How a Voltage Rectifier Changes AC current to DC current
By Sean Leister
Voltage Rectifiers are an essential component in the conversion of
alternating current to direct current. The primary reason that this conversion is
necessary is due to the fact that the best method for transmitting electricity is with a
sine (or cosine) wave. As engineers, we want to maintain a high level of efficiency
during transmission so that we don’t waste resources, but we need a method of
conversion due to the fact that many electronics operate on, or are charged with
direct current.
Overview of Composition and Function:
Voltage rectifiers are circuits that convert alternating current to direct
current. These circuits can be constructed in numerous ways in order to satisfy
requirements that are often things such as keeping costs low or creating a smaller
margin for error. The three main types of rectifiers are the half-wave rectifier, the
full-wave rectifier, and the bridge rectifier. Whatever the case may be, the basic
principles are universal throughout all rectifiers. The rectifier will use a sequence of
circuit components that consists of a transformer, a capacitor, and a particular
orientation of diodes that is appropriate for the style of rectifier that is desired.
Transformer:
The transformer is the first component in the sequence of the circuit, but
should be one of the final things that the engineer designs. The transformer allows
the input voltage to be manipulated through a ratio of turns of copper or other
conductive material. This ratio of turns is called N, where N is the initial amount of
turns divided by the secondary amount of turns. The equation below illustrates this
more clearly. The variables n1 and n2 are the primary windings and the secondary
windings respectively, and V1 and V2 are the primary voltage and secondary voltage
respectively.
𝑛1
𝑉1
=
=𝑁
𝑛2
𝑉2
This shows that if we place more windings on the secondary coil than are placed on
the first coil, there is a reduction in voltage, and vice versa. The following schematic
illustrates what the transformer normally looks like in a schematic.
Rectifiers:
The rectifier is the most important piece in the transition of alternating
current to direct current. Their primary function is to filter out the unwanted
portion of the waveform. This allows only the positive portion of the waveform to
propagate through. To properly understand how the rectifier functions, we must
first cover how a diode works. The diode is a circuit component that ideally only
allows current to flow in one direction, and blocks current in the reverse direction.
One of the only problems with diodes is there is a required voltage drop associated
with turning them on. The voltage drop must occur to allow current to flow through
the diode, and circuits operating under small voltages can be affected substantially
as a result. A diode is depicted below as a schematic component.
Half-Wave Rectifier:
The half wave rectifier circuit is the most basic of the rectifier
circuits. Its overall function is to filter out any current that flows
against the diode within the circuit. This filtration process creates a
half sine wave when dealing with transmitted current in the United
States. The result of the filtration is pictured below, but is preceded by
the schematic of the circuit that produces it.
Full-Wave Rectifier:
The standard full wave rectifier is very similar in construction
to the half wave rectifier. The major differences are that the full wave
rectifier requires two diodes and two secondary transformers to
operate instead of just one. For this reason, this rectifier is sometimes
not chosen due to increased production costs associated with adding
these extra components. The advantage of the full wave rectifier lies
in the fact that it creates a positive waveform with twice the
frequency as the half wave rectifier. This increased frequency allows
for a more consistent final output voltage, therefore is often used in
more sensitive circuitry. A typical full wave rectifier is pictured below
along with its output waveform.
Bridge Rectifier:
A bridge rectifier is a full-wave rectifier variation that is often
used because it allows for the same frequency output as the standard
full wave rectifier, but it does not require another secondary
transformer. This reduces costs substantially in the overall
composition. The downside to this is the amount of diodes required to
construct the bridge rectifier. The four-diode construction creates an
issue because of the amount of voltage that it takes to turn on two
diodes at once. This voltage, which would be around 1.5-2V typically,
could be substantial in a circuit that is designed to operate under low
voltage. Shown below is the schematic of the bridge rectifier and its
output waveform.
Filter Capacitor:
The final major component in the construction of the rectifier circuit is the
filter capacitor. Capacitors are useful because voltage cannot change
instantaneously through this component. This nature of the component creates a
voltage that rises and falls slower as a result of the capacitor charging and
discharging. If the correct capacitor is picked the output voltage can be essentially
viewed as a dc current with a very small fluctuation (ripple voltage), and is now
capable of powering circuits that require this form of electricity to operate. The
output waveform of a half wave rectifier voltage through a filter capacitor is
pictured below.
(www.hobbyprojects.com)
This waveform is only the result of using a half wave rectifier. If we use a full wave
or bridge rectifier we obtain an even more consistent output, which is why these are
necessary for use in some more sensitive circuits.
Conclusion:
In conclusion the voltage rectifier takes an input voltage that is an entirely
Alternating Current produced voltage and transforms it into a voltage source that is
useful for applications in DC circuits. This occurs in three phases: The
transformation phase, the rectification phase and then the filtering or smoothing
phase. The transformation phase creates a more functional voltage for the
application by reducing or increasing the input voltage as needed. The rectification
phase then builds upon that by eliminating the unwanted, negative portion of the
waveform. The smoothing or filtering phase is the final phase in which the
characteristics of the capacitor component smooth out the positive waveforms and
make them so that they are useful in application in many electronics today.