Transistor

Transistors are electronic switching devices, which are the basis of nearly all electronic circuits. This page will give a brief outline of what they are, as well as different methods to interface analog transistors from digital circuitry.

Introduction

The simple explanation of a transistor is that it is a combination of three 'doped' pieces of semi-conductor material.

The piece in the middle is called the Base (in Bipolar Junction Transistors), and the outside edges are the Collector, and the Emitter.

When current is put into the Base, it changes the voltage characteristics of the entire transistor, and so it is possible to control the current flowing from the Collector to the Emitter. So a small change of current on the base, results in a large change between the Collector and Emitter.

Bi-Polar Junction Transistors (BJT)
NPN

This is the simplest type of BJT to understand. As you can see in the diagram below, when you apply voltage to the base of the BJT, it turns on the transistor.

A more detailed explanation is that when current is applied onto the base, it changes the voltage difference between the collector and the base. This difference changes the bias within the transistor, causing current to flow from the collector to the emitter.

When there isn't a lot of charge on the base, there are areas within the semiconductor that aren't capable of carrying current from collector to emitter. This means that a lot of power is dissipated to drive the current through. When there is so much charge on the base that no more will fit, the transistor is said to be saturated. There are plenty of carriers for the current, and not much power is dissipated, making the transistor more efficient. This is only true when the transistors Emitter is connected directly to ground (Common Emitter).

This diagram shows how an NPN is turned on. When the base is turned off (connected to ground), there is no way to put current through the transistor, so the transistor is off. When the base voltage is raised, driving charge onto the base, it turns the transistor on.

PNP

The PNP isn't quite as simple. The base still controls the flow of current, but it is more or less opposite. In order to turn the transistor on the base is connected to ground (turned off). To turn the transistor off, voltage is applied to the base.

The reason for this is because of the type of semi-conductor used. When the base is connected to ground, loose electrons are taken away, creating 'holes'. These holes can be thought of as positive charges, and are capable of carrying current from the Emitter to the Collector.

A PNP transistor will saturate only when it is set up as a Common Emitter

The diagram below shows how this works.

This PNP is set up as a Common Emitter configuration. When the input to the base is turned off, the transistor is turned on, and current flows through the load (resistor) to ground. When the base is 'turned on', it removes the 'holes' from the base, causing current to stop flowing in the transistor.

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Dioda

Figure 1.1: Diode Schematic Symbol and Casing

A semiconductor diode consists of a semiconductor PN junction and has two terminals, an anode (+) and a cathode (-). Current flows from anode to cathode within the diode (according to the high to low circuit analysis method), but only when there is at least a certain amount of forward voltage applied. When positive voltage is applied across the diode, it is called a forward bias, whereas a negative voltage is called a reverse bias.

A diode is best described as a one way valve, since it only allows current to flow from anode to cathode. For example, if you applied a reverse bias to the diode with a magnitude of 5 volts, current would not flow.

If you applied 5 volts with a positive bias, current would flow.

This seems pretty simple, but there are exceptions to the one way valve analogy. For example, diodes have a minimum forward voltage level to allow current to flow. In most cases, about .7 volts are needed to trigger current flow. You can see this from figure 1.3 below. The current does not start to flow until a certain amount of forward voltage is applied.

Another exception is the breakdown voltage. All diodes have a point where, if the reverse voltage is high enough, the semiconductor structure will break down, allowing current to flow. This value is usually fifty volts or higher and when the breakdown voltage is reached, it generally damages or destroys the diode.

Why is a diode useful? Because it can be used for rectification, protection of components from reverse voltage, and creating interesting wave shapes. For example, say you have an electrolytic capacitor that can only withstand 10V of reverse bias voltage. All you have to do is place a diode in front of it and it will block most reverse voltages from destroying the capacitor. Rectification is the process of converting an alternating current signal into a direct current signal and is used in all AC to DC converters and power supplies.

Figure 1.2: The Operation of an Ideal Diode

Figure 1.3: The Operation of a Real Diode

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Inductor

An inductor is a coil of wire which may have a core of air, iron or other ferrous materials. Its electrical property is called inductance and the unit for this is the henry, symbol H. 1 Henry is very large so mH and µH are often used, 1000µH = 1mH and 1000mH = 1H. Iron and ferrite cores increase the inductance since they can become magnetized. Inductors are mainly used in tuned circuits and to block high frequency AC signals (they are sometimes called chokes).

They pass DC easily, but block AC signals, exactly the opposite of capacitors.

Inductance is a property that is possessed by all coils of wire containing electrical current. The current creates a magnetic field, which can in turn induce current flow if the original current decreases in magnitude or stops. Essentially, an inductor is like a capacitor, only stores energy in a magnetic field instead of an electric field. This makes it very useful for power supply filters that help maintain a fairly noiseless current. A transformer is essentially two inductors, where current flow through one inductor induces current flow in the second as a result of the magnetic field.

Inductors are most often found in audio electronics, power supplies, and radio tuning circuits. An inductor can easily be made by winding insulated wire around a ferrous rod. Thin gauge wire is easiest, since it can bend into smaller loops and is cheaper than large gauge wire.

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Resistors

Resistors are components that just about every electronic device uses. A resistor is a component that resists the flow of current. They do this by either extending the length of wire that the electricity must flow through or forcing the current to pass through a poor conductor, such as carbon. The reduction in current flow can be useful in a number of ways:

It can protect components that have a specific current rating

It can change the function of the circuit

It can create a "dummy load" for a circuit in order for testing purposes

It can create by products such as heat that can be utilized for special purposes

The protection is necessary in order to prevent destruction of certain components that have a maximum current that can be passed through them. This is particularly true in components such as LEDs, which can suffer permanent damage and/or destruction if excessive current is passed through them.

The resistance can change the function of certain circuits, such as oscillators. Certain circuits use the level of current as a control for specific functions. For example, the 555 timer IC outputs pulsed electricity, the frequency of which is determined by the current level sent to one of its leads. This is very important.

The "dummy load" is useful when testing circuits in the lab since the actual load can be impractical for testing (such as a very large antenna). The resistor duplicates the resistance of the real load and makes the circuit act as though it is connected as it normally would.

Resistance creates heat losses in electrical circuits, but that is not always a bad thing. Most electrical heaters utilize this by running electricity through resistors with very low resistance, producing a lot of heat.

On a circuit diagram normally a resistor will have a letter after the value, for values less than 1,000, an ‘R’ is used. So 100 ohms will read 100R. From 1,000 ohms a ‘K’ is used and the number is divided by 1,000. So 1,000 ohms is read as 1K, 22,000 as 22K and 100,000 ohms as 100K. Lastly, from 1,000,000 ohms a ‘M’ is used and the number is divided by 1,000,000. So 1,000,000 ohms is 1M of course. Resistors are too small to have these numbers printed on them, instead they have coloured bands, which is explained further down.

Resistor values with a decimal point in circuit diagrams are expressed in 2 ways, say the circuit requires a 1.2k ohm resistor. The diagram might have it as 1.2K or it might appear as 1K2. The ‘K’ is put in place of the decimal point to prevent the value from being misread as 12K ohms. For resistors below 1k ohm an ‘R’ is used in place of the ‘K’. So 5.6 ohm resistor on the diagram would appear as 5.6R or 5R6.

The above is a picture of a 4-band 1/4w carbon film resistor. These are the most commonly used in electronic circuits due to their low cost and versatility. They come in 1/4w, 1/2w and 1w. You can tell the difference in power handling by the physical size of the package. A 1/4w resistor 7mm long by 2mm diameter, a 1/2w is about 9mm long by 3mm diameter and a 1w is 11mm long and 4mm in diameter. They usually have a tolerance of 5%.

These are 5-band metal film resistors, they have a much smaller tolerance than carbon film resistors, these have a tolerance of 1%. These are used where you need an exact value, such as a high quality audio preamplifier.
This is a ceramic wire wound resistor. They usually come with a power rating of 5w and 10w. These are used where a lot of power is going to be dissipated, such as that of a dummy load. They will be used in a high power audio amplifier.



This is wire wound nichrome wire. The purpose of nichrome wire is to produce heat and hence it’s used in electric heaters and stoves. Nichrome wire normally has a resistance of about 13.8 Ohms per metre. This coil came from a 2400w, 240v fan heater, it had 8 lengths just like the one pictured and they were used in series and parallel combinations to achieve low, medium and high power. The wire actually had a faint red glow on full power. Nichrome wire in heaters should be protected so stray hands don’t touch them, because that stray hand will get burnt.

Because carbon resistors are so small it’s impractical to print the resistance on it so instead they have 4 or 5 coloured bands. The number of bands relates to the tolerance of the resistor, the tolerance is how much variation there is likely to be. At 5% a 100 ohm resistor can be as low as 95 ohms or as high as 105 ohms. 4 bands are used when the tolerance is 5% or 10% and 5 bands are used when the tolerance is 1% or 2%. The 5th band is used to achieve more precision.


The 1st, 2nd (3rd) and multiplier bands are bunched together so you can see where to start from. This is helpful especially with 5-band resistors, which is harder to tell because the tolerance band is brown or red.

Article written by Mojo'D

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Capasitor

Capacitors are among the most commonly used components in electronics.
Their construction is fairly simple, two metal plates and a dielectric layer
separating them. Capacitors are very similar to batteries since they store
electrical charge. However, capacitors must be charged with electricity, unlike
batteries which produce their own using chemicals.

The capacitor's charge capacity depends upon the size of the metal plates.
The larger the plates, the higher the charge and vice versa. The dielectric
can be anything that disallows the plates from touching each other and discharging,
but still allows the electric force to pass through. When charged, a capacitor
gains the same voltage as the power source that was used to charge it.

The storage rating of a capacitor is based on the Farad unit. A capacitor
with a capacitance rating of one Farad is capable of storing one coulomb of
charge (6.25 x 10 ^ 18 electrons) at 1 volt. Although that many electrons
seems like a lot, it can only power an average incandescent light bulb for
about a minute.

The reason capacitors are used is often because of their quick discharge ability.
A chemical reaction in a battery takes time, while the capacitor requires
no chemical reaction to discharge electricity. This makes the capacitor a
lot faster when it comes to discharging. That is why capacitors are used in
cameras and lasers to create a bright flash, rather than batteries.

Capacitors are also used to make DC voltage constant. In power supplies, the
voltage can vary. With a capacitor included, it makes up for a lack of voltage
and absorbs the excessive voltage. This is necessary in sensitive electronic
devices that require constant voltage supplies.

Capacitors are also used to block direct current. Since a capacitor connected
in series with a power source is essentially a broken circuit, current cannot
flow, once the capacitor is charged. However, alternating current can still
flow when connected to a capacitor, since the voltage shifts and the capacitor
charges and discharges. When capacitors are connected in parallel the total
capacitance in the network is the sum of all the capacitance, Ct = C1+C2…+Cn.
For example if C1 was 10uF and C2 is 47uF the total capacitance is 57uF.

Two capacitors in parallel

When capacitors are connected in series the capacitance is
given by 1/Ct = 1/C1+1/C2…+1/Cn.

Two capacitors in series
Capacitors are usually connected in series to increase the total

voltage that can be connected between them; this is common with Tesla Coil
Circuits as finding a capacitor with the exact capacitance and voltage would
be almost impossible to find.

Special care must be taken with high voltage capacitors, such as capacitors
where mains voltages (110-120v and 220-240) or the capacitors used in microwaves
and TV sets and they can store enough charge to kill. Capacitors can store
a charge for years after the power supply has been disconnected and the terminals
should be shorted to remove the charge, some high voltage capacitors have
‘bleed resistors’ in them to drain the
charge when the power is disconnected.
The different types of capacitors are generally named by the dielectric used
in them, and have different purposes.

Aluminium electrolytic capacitors consist of one plate that is a chemical
electrolyte and a dielectric that is an oxide on one side of the other metal
plate. Aluminium electrolytic capacitors store the most charge in the smallest
space with respect to other types of capacitors due to the oxide dielectric's
amazing properties as an insulator. There are two main types of capacitor
structural designs that you will run into when working with electronics. The
two types are radial and axial. The radial design has both leads coming out
of the same side of the capacitor. The axial design has one lead coming out
of the center of each side, creating an axis.

An axial capacitor

Electrolytic capacitors are polarised, they can only be connected
one way around. The polarity is indicated on the case of the capacitor, in
most cases it will have an arrow pointing to the negative lead, but there
are capacitors with arrows pointing to the positive as well. In the picture
above the polarity arrow can be seen and is pointing to the negative terminal.
The negative lead will also be shorter than the positive lead.

A radial capacitor

These capacitors are used in power supplies to smooth the voltage
and anything that requires large energy storage, their capacity can range
from as little as 0.22uF for filtering in audio circuits and they can have
capacities beyond 10,000uF and even 100,000uF for filtering power supplies.
Its impractical to use anything beyond 10,000uF capacitors in most cases as
they are quite large and heavy. Almost all power supply circuits work satisfactorily
with 2200uF.

The 100,000uF capacitor dwarfs the 33uF capacitor

Care must be taken to ensure electrolytic capacitors are not
connected in reverse polarity, if they are the dielectric dissolves which
allows high current to pass though the electrolyte which will vaporise and
the built up pressure will be released with the capacitor bursting open with
a loud bang if the capacitor is relatively small to the sound of an explosive
detonating for large filter capacitors (3300uF or so). In some situations
where reverse polarity will occur a special Bi-Polar electrolytic capacitor
is used. They can be identified by having no polarity markings and have the
letters ‘BP’ printed on the case.

A variation on the electrolytic capacitor is the Tantalum capacitor, which
uses tantalum film instead of aluminium. Tantalums contain electrolyte in
dry form and are more resistant to reverse polarity than electrolytic but
the polarity must still be correct.

A Tantalum capacitor

Ceramic capacitors also known as disc capacitors as they look
like small discs offer small capacitances, the lowest being 1pF which is an
extremely small storage capacity. They are used in bypassing and filtering
circuits.

Polyester capacitors, also known as ‘Greencaps’
because of their appearance are the most common general purpose capacitor.
Their values range from 10nF to 0.33uF or green caps and up to 10uF for MKT
polyester capacitors.

Left: A polyester greencap capacitor; Right: Two ceramic disc capacitors

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