert alternating voltage to a pulsating direct voltage, 
followed by a filter, comprising one or more capacitors, resistors, and sometimes inductors, to filter out 
(smooth) most of the pulsation. A small remaining unwanted alternating voltage component at mains or 
twice mains power frequency (depending upon whether half- or full-wave rectification is used)—
ripple—is unavoidably superimposed on the direct output voltage. 
                       BLOCK DIAGRAM OF POWER SUPPLY 
 
 
                                 
 
                            
                            
                                    TRANSFORMER 
A transformer is a static electrical device that transfers energy by inductive coupling between 
its winding circuits. A varying current in the primary winding creates a varying magnetic flux in 
the transformer's core and thus a varying magnetic flux through the secondary winding. This 
varying magnetic flux induces a varying electromotive force (EMF) or voltage in the secondary 
winding. 
Transformers range in size from thumbnail-sized used in microphones to units weighing 
hundreds of tons interconnecting the power grid. A wide range of transformer designs are used in 
electronic and electric power applications. Transformers are essential for the transmission, 
distribution, and utilization of electrical energy. 
 
                              CENTER T AP TRANSFORMER 
In electronics, a center tap is a contact made to a point halfway along a winding of a transformer or 
inductor, or along the element of a resistor or a potentiometer. Taps are sometimes used on inductors 
for the coupling of signals, and may not necessarily be at the half-way point, but rather, closer to one 
end. A common application of this is in the Hartley oscillator. Inductors with taps also permit the 
transformation of the amplitude of alternating current (AC) voltages for the purpose of power 
conversion, in which case, they are referred to as autotransformers, since there is only one winding. An 
example of an autotransformer is an automobile ignition coil. Potentiometer tapping provides one or 
more connections along the device's element, along with the usual connections at each of the two ends 
of the element, and the slider connection. Potentiometer taps allow for circuit functions that would 
otherwise not be available with the usual construction of just the two end connections and one slider 
connection. 
• In a rectifier, a center-tapped transformer and two diodes can form a full-wave rectifier that 
allows both half-cycles of the AC waveform to contribute to the direct current, making it 
smoother than a half-wave rectifier. This form of circuit saves on rectifier diodes compared to a 
diode bridge, but has poorer utilization of the transformer windings. Center-tapped two-diode 
rectifiers were a common feature of power supplies in vacuum tube equipment. Modern 
semiconductor diodes are low-cost and compact so usually a four-diode bridge is used (up to a 
few hundred watts total output) which produces the same quality of DC as the center-tapped 
configuration with a more compact and cheaper power transformer. Center-tapped 
configurations may still be used in high-current applications, such as large automotive battery 
chargers, where the extra transformer cost is offset by less costly rectifiers. Center-tapped 
transformers are also used for dual-voltage power supplies. When a center-tapped transformer 
is combined with a bridge (four diode) rectifier, it is possible to produce a positive and a 
negative voltage with respect to a ground at the tap. Dual voltage supplies are important for all 
sorts of electronics equipment.  
 
                            
                            
 
 
                  A full-wave rectifier using two diodes and a center tap transformer. 
                             
                                              RECTIFIER 
A rectifier is an electrical device that converts alternating current (AC), which periodically 
reverses direction, to direct current (DC), which flows in only one direction. The process is 
known as rectification. Physically, rectifiers take a number of forms, including vacuum tube 
diodes, mercury-arc valves, copper and selenium oxide rectifiers, solid-state diodes, silicon-
controlled rectifiers and other silicon-based semiconductor switches. Historically, even 
synchronous electromechanical switches and motors have been used. Early radio receivers, 
called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead 
sulfide) to serve as a point-contact rectifier or "crystal detector". 
Rectifiers have many uses, but are often found serving as components of DC power supplies and 
high-voltage direct current power transmission systems. Rectification may serve in roles other 
than to generate direct current for use as a source of power. As noted, detectors of radio signals 
serve as rectifiers. In gas heating systems flame rectification is used to detect presence of flame. 
The simple process of rectification produces a type of DC characterized by pulsating voltages 
and currents (although still unidirectional). Depending upon the type of end-use, this type of DC 
current may then be further modified into the type of relatively constant voltage DC 
characteristically produced by such sources as batteries and solar cells. 
Full-wave rectification 
A full-wave rectifier converts the whole of the input waveform to one of constant polarity 
(positive or negative) at its output. Full-wave rectification converts both polarities of the input 
waveform to pulsating DC (direct current), and yields a higher average output voltage. Two 
diodes and a center tapped transformer, or four diodes in a bridge configuration and any AC 
source (including a transformer without center tap), are needed.
[3]
 Single semiconductor diodes, 
double diodes with common cathode or common anode, and four-diode bridges, are 
manufactured as single components. 
Twice as many turns are required on the transformer For single-phase AC, if the transformer is 
center-tapped, then two diodes back-to-back (cathode-to-cathode or anode-to-anode, depending 
                            
                            
upon output polarity required) can form a secondary to obtain the same output voltage than for a 
bridge rectifier, but the power rating is unchanged. 
 
 
                  Full-wave rectifier using a center tap transformer and 2 diodes. 
                                                                       CAPACITOR 
Filter capacitors are capacitors used for filtering of undesirable frequencies. They are common 
in electrical and electronic equipment, and cover a number of applications, such as: 
• Glitch removal on Direct current (DC) power rails 
• Radio frequency interference (RFI) removal for signal or power lines entering or leaving 
equipment 
• Capacitors used after a voltage regulator to further smooth dc power supplies 
• Capacitors used in audio, intermediate frequency (IF) or radio frequency (RF) frequency 
filters (e.g. low pass, high pass, notch, etc.) 
• Arc suppression, such as across the contact breaker or 'points' in a spark-ignition engine 
Filter capacitors are not the same as reservoir capacitors, the tasks the two perform are different, 
albeit related 
 
The simple capacitor filter is the most basic type of power supply filter. The application of the simple 
capacitor filter is very limited. It is sometimes used on extremely high-voltage, low-current power 
supplies for cathode-ray and similar electron tubes, which require very little load current from the 
supply. The capacitor filter is also used where the power-supply ripple frequency is not critical; this 
frequency can be relatively high. The capacitor (C1) shown in figure 4-15 is a simple filter connected 
across the output of the rectifier in parallel with the load. 
Figure 4-15. - Full-wave rectifier with a capacitor filter. 
                            
Page 5


                            
                                           DC POWER SUPPLY 
A power supply is a device that supplies electric power  to an electrical load. The term is most 
commonly applied to electric power converters that convert one form of electrical energy to another, 
though it may also refer to devices that convert another form of energy (mechanical, chemical, solar) to 
electrical energy. A regulated power supply is one that controls the output voltage or current to a 
specific value; the controlled value is held nearly constant despite variations in either load current or the 
voltage supplied by the power supply's energy source. 
A compound DC motor connects the armature and fields windings in a shunt and a series combination to 
give it characteristics of both a shunt and a series DC motor. This motor is used when both a high 
starting torque and good speed regulation is needed. The motor can be connected in two arrangements: 
cumulatively or differentially. An AC powered unregulated power supply usually uses a transformer to 
convert the voltage from the wall outlet (mains) to a different, nowadays usually lower, voltage. If it is 
used to produce DC, a rectifier is used to convert alternating voltage to a pulsating direct voltage, 
followed by a filter, comprising one or more capacitors, resistors, and sometimes inductors, to filter out 
(smooth) most of the pulsation. A small remaining unwanted alternating voltage component at mains or 
twice mains power frequency (depending upon whether half- or full-wave rectification is used)—
ripple—is unavoidably superimposed on the direct output voltage. 
                       BLOCK DIAGRAM OF POWER SUPPLY 
 
 
                                 
 
                            
                            
                                    TRANSFORMER 
A transformer is a static electrical device that transfers energy by inductive coupling between 
its winding circuits. A varying current in the primary winding creates a varying magnetic flux in 
the transformer's core and thus a varying magnetic flux through the secondary winding. This 
varying magnetic flux induces a varying electromotive force (EMF) or voltage in the secondary 
winding. 
Transformers range in size from thumbnail-sized used in microphones to units weighing 
hundreds of tons interconnecting the power grid. A wide range of transformer designs are used in 
electronic and electric power applications. Transformers are essential for the transmission, 
distribution, and utilization of electrical energy. 
 
                              CENTER T AP TRANSFORMER 
In electronics, a center tap is a contact made to a point halfway along a winding of a transformer or 
inductor, or along the element of a resistor or a potentiometer. Taps are sometimes used on inductors 
for the coupling of signals, and may not necessarily be at the half-way point, but rather, closer to one 
end. A common application of this is in the Hartley oscillator. Inductors with taps also permit the 
transformation of the amplitude of alternating current (AC) voltages for the purpose of power 
conversion, in which case, they are referred to as autotransformers, since there is only one winding. An 
example of an autotransformer is an automobile ignition coil. Potentiometer tapping provides one or 
more connections along the device's element, along with the usual connections at each of the two ends 
of the element, and the slider connection. Potentiometer taps allow for circuit functions that would 
otherwise not be available with the usual construction of just the two end connections and one slider 
connection. 
• In a rectifier, a center-tapped transformer and two diodes can form a full-wave rectifier that 
allows both half-cycles of the AC waveform to contribute to the direct current, making it 
smoother than a half-wave rectifier. This form of circuit saves on rectifier diodes compared to a 
diode bridge, but has poorer utilization of the transformer windings. Center-tapped two-diode 
rectifiers were a common feature of power supplies in vacuum tube equipment. Modern 
semiconductor diodes are low-cost and compact so usually a four-diode bridge is used (up to a 
few hundred watts total output) which produces the same quality of DC as the center-tapped 
configuration with a more compact and cheaper power transformer. Center-tapped 
configurations may still be used in high-current applications, such as large automotive battery 
chargers, where the extra transformer cost is offset by less costly rectifiers. Center-tapped 
transformers are also used for dual-voltage power supplies. When a center-tapped transformer 
is combined with a bridge (four diode) rectifier, it is possible to produce a positive and a 
negative voltage with respect to a ground at the tap. Dual voltage supplies are important for all 
sorts of electronics equipment.  
 
                            
                            
 
 
                  A full-wave rectifier using two diodes and a center tap transformer. 
                             
                                              RECTIFIER 
A rectifier is an electrical device that converts alternating current (AC), which periodically 
reverses direction, to direct current (DC), which flows in only one direction. The process is 
known as rectification. Physically, rectifiers take a number of forms, including vacuum tube 
diodes, mercury-arc valves, copper and selenium oxide rectifiers, solid-state diodes, silicon-
controlled rectifiers and other silicon-based semiconductor switches. Historically, even 
synchronous electromechanical switches and motors have been used. Early radio receivers, 
called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead 
sulfide) to serve as a point-contact rectifier or "crystal detector". 
Rectifiers have many uses, but are often found serving as components of DC power supplies and 
high-voltage direct current power transmission systems. Rectification may serve in roles other 
than to generate direct current for use as a source of power. As noted, detectors of radio signals 
serve as rectifiers. In gas heating systems flame rectification is used to detect presence of flame. 
The simple process of rectification produces a type of DC characterized by pulsating voltages 
and currents (although still unidirectional). Depending upon the type of end-use, this type of DC 
current may then be further modified into the type of relatively constant voltage DC 
characteristically produced by such sources as batteries and solar cells. 
Full-wave rectification 
A full-wave rectifier converts the whole of the input waveform to one of constant polarity 
(positive or negative) at its output. Full-wave rectification converts both polarities of the input 
waveform to pulsating DC (direct current), and yields a higher average output voltage. Two 
diodes and a center tapped transformer, or four diodes in a bridge configuration and any AC 
source (including a transformer without center tap), are needed.
[3]
 Single semiconductor diodes, 
double diodes with common cathode or common anode, and four-diode bridges, are 
manufactured as single components. 
Twice as many turns are required on the transformer For single-phase AC, if the transformer is 
center-tapped, then two diodes back-to-back (cathode-to-cathode or anode-to-anode, depending 
                            
                            
upon output polarity required) can form a secondary to obtain the same output voltage than for a 
bridge rectifier, but the power rating is unchanged. 
 
 
                  Full-wave rectifier using a center tap transformer and 2 diodes. 
                                                                       CAPACITOR 
Filter capacitors are capacitors used for filtering of undesirable frequencies. They are common 
in electrical and electronic equipment, and cover a number of applications, such as: 
• Glitch removal on Direct current (DC) power rails 
• Radio frequency interference (RFI) removal for signal or power lines entering or leaving 
equipment 
• Capacitors used after a voltage regulator to further smooth dc power supplies 
• Capacitors used in audio, intermediate frequency (IF) or radio frequency (RF) frequency 
filters (e.g. low pass, high pass, notch, etc.) 
• Arc suppression, such as across the contact breaker or 'points' in a spark-ignition engine 
Filter capacitors are not the same as reservoir capacitors, the tasks the two perform are different, 
albeit related 
 
The simple capacitor filter is the most basic type of power supply filter. The application of the simple 
capacitor filter is very limited. It is sometimes used on extremely high-voltage, low-current power 
supplies for cathode-ray and similar electron tubes, which require very little load current from the 
supply. The capacitor filter is also used where the power-supply ripple frequency is not critical; this 
frequency can be relatively high. The capacitor (C1) shown in figure 4-15 is a simple filter connected 
across the output of the rectifier in parallel with the load. 
Figure 4-15. - Full-wave rectifier with a capacitor filter. 
                            
                            
 
When this filter is used, the RC charge time of the filter capacitor (C1) must be short and the RC 
discharge time must be long to eliminate ripple action. In other words, the capacitor must charge up 
fast, preferably with no discharge at all. Better filtering also results when the input frequency is high; 
therefore, the full-wave rectifier output is easier to filter than that of the half-wave rectifier because of 
its higher frequency. 
For you to have a better understanding of the effect that filtering has on E
avg
, a comparison of a rectifier 
circuit with a filter and one without a filter is illustrated in views A and B of figure 4-16. The output 
waveforms in figure 4-16 represent the unfiltered and filtered outputs of the half-wave rectifier circuit. 
Current pulses flow through the load resistance (R
L
) each time a diode conducts. The dashed line 
indicates the average value of output voltage. For the half-wave rectifier, E
avg
 is less than half (or 
approximately 0.318) of the peak output voltage. This value is still much less than that of the applied 
voltage. With no capacitor connected across the output of the rectifier circuit, the waveform in view A 
has a large pulsating component (ripple) compared with the average or dc component. When a 
capacitor is connected across the output (view B), the average value of output voltage (E
avg
) is increased 
due to the filtering action of capacitor C1. 
Operation of the simple capacitor filter using a full-wave rectifier is basically the same as that discussed 
for the half-wave rectifier. Referring to figure 4-18, you should notice that because one of the diodes is 
always conducting on. either alternation, the filter capacitor charges and discharges during each half 
cycle. (Note that each diode conducts only for that portion of time when the peak secondary voltage is 
greater than the charge across the capacitor.) 
Figure 4-18. - Full-wave rectifier (with capacitor filter).