RECTIFIER CIRCUITS
INTRODUCTION
Direct current is the flow of electrons in one direction. Alternating current is the flow of electrons first
in one direction and then in the opposite direction.
Some electrical machines require DC for proper operation. Some require AC. Some will operate on
either AC or DC. However, there are still others that require both AC and DC for proper operation.
It would seem at first that this would cause much confusion to electricians, but it does not. Particularly
since alternating current has the advantage of being converted with relative ease to direct current.
An electrical device used for converting alternating current to direct current is called a rectifier. The
most common type rectifiers are the mechanical rectifier (commutator), the electron diode tube, and the
chemical rectifier (diode).
The commutator was covered in a previous lesson. The vacuum tube type of rectifier is more common
to electronics than to electrical power equipment. The chemical type of rectifier is used extensively of motors
and generators and is discussed here. After completing this lesson you will be able to:
1.
2.
3.
4.
5.
State the definition of a rectifier.
Identify rectifiers and their electrical symbols.
Test diodes and transformers for serviceability.
Construct and analyze, half-wave, full-wave and bridge rectifier circuits.
All objectives will be completed IAW Student Workbook II and all applicable references.
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1.
RECTIFIERS
A rectifier (sometimes called a diode) is a device that changes alternating current to pulsating direct
current. It operates much as a check valve in a water circuit. The check valve will allow water to flow in
one direction in a water circuit, but will prevent it from flowing in the opposite direction (A of Figure 1).
The rectifier will allow electrons to flow in one direction in an electric circuit, but will prevent them from
flowing in the opposite direction (B of Figure 1).
Check
Valve
PUMP
(A)
Electrodes can flow
RECTIFIER
BATTERY
FIGURE 1 – COMPARISON OF CHECK VALVE & RECTIFIER
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a.
Semiconductors
The electron theory states that all matter is composed of atoms. The atoms are composed of
subatomic particles called electrons (-), protons (+) and neutrons (). Previously we classified
matter into two categories, conductors and non-conductors. Now let us add a third category –
semiconductors. A semiconductor has the property of both conductors and insulators and is the
basis of chemical rectifiers. Two common types of semiconductor material used in the
construction of chemical rectifiers are geranium and selenium.
BASE PLATE
Effective Rectifying
Area
Selenium
Insulating washer
Barrier
Layer
Front
Electrode
FIGURE 2 – CONSTRUCTION OF A SELENIUM RECTIFIER
b.
Selenium Rectifier
The selenium rectifier has attained wide spread use on electrical generator power equipment and
will be discussed here. In the construction of the selenium rectifier, a layer of selenium is
deposited on a thin aluminum or steel base plate. The entire surface of the plate is covered with
selenium except a small area at the center and a narrow edge around the rim of the disc. These
points later are covered with insulating compound. After the selenium is deposited, a conducting
alloy of low melting point is sprayed over the surface and becomes the front electrode (Figure 2).
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(1) Voltage Limitation
Since the deposited films on the individual metal plates is microscopic in thickness, the
voltage that each single plate can handle without rupture is relatively low, being
approximately 25 volts peak. To overcome this voltage limitation per plate, when higher
voltages are to be rectified, several plates are connected in series. The total number of plates
in series determines the peak voltage that the complete rectifier assembly can sustain.
FIGURE 3 – SELENIUM RECTIFIER COMBINATIONS
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(2) Current Capacity
The current carrying capability of each individual plate is determined by its size. To rectify
large amounts of alternating current the plates are made larger or more than one plate is
connected in parallel. The plates of a selenium rectifier can be assembled in series-parallel
combinations to meet a great variety of voltage and current requirements (Figure 3).
c.
Schematic Symbol
The schematic symbol for a rectifier is shown in Figure 4. The direction that electrons can flow
through the rectifier is against the arrowhead of the symbol.
DIRECTION OF GREATEST CONDUCTIVITY
FIGURE 4 – SCHEMATIC SYMBOL, RECTIFIER
d.
Reverse Bias
How does a rectifier stop current flow in a circuit? It does this by offering a very high resistance
to the flow of electrons (A of Figure 5). If a rectifier is placed into a circuit in such a manner as to
stop current, it is said to be reverse biased (B of Figure 5).
e.
Forward Bias
When a rectifier is placed in a circuit with forward bias, it offers very little resistance to the flow
of electrons (Figure 6).
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(A)
(B)
(C)
(D)
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FIGURE 6 – FORWARD BIAS
2.
RECTIFIER CIRCUITS
Now that we understand the “check valve” operation of a rectifier, let us examine how it can be used to
rectify alternating current into a pulsating direct current.
a.
Half-Wave Rectification
The simplest rectifier circuit is called a half-wave rectifier circuit. The circuit consists of a load (R),
one rectifier (S), and usually a transformer as shown in Figure 7.
FIGURE 7 – HALF-WAVE RECTIFIER CIRCUIT
(1) Forward Bias
As the alternating current changes direction, the voltage polarity of the transformer terminals also
changes. When the top transformer terminal (A) is positive and the bottom terminal (B) negative,
as shown in Figure 7, current will flow in the secondary circuit. The rectifier (S) offers very little
resistance to the current flow. This is referred to as forward bias.
(2) Reverse Bias
When the top transformer terminal (A) is negative and the bottom terminal (B) is a positive, no
current will flow in the bottom circuit. The rectifier (S) offers maximum opposition to current
flow. This is referred to as reverse bias.
(3) Pulsating Direct Current
It must be stressed that a rectifier changes alternating current to a pulsating direct current. The
output is not the smooth DC that a battery provides. As indicated in Figure 7, voltage is applied to
the load only with half of each cycle of the input voltage, so the circuit is called a half-wave
rectifier circuit.
(4) Advantages and Disadvantages
The advantages of using a half-wave rectifier circuit are that it requires a minimum number of
components and, therefore is simple and inexpensive. Its disadvantages are that it is difficult to
filter and its ratio of power output to power input if small. In order to get smooth DC, the
pulsating DC must be filtered, a process that will be discussed in subsequent paragraphs.
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b.
Full-Wave Rectification
A more complicated common type of rectifier circuit is the full-wave rectifier circuit. As shown in
Figure 8, it consists of a center-tapped transformer, two rectifiers (S1 and S2) and a load (R).
FIGURE 8 – FULL-WAVE RECTIFIER CIRCUIT
(1) First half cycle
During the first half cycle, the top terminal (A) of the transformer is positive as compared to the
center terminal. The top rectifier (S1) is forward biased and current flows through the load (R) in
the direction shown. At the same instant of time, the center terminal is positive compared to the
bottom terminal (B). The bottom rectifier (S2) is reverse biased so no current flows in the bottom
part of the circuit.
(2) Second half cycle
During the second half cycle, the polarities on the transformer are reversed. The bottom terminal
(B) of the transformer is positive compared to the center terminal. The bottom rectifier (S2) is
forward biased and current flows through the load (R) in the direction shown. At the same instant
of time, the center terminal is positive as compared to the top terminal (A). The top rectifier (S1)
is reversed biased so no current flows in the top part of the circuit.
(3) Pulsating Direct Current
The direct current pulsates across the load twice as often as in the half-wave rectifier circuit.
There is current flow during both half cycles so there is full-wave rectification. Because the
current passes through the load in the same direction during both half cycles, it is direct current.
(4) Advantages and Disadvantages
The advantages of the full-wave rectifier circuit are that it produces power during the full AC
cycle and is therefore twice as efficient as a half-wave rectifier circuit. It is also easier to filter the
output of a full-wave rectifier circuit into smooth DC than the output of a half-wave rectifier
circuit. The disadvantages of this type of full-wave rectifier circuit are that it requires a
transformer with a connection in the center of the winding, and an additional rectifier.
c.
Bridge Rectifier Circuit
The last common rectifier circuit discussed here is the bridge rectifier circuit. A transformer is not
always required for the operation of this full-wave circuit, however, one is included in Figure 9 so that
the bridge rectifier circuit can be easily compared with the half-wave rectifier circuit shown in Figure 7
and the full-wave circuit shown in Figure 8.
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(1) Operation
The bridge rectifier circuit operates as follows:
(a) First half cycle
When the top of the AC input is positive as compared to the lower terminal, rectifiers S1 and
S3 are forward biased. Current flows through these rectifiers and the load. At the same
instant of time, rectifiers S2 and S4 are reversed biased so that no current flows through them.
(b) Second half cycle
When the bottom terminal of the AC input is positive as compared to the top terminal,
rectifiers S2 and S4 are forward biased. Current flows through these rectifiers and the load.
At the same instant of time, rectifiers S1 and S3 are reversed biased so that no current flows
through them.
FIGURE 9 – BRIDGE RECTIFIER CIRCUIT
(1) Pulsating Direct Current
Note that current passes through the load in the same direction during both half cycles and is,
therefore, direct current. The DC output of the bridge rectifier circuit is the same as the full-wave
rectifier circuit using a center-tap transformer.
(2) Advantages and Disadvantages
The advantage of the bridge rectifier circuit is that it is not necessary to use a transformer if the
voltage input is of the desired value and it is highly efficient. A disadvantage is that it requires the
use of four rectifiers instead of one or two as required by the other types of rectifier circuits.
1.
FILTER CIRCUITS
An examination of the various rectified waveforms reveals a fluctuating or pulsating current unlike the
smooth even direct current required for most circuits. The pulsating waveforms for both half-wave and
full-wave rectification are shown in B and C of Figure 10. In contrast with this violently fluctuating current
is the smooth, even output wave-form of a battery as shown in A of Figure 10. The output from a rectifier
must be modified to approach, as nearly as possible, the steady state of the battery voltage. To accomplish
this, the fluctuations in the rectified pulses must be smoothed out by a process called filtering.
a.
Basic Parts of Filter Circuits
6-9
Filtering is accomplished by means of capacitors, inductors, and resistors. The inductors commonly
are referred to as filter chokes, chokes, or choke coils. These components can be connected in many
different circuit arrangements, three of which are shown in Figure 11. Modifications of these three
basic circuit arrangements consist mainly of additional capacitors, choke coils, or resistors.
FIGURE 10 - WAVEFORMS
(1) Capacitance Filter
The ability of a capacitor to function as a filter element depends on its ability to hold a charge.
Another important factor is that a capacitor will not pass direct current.
(2) Inductance Filter
The ability of a choke coil to function as a filter element depends on its basic property, inductance.
This can be defined as the property of a coil that tends to oppose a change in the value of a current
flowing through it. In other words, inductance tends to hold back the current when it rises in value
and to keep it flowing when it decreases.
(3) Resistance Filter
Sometimes resistors are substituted for inductors in filter circuits. Although the resistor affords
some filtering action, it is not as effective as a choke coil.
6-10
FIGURE 11 – FILTER CIRCUITS
a.
Circuit Operation
Assume that the output of a half-wave rectifier (B of Figure 10) is applied to the filter circuit shown in
A of Figure 11, and a load is placed across the output.
(1) Capacitor (C1) action
When the first half-wave pulse appears across capacitor C1, current flows through the circuit and
the capacitor charges to the peak value of the pulse. During the half-cycle when the rectifier is not
conducting C1 discharges into the line through choke L. On the next alternation when current
flows through the circuit, C1 again charges to peak value. When the current flow is cut off on the
next half-cycle, C1 returns some of its charge to the line. Although no current is delivered from
the rectifier to the filter circuit on alternate half-cycles, a current caused by the charging and
discharging of capacitor C1 flows through the circuit during these intervals. In this manner, the
gaps in the wave-form in B of Figure 10 are partly filled in and more uniform current flows in the
output circuit.
(2) Inductor (L) action
Although the output current has been made more uniform by the action of C1, it is still far from
the smooth DC required, and L and C2 provide additional filtering action. An inductor functions
to retard or restrict any change in the current flowing through it. Consequently when the rectified
current pulse is applied to choke L, it immediately tends to retard or limit the current through it.
The effect is to choke back or flatten the peaks of the rectified pulse, which gives it the name
choke coil. The action of C1 filling in the gaps and the action of choke L restricting the peaks of
the rectified half-wave pulse result in a fairly steady flow of current through the output circuit.
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(3) Capacitor (C2) action
Because of the charging and discharging action of C1 and the retarding action of L, the current
applied to C2 is flowing steadily; however it still contains pulsations that are too high in amplitude
for most applications. By charging and discharging on these pulsations in exactly the same
manner as C1, capacitor C2 further filters the output. By connecting additional capacitors and
chokes in the basic circuit, further filtering action is obtained, resulting in the pure DC required in
the output of the power supply.
(4) Resistor (R) action
Resistor R in B of Figure 11 is substituted for the choke coil in cases where restrictions on cost or
space make use of an inductor inadvisable or where less filtering can be tolerated. Although a
certain amount of filtering is obtained with resistor R at low power line frequencies, the efficiency
of a resistor as a filtering device is much less than that of an inductor.
(5) Complete Rectifier and Filter Circuit
To illustrate how alternating current can be rectified and filtered into direct current, let us analyze
the circuit shown in A of Figure 12. The circuit consists of an AC source, a transformer, a bridge
rectifier, a filter circuit, and a load. The output voltage of the AC source (E1) is a sine wave (B of
Figure 12). This AC voltage may be stepped up or down by a transformer and is reversed in
polarity (E2). The voltage is then rectified to a full-wave pulsating DC voltage (E3) by the bridge
rectifier. The pulsating DC voltage is filled in some by the first capacitor but is still a pulsating
voltage (E4). The choke coil then rounds off the peaks of the pulsating voltage (E5). Last, the
second capacitor fills in most of the remaining ripple of the DC voltage (E6). The final output
voltage applied to the load, though not a pure DC, is smooth enough for most practical purposes.
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6-13
FIGURE 12 – RECTIFIER AND FILTER CIRCUIT
4.
The silicon-controlled rectifier, usually referred to as a SCR, is one of the families of semiconductors that
include transistors and diodes. A drawing of a SCR and its schematic representation is shown in the figure.
Not all SCR’s use the casing shown, but this is typical of most of the high-power units.
Although it is not the same as either a diode or a transistor, the SCR combines features of both. Circuits
using transistors of rectifier diodes may be greatly improved in some instances through the use of SCR’s.
The basic purpose of the SCR is to function as a switch that can turn on or off small or large amounts of
power. It performs this function with no moving parts that wear out and no points that require replacing.
There can be a tremendous power gain in the SCR, in some units a very small triggering current is able to
switch several hundred amperes without exceeding its rated abilities. The SCR can often replace much
slower and large mechanical switches.
The SCR is an extremely fast switch. It is difficult to cycle mechanical switch several hundred times a
minute; yet, some SCR’s can be switched 25,000 times a second. It takes just microseconds (millionths of
a second) to turn on or off these units. Varying the time that a switch is on as compared to the time that it
is off regulates the amount of power flowing through the switch. Since most devices can operate on pules
of power (alternating current is a special form of alternating positive and negative pulses), the SCR can be
used readily in control application. Motor-speed controllers, inverters, remote-switching units, controlled
rectifiers, circuit overload protectors, latching relays, and computer logic circuits all the SCR. A small
current from gate to cathode can fire or trigger the SCR, changing it from practically an open circuit to a
short circuit. The only way to change it back again (to commutate it) is to reduce the load current to value
less than the minimum forward-bias current has completely built up to point sufficient to sustain
conduction. After conduction from cathode to anode begins, removing the current has no effect.
6-14
STUDY QUESTIONS
1.
Electrons flow first in one direction and then in the opposite direction are called ____________________
current.
2.
Electron flow in one direction at a varying value is called _______________ ________________ current.
3.
An electrical device that changes alternating current into pulsating DC is called a ____________________.
4.
The schematic symbol for a rectifier is ____________________.
5.
One rectifier is used in a ____________________ rectifier circuit.
6.
The output waveform of a half-wave rectifier circuit is ____________________.
7.
Two rectifiers and a center-tapped transformer are used in a ____________________ rectifier circuit.
8.
Four rectifiers are required for a ____________________ rectifier circuit.
9.
The output waveform of a full-wave or bridge rectifier circuit is ____________________.
10. Filter circuits components are ____________________, ____________________, and
____________________.
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STUDENT NOTES
6-16
PRACTICAL EXERCISE 1
STUDENT PRACTICAL EXERCISE
LESSON: RECTIFIER CIRCUITS
STUDENT REQUIREMENTS:
1.
Position multimeter selector switch on ohm (x100).
2.
Check all rectifiers on the board. Report any defective rectifier to the instructor.
3.
Position multimeter selector switch on DC volts.
4.
Using power transformer, one rectifier, resistor, and snap leads construct a Half-Wave Rectifier Circuit as
shown on page 6-7.
5.
Connect primary of transformer to suitable AC power source for operation.
6.
Apply AC voltage, measure and record pulsating DC voltage across the resistor. Turn power off, and
dismantle circuit.
7.
Using power transformer with center-tap, two rectifiers, resistor, and snap leads construct a Full-Wave
Rectifier Circuit as shown on page 6-8.
8.
Apply AC voltage, measure and record pulsating DC voltage across the resistor. Turn power off, and
dismantle circuit.
9.
Using power transformer, four rectifiers, resistor, and snap leads construct a Full-Wave Rectifier (bridge)
Circuit as shown on page 6-10.
10. Apply AC voltage, measure and record pulsating DC voltage across the resistor. Turn power off, remove
the plug from the socket, and dismantle the circuit.
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STUDENT WORKSHEET
LESSON: RECTIFIER CIRCUITS
6-18