Technically, radio is a small part of theelectromagnetic spectrum , so-called because electromagnetic waves are driven across space at the speed of light byelectricand magnetic forces. Gamma rays, x-rays, ultra-violet, visible light and infra-red are also parts of the same spectrum. They all travel at the identical speed but have different properties and uses, depending on theirfrequency or wavelength.
If equipment generating radio frequency energy (a transmitter) is connected to an antenna (or aerial) the energy is radiated in the form of radio waves. Some of the radiated energy can then be collected by another antenna which, when coupled to a radio receiver, enables a link to be made between the transmitter and receiver. Adding information to the radio carrier wave enables messages to be transmitted. These messages may take the form of television program-mes, voice messages, data or other forms of radio communication. Radio is an immensely versatile medium.
The way in which radio waves propagate or travel through the atmosphere varies with the frequency or wavelength of the radio signal. In general, the lower the frequency the further the distance that radio waves will travel. As the frequency increases, the waves may be obstructed or deflected by hills and buildings. At extremely high frequencies, rainfall and other weather conditions can significantly limit the effective operating range of radio communications. These propagation characteristics mean that different bands of frequencies are suited to particular types of radio use. For example, for national and international broadcasting the long-wave (approximately 148-283 kHz) and medium-wave (approximately 526-1606 kHz) bands are used. At the higher frequencies such as VHF (Very High Frequency - 30 to 300 MHz) and UHF (Ultra High Frequency - 300 to 3000 MHz), where the effective transmission range is shorter, the bands are more suited to local broadcasting or local coverage for mobile radio systems. At higher frequencies it is possible to allocate wider channels (i.e., more bandwidth) and this allows more information to be transmitted per channel than at lower frequencies. For example, at VHF, because of the greater bandwidth available, high quality stereo music can be transmitted, but a typical channel in the long wave bands will support only a low quality music broadcast.
Basic radio block diagram:
The power supply (not shown) is connected to the pre-amplifier and power amplifier blocks.
Microphone - a transducer which converts sound to voltage.
Pre-Amplifier - amplifies the small audio signal (voltage) from the microphone.
Tone and Volume Controls - adjust the nature of the audio signal. The tone control adjusts the balance of high and low frequencies. The volume control adjusts the strength of the signal.
Power Amplifier - increases the strength (power) of the audio signal.
Loudspeaker - a transducer which converts the audio signal to sound.
Radio Receiver System
The power supply (not shown) is connected to the audio amplifier block.
Aerial - picks up radio signals from many stations.
Tuner - selects the signal from just one radio station.
Detector - extracts the audio signal carried by the radio signal.
Audio Amplifier - increases the strength (power) of the audio signal. This could be broken down into the blocks like the Audio Amplifier System shown above.
Loudspeaker - a transducer which converts the audio signal to sound.
Regulated Power Supply System
Transformer - steps down 230V AC mains to low voltage AC.
Rectifier - converts AC to DC, but the DC output is varying.
Smoothing - smooths the DC from varying greatly to a small ripple.
Regulator - eliminates ripple by setting DC output to a fixed voltage.
Feedback Control System
The power supply (not shown) is connected to the control circuit block.
Sensor - a transducer which converts the state of the controlled quantity to an electrical signal.
Selector (control input) - selects the desired state of the output. Usually it is a variable resistor.
Control Circuit - compares the desired state (control input) with the actual state (sensor) of the controlled quantity and sends an appropriate signal to the output transducer.
Output Transducer - converts the electrical signal to the controlled quantity.
Controlled Quantity - usually not an electrical quantity, e.g. motor speed.
Feedback Path - usually not electrical, the Sensor detects the state of the controlled quantity.
A transformer is an electrical device that takes electricity of one voltage and changes it into another voltage. You'll see transformers at the top of utility poles and even changing the voltage in a toy train set. Basically, a transformer changes electricity from high to low voltage using two properties of electricity. In an electric circuit, there is magnetism around it. Second, whenever a magnetic field changes (by moving or by changing strength) a voltage is made. Voltage is the measure of the electric force or "pressure" that "pushes" electrons around a circuit.
If there's another wire close to an electric current that is changing strength, the current of electricity will also flow into that other wire as the magnetism changes.
A transformer takes in electricity at a higher voltage and lets it run through lots of coils wound around an iron core. Because the current is alternating, the magnetism in the core is also alternating. Also around the core is an output wire with fewer coils. The magnetism changing back and forth makes a current in the wire. Having fewer coils means less voltage. So the voltage is "stepped-down."
Transformers on the Electrical Grid
Let's look at the electricity that comes to your home. When electricity moves from a power plant it is put into a very high voltage to be able to travel long distances. The high voltage lines can be as high 155,000 to 765,000 volts to travel many hundreds of miles.
In order for your home or a store to use the electricity, it has to be at a lower voltage than on the long-distance lines. So, the electricity is "stepped-down to a lower level using a transformer. This lower voltage electricity is put into the local electric wires at a substation. The substation breaks the larger amount of power down into smaller pieces at lower voltage. It then is stepped down again and again.
Once smaller transformers take that voltage down to usually 7,200, the power leaves this substation.
In your neighborhood, a transformer on top of a utility pole, or one connected to underground wires, transforms the 7,200 volts into 220-240 volts. This is then sent into your home over three wires. The three wires go through the electric meter, which measures how much electricity you use. One of the three wires is the ground, and the other two are the positives.
Some of the electrical appliances in your home use the 220-240 volts. These are things like a water heater, stove and oven, or air conditioner. They have very special connections and plugs. Other devices, like your TV or computer only use one-half of the electricity -- 110-120 volts.
In a toy train set, the voltage is reduced even more from 110-120 and is changed from alternating current into direct current.
Some businesses use higher voltage power to run big machines. So, they don't need to have the voltage reduced as much.
Step-up and step-down transformers:
So far, we've observed simulations of transformers where the primary and secondary windings were of identical inductance, giving approximately equal voltage and current levels in both circuits. Equality of voltage and current between the primary and secondary sides of a transformer, however, is not the norm for all transformers. If the inductances of the two windings are not equal, something interesting happens.
Notice how the secondary voltage is approximately ten times less than the primary voltage (0.9962 volts compared to 10 volts), while the secondary current is approximately ten times greater (0.9962 mA compared to 0.09975 mA). What we have here is a device that steps voltagedown by a factor of ten and current up by a factor of ten:
Turns ratio of 10:1 yields 10:1 primary:secondary voltage ratio and 1:10 primary:secondary current ratio.
This is a very useful device, indeed. With it, we can easily multiply or divide voltage and current in AC circuits. Indeed, the transformer has made long-distance transmission of electric power a practical reality, as AC voltage can be “stepped up” and current “stepped down” for reduced wire resistance power losses along power lines connecting generating stations with loads. At either end (both the generator and at the loads), voltage levels are reduced by transformers for safer operation and less expensive equipment. A transformer that increases voltage from primary to secondary (more secondary winding turns than primary winding turns) is called astep-uptransformer. Conversely, a transformer designed to do just the opposite is called astep-downtransformer.
Let's re-examine a photograph shown in the previous section:
Transformer cross-section showing primary and secondary windings is a few inches tall (approximately 10 cm).
This is a step-down transformer, as evidenced by the high turn count of the primary winding and the low turn count of the secondary. As a step-down unit, this transformer converts high-voltage, low-current power into low-voltage, high-current power. The larger-gauge wire used in the secondary winding is necessary due to the increase in current. The primary winding, which doesn't have to conduct as much current, may be made of smaller-gauge wire.
In case you were wondering, it is possible to operate either of these transformer types backwards (powering the secondary winding with an AC source and letting the primary winding power a load) to perform the opposite function: a step-up can function as a step-down and visa-versa. However, as we saw in the first section of this chapter, efficient operation of a transformer requires that the individual winding inductances be engineered for specific operating ranges of voltage and current, so if a transformer is to be used “backwards” like this it must be employed within the original design parameters of voltage and current for each winding, lest it prove to be inefficient (or lest it be damaged by excessive voltage or current!).
Transformers are often constructed in such a way that it is not obvious which wires lead to the primary winding and which lead to the secondary. One convention used in the electric power industry to help alleviate confusion is the use of “H” designations for the higher-voltage winding (the primary winding in a step-down unit; the secondary winding in a step-up) and “X” designations for the lower-voltage winding. Therefore, a simple power transformer will have wires labeled “H1”, “H2”, “X1”, and “X2”. There is usually significance to the numbering of the wires (H1 versus H2, etc.), which we'll explore a little later in this chapter.
The fact that voltage and current get “stepped” in opposite directions (one up, the other down) makes perfect sense when you recall that power is equal to voltage times current, and realize that transformers cannot produce power, only convert it. Any device that could output more power than it took in would violate the Law of Energy Conservation in physics, namely that energy cannot be created or destroyed, only converted. As with the first transformer example we looked at, power transfer efficiency is very good from the primary to the secondary sides of the device.
The practical significance of this is made more apparent when an alternative is considered: before the advent of efficient transformers, voltage/current level conversion could only be achieved through the use of motor/generator sets. A drawing of a motor/generator set reveals the basic principle involved:
Motor generator illustrates the basic principle of the transformer.
In such a machine, a motor is mechanically coupled to a generator, the generator designed to produce the desired levels of voltage and current at the rotating speed of the motor. While both motors and generators are fairly efficient devices, the use of both in this fashion compounds their inefficiencies so that the overall efficiency is in the range of 90% or less. Furthermore, because motor/generator sets obviously require moving parts, mechanical wear and balance are factors influencing both service life and performance. Transformers, on the other hand, are able to convert levels of AC voltage and current at very high efficiencies with no moving parts, making possible the widespread distribution and use of electric power we take for granted.
In all fairness it should be noted that motor/generator sets have not necessarily been obsoleted by transformers for all applications. While transformers are clearly superior over motor/generator sets for AC voltage and current level conversion, they cannot convert one frequency of AC power to another, or (by themselves) convert DC to AC or visa-versa. Motor/generator sets can do all these things with relative simplicity, albeit with the limitations of efficiency and mechanical factors already described. Motor/generator sets also have the unique property of kinetic energy storage: that is, if the motor's power supply is momentarily interrupted for any reason, its angular momentum (the inertia of that rotating mass) will maintain rotation of the generator for a short duration, thus isolating any loads powered by the generator from “glitches” in the main power system.
Looking closely at the numbers in the SPICE analysis, we should see a correspondence between the transformer's ratio and the two inductances. Notice how the primary inductor (l1) has 100 times more inductance than the secondary inductor (10000 H versus 100 H), and that the measured voltage step-down ratio was 10 to 1. The winding with more inductance will have higher voltage and less current than the other. Since the two inductors are wound around the same core material in the transformer (for the most efficient magnetic coupling between the two), the parameters affecting inductance for the two coils are equal except for the number of turns in each coil. If we take another look at our inductance formula, we see that inductance is proportional to the square of the number of coil turns:
So, it should be apparent that our two inductors in the last SPICE transformer example circuit -- with inductance ratios of 100:1 -- should have coil turn ratios of 10:1, because 10 squared equals 100. This works out to be the same ratio we found between primary and secondary voltages and currents (10:1), so we can say as a rule that the voltage and current transformation ratio is equal to the ratio of winding turns between primary and secondary.
Step-down transformer: (many turns :few turns).
The step-up/step-down effect of coil turn ratios in a transformer (Figure above) is analogous to gear tooth ratios in mechanical gear systems, transforming values of speed and torque in much the same way: (Figure below)
Step-up and step-down transformers for power distribution purposes can be gigantic in proportion to the power transformers previously shown, some units standing as tall as a home. The following photograph shows a substation transformer standing about twelve feet
Light emitting diodes, commonly called LEDs, are real unsung heroes in the electronics world. They do dozens of different jobs and are found in all kinds of devices. Among other things, they form numbers on digital clocks, transmit information from remote controls, light up watches and tell you when your appliances are turned on. Collected together, they can form images on a jumbo television screen or illuminate a traffic light.
Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike ordinary incandescent bulbs, they don't have a filament that will burn out, and they don't get especially hot. They are illuminated solely by the movement of electrons in asemiconductor material, and they last just as long as a standard transistor. The lifespan of an LED surpasses the short life of an incandescent bulb by thousands of hours. Tiny LEDs are already replacing the tubes that light up LCD HDTVs to make dramatically thinner televisions.
In this article, we'll examine the technology behind these ubiquitous blinkers, illuminating some cool principles of electricity and light in the process.
What is a Diode?
A diode is the simplest sort of semiconductordevice. Broadly speaking, a semiconductor is a material with a varying ability to conduct electrical current. Most semiconductors are made of a poor conductor that has had impurities (atoms of another material) added to it. The process of adding impurities is called doping.
In the case of LEDs, the conductor material is typically aluminum-gallium-arsenide (AlGaAs). In pure aluminum-gallium-arsenide, all of the atoms bond perfectly to their neighbors, leaving no free electrons (negatively charged particles) to conduct electric current. In doped material, additional atoms change the balance, either adding free electrons or creating holes where electrons can go. Either of these alterations make the material more conductive.
A semiconductor with extra electrons is called N-type material, since it has extra negatively charged particles. In N-type material, free electrons move from a negatively charged area to a positively charged area.
A semiconductor with extra holes is called P-type material, since it effectively has extra positively charged particles. Electrons can jump from hole to hole, moving from a negatively charged area to a positively charged area. As a result, the holes themselves appear to move from a positively charged area to a negatively charged area.
A diode consists of a section of N-type material bonded to a section of P-type material, with electrodes on each end. This arrangement conducts electricity in only one direction. When no voltage is applied to the diode, electrons from the N-type material fill holes from the P-type material along the junction between the layers, forming a depletion zone. In a depletion zone, the semiconductor material is returned to its original insulating state -- all of the holes are filled, so there are no free electrons or empty spaces for electrons, and charge can't flow.
To get rid of the depletion zone, you have to get electrons moving from the N-type area to the P-type area and holes moving in the reverse direction. To do this, you connect the N-type side of the diode to the negative end of a circuit and the P-type side to the positive end. The free electrons in the N-type material are repelled by the negative electrode and drawn to the positive electrode. The holes in the P-type material move the other way. When the voltage difference between the electrodes is high enough, the electrons in the depletion zone are boosted out of their holes and begin moving freely again. The depletion zone disappears, and charge moves across the diode.
If you try to run current the other way, with the P-type side connected to the negative end of the circuit and the N-type side connected to the positive end, current will not flow. The negative electrons in the N-type material are attracted to the positive electrode. The positive holes in the P-type material are attracted to the negative electrode. No current flows across the junction because the holes and the electrons are each moving in the wrong direction. The depletion zone increases.
How Can a Diode Produce Light?
Lightis a form of energy that can be released by anatom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called photons, are the most basic units of light.
Photons are released as a result of moving electrons. In an atom, electrons move in orbitals around the nucleus. Electrons in different orbitals have different amounts of energy. Generally speaking, electrons with greater energy move in orbitals farther away from the nucleus.
For an electron to jump from a lower orbital to a higher orbital, something has to boost its energy level. Conversely, an electron releases energy when it drops from a higher orbital to a lower one. This energy is released in the form of a photon. A greater energy drop releases a higher-energy photon, which is characterized by a higher frequency. (Check out How Light Works for a full explanation.)
As we saw in the last section, free electrons moving across a diode can fall into empty holes from the P-type layer. This involves a drop from the conduction band to a lower orbital, so the electrons release energy in the form of photons. This happens in any diode, but you can only see the photons when the diode is composed of certain material. The atoms in a standard silicon diode, for example, are arranged in such a way that the electron drops a relatively short distance. As a result, the photon's frequency is so low that it is invisible to the human eye -- it is in the infrared portion of the light spectrum. This isn't necessarily a bad thing, of course: Infrared LEDs are ideal for remote controls, among other things.
Visible light-emitting diodes (VLEDs), such as the ones that light up numbers in a digital clock, are made of materials characterized by a wider gap between the conduction band and the lower orbitals. The size of the gap determines the frequency of the photon -- in other words, it determines the color of the light. While LEDs are used in everything from remote controls to the digital displays on electronics, visible LEDs are growing in popularity and use thanks to their long lifetimes and miniature size. Depending on the materials used in LEDs, they can be built to shine in infrared, ultraviolet, and all the colors of the visible spectrum in between.
