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* Units and basic definitions

* The rheostat

* Semiconductor

* Semiconductor diode

* Conduction electrons and holes

* Dopar

* The LED (light emiter diode)

* The Zener diode

* The transistor

* Phototransistor

* The bipolar transistor

Units and basic definitions:

Ampere (Amp) (A):

Unit of measurement of electric current, is the amount of charge flowing through a conductor per unit time

I = Q / t

1 A = 1 coulomb / second

1 A = 1000 mA (milliamp)

Coulomb (coulomb):

Measuring unit of electric charge.

1Coulomb = 6.28×1018 electrons

Watts (Watt):

Power unit.


Rate at which energy is supplied or consumed.

Power = Energy / Time

Parallel circuit:

Circuit that has more than one path for the current, where elements share terminals.

Series circuit:

Circuit with only one path for the current, where the elements are one after the other.


All-purpose tool, also called Tester, VOM, DMM, etc., Used for voltage measurements (voltage), current (cc.), alternating current, and sometimes resistance: diodes, transistors, capacitors, etc..

Ohm (Ohm):

Unit electrical resistance measurement, represented by the Greek letter (W, omega).

Siemens (Mho):

UoM conductance (G)

Conductance (G):

G = 1 / R = 1 / Resistance. Is the inverse of resistance. An element (resistor) with high resistance have low conductance, a resistor with low resistance have high conductance

Volt (Volt):

Measuring unit of electric potential difference or voltage, commonly called voltage.

Alternating Current (AC):

Electricity that changes periodically its amplitude over time.

Direct Current (DC):

Is the current flowing in one direction. Batteries, solar cells, etc.. produce DC current. This type of power does not change its magnitude or its direction in time.


Number of complete cycles of a wave in a unit of time

1 Hertz = 1 cycle / sec


The variable resistor (potentiometer, the resistor)

Variable resistors are divided into two categories:

The potentiometers and rheostats are differences between them, among other things, how they connect. In the case of potentiometers, these are connected in parallel and the circuit behaves as a voltage divider. See Fig.

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The rheostat

In the case of this resistor is connected in series with the circuit and must be careful that its value (in ohms) and can withstand the power (in Watts (W)) is adequate to carry the current (I in amp (ampere) than by going to run through it

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The resistors can also be divided taking into account other characteristics:

* If they are wound.

* If they are wound.

* Of weak dissipation.

* Strong dissipation.

* Precision


solid or liquid material able to conduct electricity better than an insulator, but worse than a metal. The electrical conductivity, which is the ability to conduct an electric current when applying a potential difference is one of the most important physical properties. Certain metals, such as copper, silver and aluminum are excellent conductors. On the other hand, certain insulation as diamond or glass are very poor conductors. At very low temperatures, pure semiconductors behave as insulators. Subjected to high temperatures, mixed with impurities or in the presence of light, the conductivity of semiconductors can increase dramatically and reach reach levels close to those of metals. The properties of semiconductors are studied in solid state physics.


A pn junction (also called diode) allows current flow in one direction only. The electrons in n-type material may flow to the left, through the p-type material, but the absence of an excess of electrons in the p-type material will prevent any flow of electrons to the right. Note that it is defined that the current flows in a direction opposite to the flow of electrons.

Conduction electrons and holes

Among the common semiconductor elements are chemical compounds, such as silicon, germanium, selenium, gallium arsenide, zinc selenide and lead telluride. The increase in conductivity caused by temperature changes, light impurities or due to the increased number of electrons conductors that carry electrical current. In a characteristic or pure semiconductor such as silicon, the valence electrons (or outer electrons) of an atom are paired and are shared by other atoms to form a covalent bond that holds the crystal together. These valence electrons are not free to carry electrical current. To produce conduction electrons, using light or temperature, which excites the valence electrons and causes release of the links, so that current can pass. The deficiencies or gaps that are contribute to the flow of electricity (it is said that these holes carry positive charge). This is the physical source of the increase of electrical conductivity of semiconductors caused by the temperature.


Another method for transporting electrons for electricity is to add impurities to the semiconductor or doparlo. The difference in the number of valence electrons between the dopant material (whether accepted as if giving electrons) and the receiving material causes it to grow the number of negative conduction electrons (n-type) or positive (p-type). This concept is illustrated in the diagram below, which represents a doped silicon crystal. Each silicon atom has four valence electrons (represented by dots). It takes two to form the covalent bond. In n-type silicon, an atom such as phosphorus (P), with five valence electrons, replaces the silicon and provides additional electrons. In the p type silicon, atoms of three valence electrons as aluminum (Al) lead to a deficiency of electrons or holes which behave as positive electrons. The electrons or holes can conduct electricity.

When certain layers of p-type and n-type semiconductors are adjacent, form a semiconductor diode, and the contact region is called a pn junction. A diode is a two-terminal device that has a high resistance to electric current flow in one direction and a low resistance on the other. The conductivity properties of the pn junction depending on the direction of voltage, which can in turn be used to control the electrical nature of the device. Some series of these bonds are used to make transistors and other semiconductor devices such as solar cells, lasers and pn junction rectifiers.

Semiconductor devices have many applications in electrical engineering. Recent engineering advances have been small semiconductor chips containing hundreds of thousands of transistors. These chips have made possible an enormous degree of miniaturization in electronic devices. The efficient application of such chips is the manufacture of semiconductor circuits complementary metal-oxide or CMOS, which are formed by pairs of p and n channel transistors controlled by a single circuit. Furthermore, devices are being manufactured using extremely small molecular beam epitaxial technique.

The LED (Light Emiter Diode)

(Emiter Diode Light – Light Emitting Diode)

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LED symbol

The LED is a special type of diode, which serves as a common diode, but which when electrical current passes through it emits light.

There are multi-colored LEDs and these depend on the material with which they were constructed. There are red, green, yellow, amber, infrared.

Must be chosen well the current through the LED to obtain good brightness. The LED has an operating voltage ranging from 1.5 V to 2.2 volts. about and the range of currents that must flow through the range from 10 mA to 20 mA LEDs in red and between 20 mA and 40 mA for other LEDs. It has huge advantages over common indicator lamps, such as its low power consumption, and near-zero maintenance with an average lifespan of 100,000 hours.

Applications have the LED

It is widely used in visual applications, as indicators of operating certain specific situation.


Counters are used to display

To indicate the polarity of a power source of direct current.

To indicate the activity of a source of AC power.

The Zener diode

It is a special type of diode operation unlike common diodes, and the diode rectifier (where exploit its characteristics and polarization reverse bias), the Zener diode is always used in reverse bias, where the desired current circular against the arrow representing the same diode.

Here we analyze the Zener diode, but not as an ideal, if not as a real and must take into account when it is biased in reverse if there is a current flowing in the opposite direction to the arrow on the diode but of little value.

Zener diode symbol

(A – K anode – cathode)

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Analyzing the zener diode curve, we see that in the place mark as operating region, the current (Ir, in the lower vertical line) can vary within a wide margin, but the voltage (Vz) is not changed. Remains approximately at 5.6 V. (For a 5.6 V zener diode)

Zener diode applications

The main application that gives the Zener diode is the regulator.

What does a Zener regulator

A perfect zener regulator maintains a predetermined fixed voltage at its output, regardless of whether the voltage varies in power and regardless as to vary the load to be fed with this controller.

Note: In the ideal voltage sources (some used, among other things the zener diode), the output voltage does not vary as the load varies. But sources are not ideal and it is normal that the output voltage decreases as the load is increased, or as demand load current increases.

To know if a voltage source is of good quality using the following formula:

Percentage of control = V (no load) – V (full load) / V (full load) * 100%

A smaller percentage of control value, better quality video.


A semiconductor device which can control and regulate a large flow through a very small signal. There are a variety of transistors. In principle, the pole will be explained. The symbols corresponding to this type of transistor are:

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We will see later as a circuit with an NPN transistor can be adapted to PNP. The name refers to this construction as semiconductor.


When the switch SW1 is open no current flows through the transistor base so the lamp will not light, because, all the tension is between collector and emitter. (Figure 1).

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When SW1 is closed, a current will flow through the very small base. So lower your resistance transistor collector-emitter so spend a great intensity, causing it to turn on the lamp. (Figure 2).

Overall: IE> IC> IB, IE = IB + IC, VCE = VCB + VBE


A correct polarization allows the operation of this component. Not the same polarizing NPN PNP transistor.

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Generally we can say that the union base – emitter is forward biased and the base – collector inversely.


COURT. – No current flows through the base, so that the collector and emitter current is also zero. The collector-emitter voltage is the battery. The transistor collector-emitter behaves like an open switch.

IB = IC = IE = 0, VCE = Vbat

SATURATION. – When a current flows Base, is an increase of the collector current considerable. In this case the collector-emitter transistor behaves as a closed switch. Thus, one can say that the battery voltage is in the load connected to the collector.

– IB – IC; Vbat = RC X IC.

ACTIVE. – It acts as an amplifier. You can pass more or less current.

When working in the cutting zone and saturation is said to work on switching. In short, like a switch.

The current gain also is an important parameter as it relates transistors suffering variation collector current to a change of the base current. Manufacturers often specify in their data sheets, also appears under the name hFE. It is expressed as follows:

= IC / IB


The encapsulated transistors depend on the function being performed and the power dissipated, and we find that small signal transistors have a plastic case, usually the smaller (TO-18, TO-39, TO-92 , TO-226 …), the medium power are taller and have on the back of a metal sheet that serves to evacuate the heat dissipated by suitably cooled radiator (TO-220, TO-218, TO-247. ..), the large power are those that have a larger dimension being entirely metallic encapsulation. This favors, largely heat dissipation therethrough and a radiator (TO-3, TO-66, TO-123, TO-213 …).


is essentially the same as a normal transistor, only can work in 2 different ways:

As a normal transistor base current (IB) (common mode)

* As phototransistor, when the light incident on this element acts as the base current. (IP) (lighting mode).

* Note: SS is the current gain of the phototransistor.

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You can use both simultaneously, although the phototransistor is used primarily with the leg of the base unconnected. (IB = 0)

If it is desired to increase the sensitivity of the transistor, because of low illumination, it can increase the base current (IB), using external polarization

The equivalent circuit of a phototransistor, is a common transistor with a photodiode connected between the base and collector, with the cathode of the diode connected to the collector of transistor and the anode to the base.

The phototransistor is widely used for detection applications where lighting is very important. As the photodiode, has a very short response time, only your delivery of electrical current is much higher.

In the chart below you can see the equivalent circuit of a phototransistor. It is observed that consists of a photodiode and a transistor. The current delivered by the photodiode (flowing towards the base of the transistor) is amplified ss times, and is the current that can deliver the phototransistor.

The bipolar transistor

There are two types of bipolar transistors:

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The bipolar transistor is the most common of the transistors, diodes and as may be germanium or silicon.

There are two types transistors, the NPN and PNP and the direction of current flow in each case indicated by the arrow is seen in the graph of each type of transistor.

The transistor is a 3-pin device with the following names: base (B), collector (C) and emitter (E), always coinciding, the issuer, with the pin that has the arrow in the graph transistor.

The transistor is a current amplifier, this means that if we introduce a number of current through one of its legs (base), delivered by another (transmitter), an amount greater than this, a factor called amplification. This factor is called b (beta) and is data specific to each transistor.


* Ic (current through the collector pin) is equal to b (amplification factor) by Ib (current flowing through the pin base).

* Ic = * Ib

* Ie (current through pin emitter) is the same value Ic, except that in one case the current enters the transistor and in the other case the exit, or vice versa.

According to the above formula do not depend on the current which feeds the circuit voltage (Vcc), but actually if it does and the current Ib changes slightly when changing Vcc. See figure.

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In the second graph the base currents (Ib) are examples to understand that most current curve is higher

Transistor operating regions

Cutting Region: A transistor is off when:

collector current emitter current = 0, (Ic = Ie = 0)

In this case the voltage between the collector and emitter of the transistor is the circuit supply voltage. (As there is no current flowing, no voltage drop, see Ohm’s law). This event typically occurs when the base current = 0 (Ib = 0)

Saturation Region: A transistor is saturated when:

= collector current emitter current = maximum current (Ic = Ie = I maximum)

In this case the current magnitude depends on the voltage supply circuit and the resistors connected to the collector or emitter, or both, see Ohm’s law. This event typically occurs when the base current is large enough to induce a current of times larger manifold. (Remember that Ic = * Ib)

Active Region: When a transistor is neither in its saturation region or the region is then cut in an intermediate region, the active region. In this region, the collector current (Ic) mainly depends on the base current (Ib), (current gain of an amplifier, a manufacturer’s specification) and have resistances connected in the collector and emitter) . This region is the most important if what you want is to use the transistor as an amplifier.


There are three types of typical configurations in transistor amplifiers, each with special characteristics that make them better for certain types of application. and it is said that the transistor is not conducting. Normally this case is when no base current (Ib = 0)

* Common Emitter

* Common Collector

* Common Base

Note: collector current and emitter current are not exactly alike, but are taken as such, due to the small difference between them, and do not affect the circuit almost anything made with transistors


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