Fiber Optic Sensors

What are fiber optic sensors?

The fundamental characteristic of all fiber optic sensors is that they depend on some optical properties, such as intensity, phase, state of polarization and wavelength, to be modulated by measurands. Measurands could be pressure, temperature, electromagnetic field or displacement.

All fiber optic sensors have an optical element that is sensing these property changes. For most sensors, this element is the optical fiber itself or a non-fiber optical element.

Fiber optic sensors with optical fiber as sensor element are called "intrinsic fiber sensor" and sensors with a non-optical fiber sensing element are called "extrinsic fiber sensor".

1. Intrinsic Fiber Sensors

In the intrinsic fiber sensor, external measurands such as pressure, vibration, temperature interact with optical fiber element and cause fiber bending, fiber distortion and a change in the refractive index of the sensing fiber.

Because of the refractive index change, lights that travel through the fiber are affected accordingly. The changes in light properties, such as light intensity, light wavelength and light phase are then detected. The magnitude of measurands interacting with the fiber can then be determined.

2. Extrinsic Fiber Sensors

Birefringent crystal, intensity mask or thin film absorbers are most often used as sensor elements in extrinsic fiber sensors. Usually they are integrated into the optical path.

When the external force interacts with the sensing element, the light properties are modulated as well. The sensor has light source, optical path and photo detector parts. The magnitude of measurands is detected similar to intrinsic fiber sensors.

The Applications of Fiber Optic Sensors

Wide Area Sensing and Monitoring

Because of optical fiber's immune to electromagnetic field, fiber sensors have a big potential in these areas. They are widely used in temperature sensing in building, leakage monitoring along oil pipelines and so on.

The above mentioned applications are called wide area sensing or monitoring. The name means that the sensing covers a wide area. In this area, fiber sensors are divide into two categories: distributed sensor and quasi-distributed sensor.

1. Distributed Sensor

Distributed sensors sense measurands continuouly over the entire length of the fiber. The most important criteria is that sensor fibers must be very sensitive to measurands.

A typical example of distributed sensors is a temperature sensor utilizing Raman scattering effect in optical fibers. Another example is OTDR (Optical Time Domain Reflectometer) which senses signal reflection in the whole length of an optical path.

2. Quasi-Distributed Sensor

Quasi-distributed sensors use discrete sensor elements that are carefully arranged in the fiber network. This type if sensor needs to be small size, low cost and high reliability.

High Sensitivity Measurements

Another area for fiber sensors is the high sensitivity measurement applications. This type of sensors typically utilize light interference's extremely high sensitivity property.

A number of interferometric fiber sensors have been used for measurement of temperature, pressure, vibration and so on. The fiber optic gyroscope is one typical example of this type of applications.

Harsh Environment Measurement

Some extreme environment has no choice but fiber optic sensors. This kind of applications include high temperature, immersion into chemical reagents, radioactive rays factories and so on. The fiber optic sensor's resistant to this type of harsh environment is extremely important.

Colin Yao is an expert on fiber optic communication technologies and products. Learn about fiber optic ST, ST connectors, ST fiber connector on Fiber Optics For Sale Co. web site.

Article Source: _http://EzineArticles.com/?expert=Colin_Yao

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Logic Gates

Logic gates take binary values and perform functions on them, similar to the functions found in simple algebra. Binary algebra is the set of mathematical laws that are valid for binary values. A binary value can only be a 1 or a 0. 1 is a high value, representing true and high voltage. 0 is a low value, representing a false value and low voltage.

Logic gates are typically packaged in integrated circuits, although they can be constructed using analogue components. Integrated circuits allow multiple logic gates to be packaged in one chip and are usually quite reliable. Logic gates typically come in two flavours, TTL (transistor-transistor logic) and CMOS (Complementary Metal Oxide Semiconductor). One must be careful mixing the two types, there logic low and logic high are different voltages. A CMOS might take a TTL high as a LOW and a TTL will accept a CMOS low as a high. Because of this they are generally incompatible, but there are a few CMOS that can accept TTL inputs and vice versa.

The buffer and NOT gates are the simplest of the logic gates. The buffer would be used as a digital signal booster, if a logic signal was to travel for some distance voltage drop from wire resistance would lower a logic high voltage so low that when it reaches its destination its read as a logic low, putting this in between would solve that problem. The buffers algebra function is B = A

NOT gates simply change the input from a 1 to a 0 or vice versa. It is also called an inverter and has many uses in logic circuits. For example, you have 2 lights, but you only want 1 on at any one time, you would put a NOT gate between one light so when there is a logic high input 1 light is on and the other connected to the NOT gate is off and when there is a logic low input the second comes on because of the NOT gate. The circle on the end of the triangle indicates that it’s an inverting gate and you can recognise any inverting logic device by this circle.

The equivalent binary algebra function is B = A’, where B is the output and A is an input value.



The AND gate most commonly come in IC packages of 2 and 3 input versions. The output only produces a logical 1 when all of the inputs are 1. An AND gate could be used in an alarm circuit, where input A would be a reed switch input and B would be an armed control, So the alarm would only be activated if the alarm was active AND the reed switch was circuit was opened (opened door ect.).



The OR gate has a minimum of two inputs and produces an output of 1 if at least one of the inputs has a value of 1. An OR gate could be used to expand the number of reed switches in the previous example

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Relays

The RELAY is a device that acts upon the same fundamental principle as the solenoid. The difference between a relay and a solenoid is that a relay does not have a movable core (plunger) while the solenoid does. Where multipole relays are used, several circuits may be controlled at once. Relays are electrically operated control switches, and are classified according to their use as POWER RELAYS or CONTROL RELAYS. Power relays are called CONTACTORS; control relays are usually known simply as relays. The function of a contactor is to use a relatively small amount of electrical power to control the switching of a large amount of power.

The contactor permits you to control power at other locations in the equipment, and the heavy power cables need be run only through the power relay contacts. Only lightweight control wires are connected from the control switches to the relay coil. Safety is also an important reason for using power relays, since high power circuits can be switched remotely without danger to the operator. Control relays, as their name implies, are frequently used in the control of low power circuits or other relays, although they also have many other uses. In automatic relay circuits, a small electric signal may set off a chain reaction of successively acting relays, which then perform various functions. In general, a relay consists of a magnetic core and its associated coil, contacts, springs, armature, and the mounting. Figure 3-19 illustrates the construction of a relay. When the coil is energized, the flow of current through the coil creates a strong magnetic field which pulls the armature downward to contact C1, completing the circuit from the common terminal to C1. At the same time, the circuit to contact C2, is opened.

A relay can have many different types of contacts. The relay shown in figure 3-19 has contacts known as "break-make" contacts because they break one circuit and make another when the relay is energized. Figure 3-20 shows five different combinations of relay contacts and the names given to each.
A single relay can have several different types of contact combinations. Figure 3-21 is the contact arrangement on a single relay that has four different contact combinations. (The letters next to the contacts are the "forms" shown in figure 3-20.)
One type of relay with multiple sets of contacts is the clapper relay shown in figure 3-22. As the circuit is energized, the clapper is pulled to the magnetic coil. This physical movement of the armature of the clapper forces the pushrod and movable contacts upward. Any number of sets of contacts may be built onto the relay; thus, it is possible to control many different circuits at the same time. This type of relay can be a source of trouble because the motion of the clapper armature does not necessarily assure movement of all the movable contacts. Referring to figure 3-22, if the pushrod were broken, the clapper armature might push the lower movable contact upward but not move the upper movable contact.
Some equipment requires a "warm-up" period between the application of power and some other action. For example, vacuum tubes (covered later in this training series) require a delay between the application of filament power and high voltage. A time-delay relay will provide this required delay. A thermal time-delay relay (fig. 3-23) is constructed to produce a delayed action when energized. Its operation depends on the thermal action of a bimetallic element similar to that used in a thermal circuit breaker. A heater is mounted around or near the element. The movable contact is mounted on the element itself. As the heat causes the element to bend (because of the different thermal expansion rates), the contacts close.
Relays can be described by the method of packaging; open, semisealed, and sealed. Figure 3-24 shows several different relays and illustrates these three types of packaging. Figure 3-24 (E), (G) and (H) are open relays. The mechanical motion of the contacts can be observed and the relays are easily available for maintenance. Figure 3-24 (A), (B) and (C) are semisealed relays. The covers provide protection from dust, moisture, and other foreign material but can be removed for maintenance.
The clear plastic or glass covers provide a means of observing the operation of the relay without removal of the cover. Figure 3-24 (D) and (F) are examples of a hermetically sealed relay. These relays are protected from temperature or humidity changes as well as dust and other foreign material. Since the covers cannot be removed, the relays are also considered to be tamper-proof. With metal or other opaque covers, the operation of the relay can be "felt" by placing your finger on the cover and activating the relay.

Source: freeinfosociety.com

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Opamps: Operational Amplifiers

Operational amplifiers, often known as opamps, are nifty little integrated circuits that contain several transistors. These transistors are used to amplify or attenuate a signal, depending on the circuit that the opamp is placed in.

Why use an opamp when you can use a transistor? Because opamp packages are easier to control, generally more powerful, and also more versatile. An equivalent amplifier circuit might take numerous components, but the opamp places them all into a compact package, reducing the amplifier’s circuit board footprint and the number of components that could fail.

The classic opamp, and perhaps the most widely used, is the UA741. This device comes in several packages, including a compact 8 pin DIP as well as a 10, 14, and 20 pin version for other purposes. All of these are quite cheap, with the 8 pin version costing only $0.30 or so at most online retailers. You would pay considerably more to construct an amplifier circuit using pure transistors.

Opamps are used very frequently for signal conditioning circuits. Let’s say you have a sensor, perhaps a heartbeat sensor, and it is picking up a signal of several millivolts. You want the signal to be windowed to 0-5V so you can hook it up to an analog-to-digital converter. Sounds hard right? Not so!

The first step in designing your circuit is to determine the gain you will need. Let’s assume that the signal from your sensor has a peak of 5 millivolts. In that case, you can calculate the gain as:

To find the gain you need, you divide the desired output by the input, in this case 5 and 5E-3 (.005) volts, respectively. Here, we need a gain of one thousand. Can the opamp handle that magnitude? Yes!

Now we need to construct our circuit. The opamp is represented as a triangle:


The leads on the left are inputs. One of these has to be grounded and which one depends on whether your signal is inverted or not. To avoid inverting (flipping) your signal, you need to ground IN- and connect your signal to IN+.

The other goofy thing about the opamp is that it needs two VCC signals, unlike most other integrated circuits. They should be identical in magnitude, but opposite in polarity. For example, you might connect positive 12V to VCC+ and negative 12V to VCC-. Finally, we have Vout, which is the output signal.

Now, to perform the amplification, we need to tell the opamp what gain we want. To do this, we but two resistors in a circuit with it, forming the classic non-inverting amplifier circuit:
Here, we see the two resistors, R1 and R2. The approximate gain of the amplifier in this case would be the ratio of R2 to R1. The two resistors can be any values as long as they maintain that ratio. In our case, we could use a 1000 ohm resistor for R1 and a 1 megaohm resistor for R2.

Now if this circuit was actually constructed, Vin could be connected to the sensor and Vout could be connected to the ADC. By looking at the input and output signals on an oscilloscope, you would be able to see that the signal was clearly amplified. If no amplification is occurring or you only see noise, ensure that your circuit is connected properly, you have +12 and -12 (not ground) volts connected to the opamp’s VCC, and also make sure that everything has a common ground. These are the most common mistakes that I have committed when working with opamps.

Now you should be able to go out and use the opamp effectively. We have looked at the most common circuit configuration, but it is also possible to configure the opamp to do frequency-based filtering and other nifty things.

Artikel From : freeinfosociety.com

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LED

Light from a Light Emitting Diode (LED) is created in much the same way that light is created in a flourescent tube or neon sign. In an LED crystal the electrons of its atoms are pumped up to higher energy states, and when they fall back down again, each atom gives off a particle/wave of light. However, the electrons in an LED are not exactly the same as the gas molecules in a neon sign. They are not in orbitals stuck to individual atoms.

Instead the electrons occupy a contiguous "sea of charge," and they continually wander among all the atoms in the material. But while they do this, they maintain a particular energy level just like
they do when stuck to individual atoms. It's as if each electron in an LED crystal was "orbiting" among all the atoms of the substance as a whole, and the electron always "orbits" at a particular "height" above each of the atoms it passes.

Be aware that *all* substances contain electrons. The electrons I'm discussing here are not supplied by the battery, they instead occur naturally in the wires, crystals, etc. They are in the LED all the time, even when the battery is not connected. Don't make my original mistake by imagining that electrons are injected into the LED by the power supply. In fact, they are already in the material, and the power supply simply forces them to flow.

To create LED light, first we connect two conductive crystals of different characteristics together. Both types of crystal contain movable electrons. In one type of crystal the electrons "orbit" naturally at a high energy level, and in the other, they always "orbit" low. When a voltage is applied across the joined crystals, the electrons inside are forced to flow across the boundary between the pair of crystals. If the flow direction is correct, electrons in the "high" crystal flow into the "low" crystal and must begin orbiting at the lower energy level. As they fall to the lower energy level, they give off light. The frequency of the light (which we see as the color) is determined by the difference in energy levels between the two crystals. By manufacturing different types of crystals having different natural energy levels, various colors of light can be created. Crystals with similar levels create low-energy photons of red light or even infrared light. With a larger difference in energy levels, green light can be created. An even larger energy-step can create blue light.

The "high" and "low" crystals are usually called "n-type" and "p-type." In n-type crystals the movable electrons wander around while staying at the upper energy level of an unfilled outer atomic orbital. During an electric current they travel at this level. In "p-type" crystals the mobile electrons naturally exist at a deeper orbital level. When the two crystals are connected to each other and then connected properly in a circuit with a battery, the battery creates a current in the entire circuit. It sucks electrons out of the end of the p-type crystal and into the wire. At the same time it pushes electrons into the far end of the n-type crystal. The electrons already in the n-type crystal then are forced to flow across the crystal junction, fall down in energy, emit light, and end up back in the p-type crystal.

Where did the electrons get the energy to emit light? How do they get to a higher energy level so they can enter the n-type crystal? Well, in order for the battery to push electrons through the LED, it had to apply electrical attraction and repulsion forces to the electrons in the crystal. To apply force to the electrons in the crystal, it had to apply a force to the electrons in the negative wire. This squeezes all the electrons on the surface of the negative wire together, which raises the voltage of the entire wire. (If electrons were like water, then the wire is like a long trough. The battery pumps water into one end of the trough, and this makes the water level 'voltage' rise everywhere in the trough.) When the negative wire's electrons get to the energy level equal to the n-type crystal, they start flowing into the crystal and falling "down" the junction, emitting light as they go. (This analogy is incomplete: at the same time that the battery was pumping up the "water level" of the negative wire to match the n-type crystal level, it also was REDUCING the "water level" of the positive wire so that the low-energy electrons of the p-type crystal could be sucked into the wire.)

Here's another way to visualize LEDs. In a neon sign, the electrons around each neon atom get pumped up in energy as they're whacked by incoming high-speed electrons. In an LED the battery pumps up the electrons directly. In a neon sign, each atom emits light when an electron falls back to its original energy level. In an LED, the whole crystal junction emits light as electrons drop back to a lower level. Therefor an LED resembles a gigantic single neon atom! An LED/atom is so large that we can connect its electron cloud directly to a battery with wires. It's so large that we can build in different characteristics, and change the color of its flourescence.

Light Emitting Diodes are much like solar cells. Both devices use n-type and p-type crystals, but in a solar cell the process runs backwards: instead of falling down in energy and emitting light, light hitting the solar cell causes electrons in the p-type crystal to jump upwards in energy. If these electrons are near the crystal junction, they can end up in the n-type crystal, and they can flow through wires to the outside world, falling down in energy as they do. In fact, if light shines on an LED, the LED behaves as a tiny, inefficient solar cell. And conversely, if a battery is used to create a current in a solar cell, the solar cell can emit a very tiny amount of (mostly infrared) light. An LED gives light when charge is pumped through it, and when light shines on a solar cell, the solar cell becomes a charge pump.

Light Emitting Diodes are also like thermocouples. N-type and p-type crystals are not the only materials whose electrons "orbit" at different energy levels. Different metals have different levels too. If a copper wire is twisted together with an iron wire, a junction is formed between them which contains an energy-step like that of an LED. The energy-step in a thermocouple is much smaller than in an LED. If electrons are forced to flow across the thermocouple's energy step, they fall down in energy level and emit energy. But what do they emit? Longwave Infrared light and crystal vibrations. Together we call these by the name "heat energy". The energy step in a thermocouple is too small, so it cannot emit photons of visible light. Instead it creates "heat." And conversely, if heated, a thermocouple can create an electric current. When operated one way, a thermocouple is a bit like an LED which emits heat. When operated the other way, it acts a bit like a "solar battery" and becomes a "heat battery."

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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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Transformers

Transformers are devices that utilize the property of inductance to step-up or step-down voltages. Transformers, however, only work with alternating current, since inductance only occurs when a magnetic field is changing, which is not the case with direct current. However, transformer usage is not limited to voltage modification, it is also capable of matching impedances between different electrical circuits, mating balanced and unbalanced circuits, and isolating dc between circuits, while allowing ac to pass.

The basic construction of a transformer consists of two seperate coils of wire wrapped around a core of iron, air, or any ferromagnetic material. While iron and ferromagnetic cores provide much higher coupling (efficiency of induction transfer), there are significant losses through heat generation in the core. Cores can also be shaped differently, such as the rod and E-core designs. The voltage modification is caused by the difference in number of coils on each wire. The wire you have the supply voltage on is referred to as the primary winding. The wire that is receiving the modified voltage is the secondary winding. The level of modification is determined by the ratio of turns between the primary and secondary windings. Current limiting is also provided by the gauge of the wire used in the windings. A thicker gauge wire allows higher amounts of current to go through, while a smaller gauge allows less. Transformers are the primary reason that power transmission and household outlets utilize alternating current. This is because transformers allow for efficient changes in voltage that allow power to be transferred all over the world at high voltages and low costs.

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Optoisolators

Optoisolators are circuits designed to isolate one circuit from another using light. Why would you use light instead of implementing some diodes or voltage regulators? Because light is essentially a guaranteed protection. If an overvoltage or overcurrent is applied to the optoisolator, the optoisolator circuit is destroyed and the circuit(s) it was protecting will not be affected.

The most common optoisolators are placed in an integrated circuit for convenience and efficiency. The most popular model is the 4N33, which is housed in an 8-pin DIP package. The 4N33 internally is basically an LED next to a phototransistor. This could be called an optical modem, since it modulates the electric signal into light waves, which are demodulated by the phototransistor as it converts the light into electricity again.

To design a circuit using an optoisolator, you cannot ignore the components inside of it. On the input circuit, you must take the forward voltage of the LED into account, which can be approximated to .7 volts. On the output side, you should take the phototransistor's forward voltage into account and it can be approximated to .2 volts. If you are working with relatively high voltages, you will need a resistor on the input to protect the LED.

Optoisolators are commonly used to protect expensive circuits from voltage and current surges. One example application would be on the output of a microprocessor. Most microprocessors can't output a very high current, but you might want to be able to power a motor with an output pin. To work around this problem, you could connect the output pin to the optoisolator's input and then connect the motor in series with the output and a voltage supply that is high enough to power the motor. This way, the majority of the current is coming from the independent power supply rather than the microprocessor.

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