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|17.2||Electronic Components and Switching|
In this Physics tutorial, you will learn:
In the previous tutorial we explained how electronic devices receive, store, process and transmit the input information. In a certain sense, it was a software-related tutorial. Now, we will discuss about physical features of circuit components; this means this tutorial is more hardware-related.
We will briefly explain the operating principle of some basic components used in electronic circuits. We have dealt with some of these components in Electric Circuits, but some others are new, as they are specific only for electronic circuits.
An electronic component is a basic element that contributes for the development of an idea into a circuit for execution. Each component has a few basic properties and it behaves accordingly. The idea for all components comes from developers and is applied by electronic engineers, in order to use them for the construction of the intended circuit. The following list shows a few examples of electronic components that are used in different electronic circuits.
The following table includes a real image and the circuit symbol for each of these components.
The components in an electric circuit are classified into two categories: input and output components (devices). Some examples of input components include:
As for the output components, we can mention:
An input device or sensor activates one or more output devices. For example, a microphone may activate a loudspeaker; a pressure switch may activate a bulb and so on.
The input devices are also known as transducers, i.e. devices that convert electric signals into other forms or vice-versa.
Electronic devices such as diodes and transistors are made from special materials called semiconductors. A semiconductor is a solid substance that has a conductivity between that of an insulator and that of most metals, either due to the addition of an impurity or because of temperature effects. Silicon (Si) and Germanium (Ge) [mainly Silicon] are the two most notorious examples of semiconductors used in electronics. Devices made of semiconductors, are essential components of most electronic circuits.
The reason why semiconductors are so widely used in electronics is the ability of such materials to host "impurities", which are extra electrons or extra "holes" (absence of electrons) in the semiconductors - a factor which contributes in the increase in their conducting ability. In other words, the addition of impurities to a semiconductor material leads to cause variation in the conducting nature of the material.
There are two types of impurities added to a semiconductor: donors and acceptors. The fundamental factor of difference between donor and acceptor impurities is that a donor impurity donates charges to the semiconductor while an acceptor impurity accepts the charges from the semiconductor material. The process of adding impurities to a semiconductor to increase its electricity conducting ability is known as "doping".
Semiconductors come in two primary types: intrinsic and extrinsic. Intrinsic semiconductors are semiconductors that are pure, i.e. they don't contain any doping agents added, whereas the extrinsic semiconductors do.
The adding of the doping agents changes the electron and hole carrier concentrations of the semiconductor at thermal equilibrium, i.e. at the temperature in which two adjacent substances do not exchange heat energy. Basically, it allows us manipulate the semiconductor to lower its resistance.
Extrinsic semiconductors have very common uses; they are mainly used as components in electrical devices where they provide high electrical resistance. Extrinsic semiconductors on the other hand, are essentially divided into two types: P-Type and N-Type. The P-Type semiconductors carry a positive charge, while the N-type ones carry a negative charge. This extra charge depends on the hole concentration and the electron concentration. Thus, a P-type semiconductor has a larger hole concentration, which results in a net positive charge. Similarly, a N-type semiconductor has a larger electron concentration than hole concentration, which results in a net negative charge.
Said this, it is obvious that a besides the main element, a P-type semiconductor contains a number of extra particles that belong to an acceptor-type impurity while a N-type semiconductor contains some extra particles that belong to a donor-type impurity.
For example, when we add a Boron atom to Silicon, the Boron needs an extra electron to fill its outer layer because it is trivalent. It takes this extra electron from Silicon (or more exactly, a Silicon atom shares with the Boron atom an electron). In this way, a hole (lack of electron) is created in the material. This hole - which can shift throughout the material as this is a dynamic process - represents an extra positive charge created in the material, which acts as an electricity carrier, similarly to electrolyte, in which positive ions act as electricity carriers.
Likewise, when we add an Antimony (Sb) atom to Silicon, it brings an extra electron to the material (because it is pentavalent), which makes it negatively charged. This extra charge (electron) is a travelling charge, which displaces throughout the material.
The following table summarizes the main properties of semiconductors in regard to the extra charge they contain due to impurities.
Semiconductors are more suitable than conductors in electronic circuits, as they need less power and can operate at very low voltage. Prior to introducing semiconductors in technology, mathematical operation and computation electronics were carried out by using "vacuum tube technology". Since the invention of transistors in 1947, semiconductors have played an important role in the electronic mathematical processing and computation.
Some of the advantages of semiconductors in electric circuit when compared to conductors include:
The difference between semiconductors and conductors is made clearer in the table below:
In the following paragraphs we will explain, in greater detail, the structure and operating principle of some electronic components.
As explained earlier, diodes are circuit components that let the current flow in only one direction. Since most electronic devices operate in AC circuits, the current produced by an AC source changes direction 50 times in second (50 Hertz operating frequency) in most countries and 60 times in second (60 hertz operating frequency) in the rest of world. Therefore, a current rectifier is necessary in the circuit, which lets the current flow in one direction but prevent it from flowing in the opposite direction. This function is carried out by the diode.
In other words, a diode is an electronic device used to change the current from AC to DC. This process is known as rectification. A diode lets the current flow in the forwards direction but blocks the backward one. The following figure shows an AC circuit containing a diode and a resistor. To make the process of current rectification visible, an oscilloscope is connected to the terminals of the input AC source so that the output waveform is shown on the screen.
Diodes can be connected in DC circuits as well. Thus, when a diode is connected in the so-called forward-biased way, it offers a very low resistance to the current and as a result, the lamp glows bright. In this case, we can appoint a direction to the current flow, similarly to the current flow in DC circuits. On the other hand, when the same diode is connected in the reverse bias way, it offers a very high resistance to the current flow. Therefore, the lamp does not glow because the diode blocks the current. Look at the figure.
However, there is still an issue to overcome. If you look carefully the graph of output signal which represents the rectified current in the AC circuit, you can see that the current is not entirely rectified. Only in one of directions the current is rectified by the diode; in the other direction the current is still sinusoidal, as shown by the output signal. To fix this issue, we connect a capacitor across the output. The capacitor acts like a shock absorber in cars, i.e. it collects charge during the surges and releases it when the current from the rectifier falls. This makes the current flowing in the circuit be more uniform, very similar to the current produced by a DC source such as a battery. This is why diodes are so useful in electronic circuits; you can create a DC-like circuit supplied by a practically sustainable source of energy such as an AC power supply instead of using normal batteries which have a limited longevity.
A potential divider is an arrangement used to deliver only a portion of the input potential difference produced by the source. For example, if we want to deliver only half of the input potential difference, we connect two identical resistors in series and the terminals of the potential divider are connected across one of resistors, as shown in the figure below.
We can also connect a variable resistor (rheostat) instead of the lower resistor in the figure above in order to obtain output voltages varying between 0 V and ΔVbat / 2, as shown in the figure below.
The last circuit can be used as a volume control in a radio or as light intensity controller in a dimmer switch.
What is the output voltage across the terminals of the potential divider shown in the figure below?
This is a series circuit, so the current is the same everywhere. Since the total resistance of circuit is
the potential difference across the potential divider is one-third of the total potential difference produced by the battery as
Thus, after substitutions, we obtain
A reed switch is a type of switch controlled through a magnetic field. The contact close if a magnet is brought near the switch shown in the figure below.
When the magnet is moved near, the reeds magnetize and as a result, they attract each other. Various alarm system contain reed switches.
A reed switch becomes a reed relay when a coil is placed round it. The current in one circuit (through the coil) switches on another circuit (through the contacts).
A transistor is a semiconductor device with three connections, capable of amplification in addition to rectification. In other words, a transistor is an electronic component used in a circuit to control a large amount of current or voltage with a small amount of voltage or current. This means that it can be used to amplify or switch (rectify) electrical signals or power, allowing it to be used in a wide range of electronic devices.
The term "transistor" derives from "transfer resistor" because the current is transferred across a material that normally has high resistance such as a semiconductor. Normally, the construction of a transistor is based upon the sandwiching of one semiconductor between two other semiconductors.
There are essentially two basic types of point-contact transistors, the N-P-N transistor and the P-N-P transistor, where the N and N stand for negative and positive, respectively, as explained earlier in this tutorial. The only difference between the two is the arrangement of bias voltages. The figure below shows the image of a transistor and the circuit symbols of the two aforementioned types of transistors.
To explain how transistors work, let's consider a N-P-N type transistor. Each end of the transistor is a N-type semiconductor material and between them is a P-type semiconductor material. If you picture such a device plugged into a battery, you'll see how the transistor works:
By varying the potential in each region, then, you can drastically affect the rate of electrons flow across the transistor.
The invention of transistors was a huge step in the history of electronics. The old vacuum tubes cannot be compared to transistors regarding effectiveness, applicability, adaptability, etc. A single integrated circuit used in PCs, laptops, etc., contains millions of tiny transistors, which help produce a large number of voltage variations necessary for digital computing.
At last, there is a large number of transistors depending on their application field. However, we will not deal with them here, as this goes beyond the scope of this tutorial.
The traditional method of switching on a lamp consists on closing two contacts by pressing a switch. We have dealt with this method when discussing electric circuits in previous sections.
Another way of switching a small lamp or a LED is by using a transistor. In normal conditions, a transistor blocks the current. It acts like an open traditional switch. If a small voltage is applied across the two terminals 1 and 3 of the transistor, it starts conducting electricity and the lamp starts glowing. (The terminal 1 of collector is otherwise known as "collector", the terminal 2 as "base" and terminal 3 as "emitter". Therefore, sometimes you will see them labelled as C, B and E instead of 1, 2 and 3).
The conducting process discussed above can be achieved through a potential divider as explained earlier. Look at the figure below.
This is a kind of switch that is controlled through the light intensity. A light sensitive resistor (LDR) is connected in the section of circuit's potential divider. The resistance of this LDR decreases when light falls on it. When the circuit containing the LDR is placed into a dark environment, the lamp starts glowing because the resistance in the potential divider section is high and most of current flows through the rest of circuit where the lamp is connected. This is because the lamp turns on automatically at night.
Look at the figure below.
Now, let's consider a circuit containing a thermistor, which - as explained earlier - is a special type of resistor sensitive to temperature, i.e. a resistor whose resistance falls when the temperature rises. For example, a thermistor can be used to turn on a lamp when the temperature in the environment rises above a certain level. Again, we can use a potential divider to materialize this idea. Look at the figure below.
The principle of operation is the same as in the previous circuit containing the light sensitive resistor LDR; we have just replaced the LDR with a thermistor, which is affected by temperature, and not by the light intensity.
A thermistor is connected in series to a 2 kΩ resistor. The two resistors are connected to a 24 V battery. The lamp turns on when the current in the circuit is at least 10 mA (0.01 A). The initial resistance of thermistor at the given temperature is 1.2 kΩ and it decreases by 50 Ω for every 10 C raise in temperature.
To have a 1mA current flowing through the circuit (necessary to turn the lamp on), the equivalent resistance must be
The actual resistance however is
Therefore, the resistance in the circuit must decrease by
to make the lamp glow as the resistance is higher than needed.
Since the resistance of thermistor falls by 50 Ω for every degree of temperature raise, we obtain for the raise in temperature needed to turn on the lamp:
Thus, the temperature must increase by 160 C to make the lamp glow.
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