A rectifier is an electrical device that converts an Alternating Current (AC) into a Direct Current (DC) by using one or more P-N junction diodes.
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Rectifier
In a large number of electronic circuits, we require DC voltage for operation. We can easily convert the AC voltage or AC current into DC voltage or DC current by using a device called P-N junction diode.
One of the most important applications of a P-N junction diode is the rectification of Alternating Current (AC) into Direct Current (DC). A P-N junction diode allows electric current in only forward bias condition and blocks electric current in reverse bias condition. In simple words, a diode allows electric current in one direction. This unique property of the diode allows it to acts like a rectifier.
Rectifier definition
A rectifier is an electrical device that converts an Alternating Current (AC) into a Direct Current (DC) by using one or more P-N junction diodes.
What is a rectifier?
When the voltage is applied to the P-N junction diode in such a way that the positive terminal of the battery is connected to the p-type semiconductor and the negative terminal of the battery is connected to the n-type semiconductor, the diode is said to be forward biased.
When
this forward bias voltage is applied to the P-N junction
diode, a large number of free
electrons (majority carriers) in the n-type
semiconductor experience a repulsive force from the
negative terminal of the battery similarly a large number of holes (majority
carriers) in the p-type
semiconductor experience a repulsive force from the
positive terminal of the battery.
As a result, the free electrons in the n-type semiconductor start moving from n-side to p-side similarly the holes in the p-type semiconductor start moving from p-side to n-side.
We know that electric current means the flow of charge carriers (free electrons and holes). Therefore, the flow of electrons from n-side to p-side and the flow of holes from p-side to n-side conduct electric current. The majority carriers produce the electric current in forward bias condition. So the electric current produced in forward bias condition is also known as majority current.
When the voltage is applied to the P-N junction diode in such a way that the positive terminal of the battery is connected to the n-type semiconductor and the negative terminal of the battery is connected to the p-type semiconductor, the diode is said to be reverse biased.
When this reverse bias voltage is applied to the P-N junction diode, a large number of free electrons (majority carriers) in the n-type semiconductor experience an attractive force from the positive terminal of the battery similarly a large number of holes (majority carriers) in the p-type semiconductor experience an attractive force from the negative terminal of the battery.
As a result, the free electrons (majority carriers) in the n-type semiconductor moves away from the P-N junction and attracted to the positive terminal of the battery similarly the holes (majority carriers) in the p-type semiconductor moves away from the P-N junction and attracted to the negative terminal of the battery.
Therefore, the electric current flow does not occur across the P-N junction. However, the minority carriers (free electrons) in the p-type semiconductor experience a repulsive force from the negative terminal of the battery similarly the minority carriers (holes) in the n-type semiconductor experience a repulsive force from the positive terminal of the battery.
As a result, the minority carriers free electrons in the p-type semiconductor and the minority carriers holes in the n-type semiconductor starts flowing across the junction. Thus, electric current is produced in the reverse bias diode due to the minority carriers. However, the electric current produced by the minority carriers is very small. So the minority carrier current in the reverse bias condition is neglected.
Thus, the P-N junction diode allows electric current in forward bias condition and blocks electric current in reverse bias condition. In simple words, a P-N junction diode allows electric current in only one direction. This unique property of the diode allows it to acts like a rectifier.
The
forward bias and reverse bias voltage applied to the diode is
nothing but a DC voltage. A DC voltage produces a current
which always flows in one direction (either forward direction
or backward direction).
But an AC voltage produces a current which always reverses its direction many times a second (forward to backward and backward to forward).
We have observed how a diode behaves when DC voltage (forward bias voltage and reverse bias voltage) is applied to it. Now let’s take look at a P-N junction diode when AC voltage is applied to it.
The
AC voltage or AC current is often represented by a sinusoidal
waveform whereas the DC current is represented by a straight
horizontal line.
In
the sinusoidal waveform, the upper half cycle represents the
positive half cycle and the lower half cycle represents the
negative half cycle.
The positive half cycle of the AC voltage is analogous to the forward bias DC voltage and the negative half cycle of the AC voltage is analogous to the reverse bias DC voltage.
The alternating current starts from zero and grows to peak forward current or peak positive current. The positive peak of the sinusoidal waveform represents the maximum or peak forward current. After reaching the peak forward current, it starts decreasing and reaches to zero.
After a short period, the alternating current starts increasing in the reverse or negative direction and grows to peak reverse current or peak negative current. The negative peak of the sinusoidal waveform represents the maximum or peak reverse current. After reaching the peak reverse current, it starts decreasing and reaches to zero. Likewise, the alternating current continuously changes its direction in a short period.
When
AC voltage or AC current is applied across the P-N junction
diode, during the positive half cycle the diode is forward
biased and allows electric current through it. However, when
the AC current reverses its direction to negative half cycle,
the diode is reverse biased and does not allow electric
current through it. In
simple words, during the positive half cycle, the diode allows
current and during the negative half cycle, the diode blocks
current. Thus, electric current flows through the diode only
during the positive half cycle of the AC current.
This current which flows across the diode is nothing but a DC current. Thus, the P-N junction diode acts like a rectifier by converting the AC current into DC current.
However,
the DC current produced by a basic rectifier (half wave
rectifier) is not a pure DC current. It is a pulsating DC
current.
The pulsating direct current is a type of DC current whose value changes over a short period.
The pulsating DC current starts from zero and grows to the maximum forward current (peak level), and decreases to zero. However, the pulsating DC current does not change its direction periodically like AC current.
The pulsating DC current always flows in one direction like the pure DC current. However, the value of pulsating DC current or pulsating DC voltage slightly changes over a given period. The electric current produced by batteries, power supplies, and solar panels is a pure DC current.
By using the combination of components such as capacitors, inductors, and resistors in the circuit, we can achieve the smoothening of pulsating DC to pure DC.
Types of rectifiers
The rectifiers are mainly classified into two types:
Half wave rectifier
Full wave rectifier
Half wave rectifier
As the name suggests, the half wave rectifier is a type of rectifier which converts half of the AC input signal (positive half cycle) into pulsating DC output signal and the remaining half signal (negative half cycle) is blocked or lost. In half wave rectifier circuit, we use only a single diode.
Full wave rectifier
The full wave rectifier is a type of rectifier which converts the full AC input signal (positive half cycle and negative half cycle) to pulsating DC output signal. Unlike the half wave rectifier, the input signal is not wasted in full wave rectifier. The efficiency of full wave rectifier is high as compared to the half wave rectifier.
Rectifier practical example
In our houses, almost all the electronic appliances use AC current. However, some electronic appliances such as laptops or notebook computers convert this AC current into DC current before they consume the power.
The
AC adapter of the laptop connected to the AC source converts
the high AC voltage or high AC current into low DC voltage or
low DC current. This low DC current is supplied to the laptop
battery and this is what we called laptop charging. However,
the laptop will not turn on unless you manually turned it on
by pressing the on button. When you press the laptop "power
on" button, the laptop battery starts supplying DC current.
We have forgotten an important step; how the AC adapters convert high AC voltage or high AC current into low DC voltage or low DC current.
The AC adapters consist of all the essential components needed for AC to DC conversion.
These components are a transformer, capacitor, and several diodes. Out of these components, the main key component is a diode which converts the alternating current into direct current.
The
transformer
in the AC adapter reduces the high AC voltage to a low AC
voltage.
The
rectifier (made up of diodes) converts this low AC voltage or
AC current into low DC voltage or DC current. However, the
converted current is not pure DC current. It is a pulsating DC
current.
The capacitor filters this pulsating DC current to pure DC current.