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The past few weeks have been refreshing for me. I have gone from how electrical signals in the brain help us to make connections between brain cells (a function that is fundamental to our ability to store and retrieve information) to electromagnetic waves, radio waves and microwaves. We have also discussed cellular communication about electromagnetic waves, the use of optical fibres to transmit data at high speeds. We moved on to vacuum tubes and transistors and how these little electronics devices played a crucial role in changing our world into a connected world.

From there, it seemed only natural to talk more about semiconductors. In my last post here, while discussing the workings of a solar cell, the concept of doping a semiconductor material came up again, and it started to feel as if I have been repeating myself. Well, science and technology are a lot like that. There are always recurring concepts, and to explain the foundation of each technology, it is incumbent that we visit the underlying idea once again.

While trying to explain how direct current (DC) obtained from the solar cells is converted to alternating current (AC), we ran across rectifiers and inverters. Incidentally, we cannot hope to cover both devices in just one post, so today, we shall spend some time on rectifiers, the materials they are made of, the underlying concepts, the applications and perhaps a little bit of math.

^{Wikipedia Creative Commons: Op Amp Precision Rectifier}

DC Power Supply

It is true that most appliances in our homes accept alternating current, but that is by design. Electronic devices end up using only direct current, but due to the low loss advantages that alternating current offers over direct current, transmission of electric power is done using AC. For the appliances in our homes to function, we must find a way to convert AC to DC. While the rectifier is responsible for rectification, the supply of DC to electronic devices in our homes go through stages. Below is a diagram that describes the stages involved in DC power supply:

The Rectifier

When one says The Rectifier, it sounds like a Hollywood action movie where one guy that has been wronged by the "System" takes it upon himself to clean stuff up and "Straighten-up" the kinks in the society. Well, that is the function of the rectifier: it takes AC voltage as an input and produces a DC as output. AC is a current that moves in two directions and is often seen as a sinusoidal wave. DC is a direct current which flows only in one direction.

A basic implementation is achieved using semiconductor diodes. Historically, before the discovery of the rectifying properties of semiconductor diodes, vacuum tube thermionic diodes and copper oxide- or selenium-based metal rectifier stacks were used. At some point, even synchronous machines were used for rectification. However, with the discovery of semiconductors, rectification is widely achieved using various types of diodes. However, it is worth mentioning that rectification is not performed using only diodes. There are other devices involved, and together, these make up a rectification circuit. We shall get back to how these devices work together to achieve rectification. First, let us take a closer look at diodes

Diodes

The periodic table shows the arrangement of elements. The elements are arranged according to their similarity in chemical properties. These chemical properties have a lot to do with how many electrons they have in their valence shell and this, in turn, determines how each atom binds with one another (valency) to form molecules. The atoms of some elements like copper or gold are closely bound in a crystalline structure, leaving some electrons free to roam in the crystal making such elements good electrical conductors. Other elements are not so good at conduction because the valency does not result to free electrons therefore little or no electricity would flow through them in the presence of an electric field: these are called electrical insulators.

In between electrical conductors and electrical insulators are semiconductors which can be manipulated or doped to improve their conductivity. The process of doping was discussed extensively here if you are interested in reading more. Moving on, the general idea is that there are two types of semiconductors based on the type of doping:

• n-type - which has excess electrons
• p-type - which has a shortage of electrons called holes which simply means that it has places where there should be electrons, but there are none.

Creation of a Junction

Unbiased Equilibrium

A diode is made by combining n-type silicon and a p-type silicon semiconductor.

Naturally, when the two types are combined, the excess electrons from the n-type scoot over to the p-type effectively resulting to a junction between the two parts where conventional silicon without roaming electrons and holes results and, as we mentioned before, this does not conduct electricity. Tragic. But what happens when we connect the two parts (n-type and p-type) to a source of DC voltage? Well, the correct answer has to be, "It depends."

Forward Bias

The result of applying a voltage to the terminals of the n-type and p-type silicon depends on how the terminals are connected. If we connect the negative terminal of a battery to the n-type and connect the positive terminal to the p-type, the diode is said to be Forward Biased.

The result of this connection is that positive and negative charges would flow to either side resulting first, in the shrinking of the depletion layer, then an electric current flows through the diode.

Reverse Bias

If, however, the terminals of the batteries are reversed, such that the positive terminal is now connected to the n-type silicon, the reverse of what happened in the forward bias would occur. The electrons and holes would move further away from the junction, thereby widening the depletion zone and no current would pass through the diode. This state of the diode is called Reverse Bias.

The ongoing describes how a diode works and why it allows an electric current will flow through it only in one direction and why it is suitable for rectification of AC voltage. Think of alternating current as a current that flows in one direction this moment just to reverse the next moment and flow in the other direction. This is why it is described as a sine wave. In the simplest terms, what happens during rectification is that the diode would allow the positive half cycle of the AC flow through it because the voltage would make the diode forward-biased at that point. When the voltage alternates and the negative half cycle comes along, the diode would be reverse-biased, and no current would flow. The output from the diode would be a pulsating DC which can be filtered to obtain a direct current.

Rectifier Circuit Analysis

In circuit representation, a diode is depicted by a shape made up of an equilateral triangle and a line that is perpendicular to the terminals of the diode. The perpendicular line is the n-type (cathode) whereas the triangle is the p-type (anode).

Types of Rectifiers.

There are two main types of rectifiers:
a. Half-wave rectifier and;
b. Full-wave rectifier: this is further divided into two -

• centre tap and,
• bridge type rectifier.

a. Half Wave Rectifier

In a half-wave rectifier, the positive half cycle of the waveform makes the diode forward biased and current is allowed to pass. The second half cycle makes the diode reverse biased and effectively creates an open circuit. Therefore, only the positive half cycle of the AC is seen in the output voltage; the output for the second half cycle is zero. The waveform for the rectified voltage is similar to the image shown below. As you may have suspected, especially given our initial claim that the output of the rectifier should be a pulsating DC, the half-wave rectifier leaves a lot to be desired especially for electronic application. The result of full wave rectifiers is more satisfying than that of half-wave. However, our discussion of them would be limited to the circuit and the waveform of the rectified voltage in line with the promise of doing as little math as possible.

^{Wikipedia Creative Commons: Waveform of a half wave rectified voltage. [ Please click on the image to see circuit analysis and worked out details of how the rectifier works]}

b. Full Wave Rectifier

As mentioned, there are two types of full wave rectifier. We shall use the bridge type rectifier to examine the circuitry and output waveform of a full wave rectifier. But why bother with the full wave when we have half wave? Well, half wave rectifiers presented us with some problems:

• We got less than 50% output from the half wave rectifier because V_o = V_i = V_msinwt for the positive half cycle and V_o = 0 for the negative half cycle
• Ripple effect which is the fluctuations in the voltage which we intended to remove, was
  % in half wave rectifiers. Therefore we did not achieve our aim.
• A third problem was low level of efficiency

Full wave rectifiers would enable us obtain 100% voltage at the output such that in an idea situation, V_i = V_o

^{Wikimedia Creative Commons: circuitry of bridge full wave rectifier and output waveform}

It is this output that is fed to a filter to remove the pulsations and output a DC.

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I'd like to thank you for taking the time to read to the end.

References

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1. Electronics Tutorials | Diodes
2. Periodic Table
3. Wikipedia | Sine Waves
4. Visionics | Have Wave Rectifiers
5. Explain that Stuff | Diodes
6. Wikipedia | Rectifier

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