Power Supplies

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In lesson 48, we examined how power came into your house or plant. It is sent to you in AC form, because it is less expensive, as well as more stable, to send it in a high voltage, low current form, and transform it to lower voltages at higher currents at the point of use.

This is a great way to get power into the homes of everyone on the planet. The big problem comes in that most electronic components don't normally work on AC - they work on DC. So we need some way to convert an Alternating Current power service, into Direct Current for your equipment and devices to run off. Fortunately, this has been taken care of in most of your equipment. Most modern electronics equipment has a power supply built in. That power supply takes the AC line voltage coming into your home, and converts it into the needed DC voltages that your equipment uses.

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Transistors

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Recall that in a semiconductor diode, we have 2 regions of DOPED semiconductive material. One region is doped positive, and the other region is doped negative. There is also a junction, where the two regions are joined.

When a diode is forward biased, it conducts electricity easily, like a ball rolling down a hill. When it is reverse biased, it is extremely resistive to current flow, as the ball is rolling uphill, and is much harder to get over the hump.

Remember also, that we had diode tubes, which operated in a similar manner. They would conduct electricity in one direction easily, but would not conduct in the opposite direction.

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Semiconductors - Diodes and Transistors

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But we've already discussed diodes. They are a simple form of vacuum tube aren't they?

Well - yes and no.

While diodes existed in tube form for many years vacuum tube diodes had their problems, and the electronics industry would try to find a way around those problems. Vacuum tube diodes did a fine job of rectifying (turning ac into dc), but they wasted a lot of electrical energy in the process, which made them inefficient and costly to operate. Quite a bit of power was lost just keeping the filament warm! They also had the problem of being physically fragile, and tended to be the main cause of an electrical equipment failure.

As early as 1874, researchers noted that a metal-lead sulfide junction had rectifying properties. They found that it would conduct electrical current in one direction, but if they reversed the current, it would not flow in the opposite direction. This "junction" was "semi-conductive" in nature. They had, without knowing it, invented the semi-conductor. The problem was, they had no practical application for it, and to be honest, didn't understand how it worked. However, In 1926, P.H. Geiger and L.O. Grondahl discovered the rectifying properties in a semiconducting copper oxide-copper junction. Armed with W. Schottky's theoretical explanation of how it worked, this was the first practical diode that didn't involve a vacuum tube.

Other materials that involved semi-conductive junctions included silicon, germanium, and selenium.

Click for Speach from ShockleyOn Tuesday, December 16, 1947, Physicists John Bardeen and Walter Brattain, while working for William B. Shockley at Bell Laboratories, invented the semiconductor transistor. With the single statement "This thing's got gain!", Brattain announced the discovery of a SOLID-STATE device that could actually amplify electrical current!. By 1956 Bardeen, Shockley, and Brattain, shared the Nobel Prize for jointly inventing the transistor. It was a grand time for the electronics industry. The invention of the transistor would mean the disappearance of the tube... or would it?

Still to this day, there are applications where the transistor isn't practical. Tubes tend to work better for high power or high frequency applications. It is a tube that you look at on your computer monitor (CRT = Cathode Ray Tube). Your TV set and microwave ovens employ tube technology. Most Television Transmitters use a tube as their final output stage. "Nanotubes" are the latest craze in electronic innovation. Audiophiles swear by the rich, full, warm, reproductive sound that can only tubes can make in audio circuits. It is my belief that we need to further understand both tube and transistor technologies, and use them in tandem. It is for this reason that I taught tubes first, and that I will emphasize them throughout the rest of the course.

But now let's take a look at semiconductors: 


The Basics of Semiconductive Materials







Without going into complete details of how a semiconductor works, there are certain things we must know about them in order to use them. Semiconductors are chemical elements, that when compounded with other elements, have certain electrical characteristics.


¿what?

There are several types of semiconductive chemicals, to include but not limited to, Silicon, Germanium, Selenium, and Copper Oxide. Semiconductors do not normally conduct electrical current. But when they are combined with other chemicals, like Boron or Arsenic, can be made to partially conduct. When we combine the second chemical to the semiconductive chemical, we say that the semiconductor is "doped". Doping can either be negative doping, or positive doping. The real magic occurs, though, when we put the two types together.

In the diagram to the right, we see a device with both a positively and negatively doped section joined. The point at which the two sections join, we will call the N - P junction ( or simply the junction ).

Think of the junction as a hill, and the electrons flowing through it as a ball. As long as the ball is rolling down hill, it is easy to push along. But try pushing the ball uphill, and it is much harder to do. The same goes with the flow of electricity through a junction of P and N doped semiconductor material.



Consider, for a moment, what happens if we connect a negative DC voltage to the N doped side, and a positive DC voltage to the P doped side. According to the electron theory of current flow, electrons move from negative to positive. The electrons leave from the negative side of the battery, moving toward the positive. They come into contact with the diode, which acts like a "hill" to the electrons. The electrons flow "downhill", and current flows easily. But what happens if we reverse the direction of current flow by reversing the battery?



If the battery is reversed, the polarity applied to the diode also changes. Electrons still try to flow from negative to positive, however, going through the diode, is more like rolling a ball up-hill. It takes much more effort to push the ball uphill. The hill is steepest at the point of the P - N Junction, where it is nearly impossible to push the ball up the hill.

Because of this, a semiconductor diode acts much like a vacuum tube diode, as it conducts in one direction, while having a high resistance to current flow in the opposite direction.


It is possible, however, to make a diode conduct electricity in reverse. If a high enough voltage is applied across the junction ( which is also sometimes called the "depletion region" ), it will conduct in the reverse direction. Just like if you kick the ball hard enough, it will eventually go over the top of the hill. However, when this happens, the diode is no longer acting quite like a normal diode.

Some diodes are actually designed to be operated in this manner. These are called " ZENER DIODES ". When an exact given reverse bias voltage ( 12 Volts for instance ) is reached, the junction of the diode begins to " break down ", and act like a piece of wire. It does not conduct electricity only in one direction, but in both directions at this point.


The semiconductor diode ( usually just called a " diode " ), is one of the most important building blocks of modern electronics.

To the right is a picture and the schematic symbol for the diode. Note that there is always a line, or band that circles around one end of the diode. This line indicates which end of the diode the cathode, or positive end is on. Sometimes instead of a line, there is a dot or some other kind of indicating marker, but the cathode is always indicated, as the proper polarity of a diode is crucial in a circuit.

In addition to what you've already learned about semiconductor diodes, most semiconductor devices have other properties that we have not discussed. For instance, many semiconductor materials will radiate light when electricity is passing through it. This is the principle behind how a Light Emitting Diode, or LED (schematic to the right) works. The first case of this may be by H. J. Rounds, an employee of Marconi Labs, who in 1907 made note of a light coming from a Cat's Whisker connected to a crystal of silicon carbide.

While knowledge of this wasn't kept a secret, and experimentation was done on it for years, it wouldn't be until 1962 when General Electric employee Nick Holonyak made a practical LED public, and from there it took off. Light Emitting Diodes generate more light than heat, so unlike a standard incandescent lamp, the LED used less electricity. This made it ideal for battery operated devices that needed some kind of light display. In addition, because it was so small, several of them could be put in an array, making a display for characters (numbers and letters) more readily available.

This gives rise to the infamous "Seven Segment Display". The seven segment display is used on all kinds of electronic equipment to display basic letters and numbers.

It works by grounding one common leg of the display, while supplying voltage (typically 5 volts) to other various legs. For instance, in the display to the left, if we wanted to create the number 3, we would ground pins 4 & 12, while supplying power to pins 2,7,8,13, and 14, which would light up segments A, B, C, D, and G.

Another interesting feature of semiconductors, although not un-related, is that it will conduct and/or generate electricity when light is applied to it. this is the basis for the photoelectric cell (photocell - you may relate to the Solar Cell). We will discuss this in more detail in the section on transistors.

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Circuits Circuits Everywhere!

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In the last section, we saw how a very simple transmitter worked. It was made up of several different types of electronic components, including capacitors, transistors, resistors, etc. When we assemble several types of electronic components in a configuration that serves some purpose, we call it a CIRCUIT.

Some common electronic components are:
Wire
Resistors
Capacitors
Coils
Transformers
Tubes
Transistors
Diodes

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The Triode as Applied to a Circuit

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Knowing what a diode is, or how a triode works is of little use unless you have some practical knowledge of how it can be applied within a circuit. We are going to begin with a VERY basic schematic of an early transmitter. Do NOT try to build this at home! It probably won't work, and you may violate Federal Laws ( FCC regulations ) or injure yourself in the process. I will let you know when it is time to begin building projects. ( Yes, this course will come with a practicum ). I will also begin to explain the theory and operation of some new components.

When ever we are looking at a schematic diagram, we must look at it from 2 directions. Top to Bottom, and Left to Right:

First: No circuit can operate without some kind of Power, In this schematic, we have 2 batteries shown operating the circuit.

    While it may* be true that electrons flow from negative to positive, it is usually easier to understand movement of voltage through a circuit using the Conventional current flow theory, otherwise known as the Franklin theory, which states that electricity flows from positive to negative. When looking at a schematic, it is customary to start with the highest positive voltage, which is normally at the top of the diagram, and work your way down to the lowest voltage, usually ground, which is located at the bottom of the diagram, removing voltage with every resistance (voltage drop). You could begin at ground and work your way up, but that would require "voltage raises" being encountered every time you run into a resistance - which frankly - while being technically correct - doesn't make a lick of sense to the average human being.


Second: The purpose of most circuits is to move some kind of signal. Signal flow normally goes from Left to Right.

    Signals can be anything from basic AC sine waves, to audio, video, radio, or data signals. They normall begin with some signal source on the left, and move to the right.


Before you can fully understand the circuit, I will have to give you a brief introduction to microphone construction and theory.

In the following illustration, I show a breakdown of how 3 different types of microphones work. In all three types, when we speak into the microphone, the sound waves from your voice create vibrations in the air. When these sound waves reach the microphone, they vibrate the diaphram of the microphone, which is usually a very thin layer of flexible plastic. This diaphram works like the eardrum in your ear, and vibrates back and forth at the same frequency ( pitch ) and amplitude ( volume level ) of the sound waves that stimulate it. The diaphram in all three types move left and right as shown. The third pictures a 3 dimensional representation of a magnetic type microphone for ease of understanding.

In the case of the Condenser type, a condenser is basically a capacitor. Here we have 2 electrodes, separated by an air dialectric. When the diaphram moves, it moves one of the plates, which changes the distance between the plates. Since the distance between the plates governs the capacitance of a capacitor, by moving one plate, we change the capacitance of the capacitor. The changes in capacitance are directly proportional to the frequency and amplitude of the sound waves picked up by the diaphram. So the electrical signal coming out of the microphone reflects the sound waves picked up by the diaphram.

Carbon microphones work in similar fashion to Condensor microphones, except that instead of an air dialectric, we have carbon, which when compressed, changes its resistance instead of its capacitance. In short, we have a variable resistor, which changes reflect the incoming sound.

Finally, a dynamic, or magnetic microphone has a moving coil, which is moved by the diaphram when it vibrates. The hair like strands of wire in the coil move back and forth with the incoming sound, cutting the magnetic field of the permanent magnet. As you recall, when a wire cuts the field of a magnet, an electrical current is induced in the wire. When the coil moves, it generates a small electric current, which changes with the incoming sound.

Many microphones, such as the Carbon and Condenser type require power, whether via battery or "phantom power" to operate. Because a Dynamic microphone actually generates an AC current, it needs no outside power source.

Now let's get back to our diagram. Here we actually have 3 circuits. Let's begin with the first, and simplest, which I hope will allow you to get the idea of how to read a schematic diagram. On the far left of the diagram is this small section, which includes 3 components. There is a microphone on the left, a coil on the right and a battery at the bottom. The battery provides power for the microphone to operate.

The positive side of the battery is connected to the microphone directly. The negative side passes DC current through the coil to the other side of the microphone. Think of the microphone as a very thin membraned capacitor, with air acting as a dilectric. As we speak into the microphone, the vibration of the sound waves from our voice generates an alternating current, which varies directly in amplitude ( volume level )and frequency ( pitch ) with the changes of our voice. This alternating current is applied to the coil, causing it to set up an induced electromagnetic field, which rises and colapses, changing along with our speach.

Now on to the second part of the circuit... the Tube Amplifier! First let's go over power, then signal. The tube, being the active device in this circuit, is powered by the battery. The Positive side of the battery produces a positive voltage, which passes through the plate coil to the PLATE of the Tube. The Plate, having a positive potential, is ready to begin attracting electrons. We'll say we have a BIG battery, and it puts out 200 Volts.

The CATHODE of the tube is directly connected to the negative side of the battery, and begins to emit an electron cloud, which is attracted to and caught by the PLATE. We now have a flow of current from CATHODE to PLATE. Note that on the bottom right hand corner of our diagram we have another symbol ( 3 horizontal lines gradually getting smaller ). This is the symbol for the "ground" potential of the chassis, and is usually considered to be "0 Volts".


ADVANCED CONCEPT

Now at this point, you might ask, "What is the voltage on the Control Grid?" This would be a very good question. Because of the combined resistances of the grid coil and grid resister, the grid must be at some voltage higher than that of the cathode. So if we say that the Plate is at 200 Volts, and the Cathode is at 0 volts, the grid must be somewhere in between. Another way of saying this is that the grid is positive with respect to the cathode. But because a coil has very little resistance at DC (battery), it wouldn't be TOO much higher than the 0 Volt Anode voltage. We'll estimate and say 1 or 2 volts.
END OF ADVANCED CONCEPT



Now we have 2 more terms to learn:


We now can say that the tube is working, but it is just sort of idling along, sort of like a car sitting at a traffic light. The motor is running, but it isn't going anywhere. When a tube has the proper voltages to operate, we say that it is "properly biased", and BIAS means voltage in most electronics work.

Also, whenever a tube is running, but in an idle state, with no signal applied, we say that it is in its QUIESCENT state. ( think "quiet" ).

Now let's apply a signal and see what happens. The signal from the microphone circuit is magnetically coupled to the amplifier circuit by way of 2 coils - the microphone coil, and the grid coil.

The mike coil is physically placed very close to the grid coil. When two coils are so close, that the magnetic field from the first coil is induced upon the second coil, we have what is called a TRANSFORMER.

Often 2 coils are actually wound on the same coil form. These coil forms can have an air dilectric, or may be ferrite ( iron ) based, which increases the amount of energy transferred from one coil to the other.

With a signal applied across the grid coil, it travels along the wire to the grid. The signal voltage alters the grid voltage at the rate of frequency and amplitude that is picked up by the microphone. This in turn increases and decreases the current which moves from the anode to the plate of the tube as described in the previous section. The output from the tube is an exact replica of the input of the microphone, except that it is greatly amplified ( made louder ), and is flipped to look like a mirror image.

So now that we have an amplified signal, what are the other components in the circuit for? Let's go over them one by one:

The Grid Biasing Resistor:
The grid resistor sets the bias voltage for the grid, so that it never reaches the two taboo states - cutoff and saturation. When a tube is in cutoff, it doesn't have current flow from anode to plate. When it is saturation, any change in signal at the grid won't be seen at the plate, because it will be at maximum current flow constantly. Both of these conditions can cause distortion of the signal, and are usually* unwanted. It is for this reason that we need to control the bias voltage to the grid by using a grid bias resistor.

The Plate Coil: Why didn't we simply connect the positive terminal of the battery directly to the plate? Answer: Because the output signal of the tube would then affect the battery current, which would affect the battery voltage, which would affect the tube operation. Bad Scene! Recall that a coil PASSES low frequencies ( D. C. is about as low as you can get ), but blocks the affects of AC. By using a plate coil, we can freely pass the DC to bias the plate, without the output signal affecting our power supply.

The Plate Coupling Capacitor: The purpose of this guy is simply to block the DC from the power supply from getting into the next circuit, while allowing the AC signal to pass on. Hence, it is called a "coupling capacitor", because it couples 2 circuits together.

After the coupling capacitor, we have the final output section of our transmitter. In the output section, there is a radio frequency signal generator, a transformer, and an antenna. Our amplified audio signal is coupled to the "final" section by way of the coupling capacitor. It then goes into the antenna output transformer. On the other side of the transformer is our radio frequency generator. The radio frequency generator creates a radio signal ( we won't go into how just yet ), which is coupled across the transformer to the audio side. The two signals mix in the transformer, and are sent out the antenna into the atmosphere. Now let's review:










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A Short Review-earlier lesson

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    Thomas Edison, famous (at least in America), for inventing the light bulb, See Note 1made many discoveries before he completed his task of lighting the path of the world. Along the way, he incidentally noted that if a filament were energized within a vacuum, that after time, a "shadow" would be left on the inside of the glass, which resembled the shape of the filament. He surmised from this, that within a vacuum, particles (we now call them electrons) were emitted around the wire, forming a cloud, or SPACE CHARGE. This effect became known as the EDISON EFFECT, which is the basic operating theory behind all vacuum tubes.
Later, J. Ambrose Fleming invented the FLEMING VALVE, when he noticed that a second ELEMENT, or ELECTRODE within the vacuum along with the filament, but not touching it, electricity would flow through the vacuum and be collected on the second element. The second element was called a PLATE. He further noted that electricity would flow from the filament to the plate, but not in the opposite direction. The Fleming valve was later dubbed the DIODE, because it has 2 elements inside the vacuum - the filament and the plate.

The AUDION came about when Lee DeForest, In 1906, added a 3rd element between the two. This third element, a control grid, allowed one to electronically control the output of the tube based directly upon the input. This was the birth of Amplification. The term AUDION was later replaced by the term TRIODE, as the tube has 3 elements within the vacuum.

The reason why a tube works is because the CATHODE is heated to a point of THERMONIC EMISSION, forming a SPACE CHARGE or cloud of electrons, which is attracted to the positive charge on the PLATE or ANODE. The Cathode ( K ) of a tube can be either directly or indirectly heated.


Diode tubes only allow electrical current to flow in one direction. By using a diode tube we can change, or RECTIFY, Alternating Current into Direct Current.

An X / Y plot of Voltage vs. Current produces the CHARACTERISTIC CURVE of a device. We looked briefly at the characteristic curve of a DIODE tube. Now let's look, for a moment, at the curve for a resistor.

Remember on page 8, we discussed Ohm's law. We used a 10 Ω resistor for an example. When we applied a 100 volt source to the resistor, 10 Amps would flow through it. What happens, though, if we reduce the voltage to 10 volts?



10 Volts / 10 Ω = Click for Answer



Now let's try another voltage value.

50 Volts / 10 Ω = Click for Answer



Finally, what happens if we apply a Zero volt source to the resistor?

0 Volts / 10 Ω = Click for Answer
If we plot this change on a chart, we will find that a resistor has a characteristic curve which equals a straight line. Not all resistors will be an exact 45� angle. The angle of the line will depend on the value of the resistor. However the characteristic curve of all resistors will be a straight line if plotted voltage vs. current.






Note 1:

Not to incur the wrath of Edison haters or Tesla enthusiests - Tesla is indeed the inventer of many things - including the AC power that the Edison Light bulb presently runs on. However, Tesla did not invent everything, and absolutely did not invent the vacuum tube or light bulb. Of noteworthy mention is the work of Swan in England, upon which many of Edison's experiments were built. In the end - we all build on the work of others, and stand on the shoulders of giants. Tesla was a giant, as was Swan, Edison, Fleming, Deforest, Ohm, and many others, and all built on knowledge obtained through the works of others. Dare I say Newton?


The TRY - ode


Up to now, you have learned something about the diode vacuum tube and how it works. The most important point to rember about the diode is that it only allows current to flow IN ONE DIRECTION.


Thus, when a diode is connected in series with other circuit components, current can flow in only one dirction in all of these components. This characteristic makes the diode a useful device for "rectifying", or converting AC into DC.



You have learned that the diode vacuum tube has only 2 electrodes - a cathode, and a plate (or anode). We briefly hinted that there were other types of tubes tried over the years, with more electrodes added. In 1906, Lee DeForest developed the Audion, later called the Triode because it had 3 electrodes

The third electrode, called the Grid, was placed between the cathode and the plate. The Grid was a piece of wire mesh, coil, perferated metal, or other shape that would allow electrons to pass through it. Physically, the Grid is much closer to the cathode than it is to the plate.

The purpose of the Grid, is to offer a way to control the flow of electrons to the plate. For this reason, the Grid is sometimes referred to as the "Control Grid". Furthermore, in jolly old England - a tube is often referred to as a Valve.

The question you are probably asking at this point is, "How does it work?"

During normal operation, the Plate is kept at a Positive DC potential in relation to the Cathode, so that it always attracts electrons. ( memory note: P = P.... Plate = Positive). The Negative electrons being boiled off the Cathode, are attracted to the Positive plate, and begin to flow in that direction. The more Positive the Plate is, the greater the attraction, and the more electrons flow. We will assume, however, for the sake of our discussion, that we have a fixed high Positive voltage on the plate, and a fixed Negative voltage on the Cathode. We have current flowing from the Cathode to the Plate at a fixed rate.

Now we apply a voltage to the Grid. If we apply a small Positive voltage, electrons flow from the cathode toward the grid. Since the Grid voltage is small, and the Plate voltage is large, the electrons continue past the Grid on to the Plate. The Grid, being closer to the Cathode than the Plate is, gets the electrons moving in the direction of the Plate, and sort of helps them along their way. Because of this, more current flows to the Plate with a Positive Grid than with an un-energized Grid or no Grid at all.

If, however, we apply a Negative voltage to the grid, it creates a Negative field between the Cathode and the Plate. This field restricts the flow of electrons moving to the plate. Sort of like pinching a garden hose. The tighter we pinch it, the less water flow there is. The same is with the Control Grid of an electron tube. The more Negative we swing the grid, the less current flows to (and the fewer electrons fly at) the plate.

So we find, then, that when the Grid Voltage swings Positive, Current flow is increased at the Plate, and when the Grid Voltage swings Negative, the Current flow is decreased at the Plate. Speaking "mathematically", we would say that the "Grid Voltage is directly proportional to the Plate Current ". In plain english, we can say that we can control the CURRENT of the PLATE, by changing the VOLTAGE of the GRID.

So then, what happens when we apply an ALTERNATING current to the grid?

If we place an AC signal on the Control Grid of a triode, the signal swings from Positive, to Negative, then back to Positive. As it does so, the Plate Current swings directly with the Grid Voltage. If we have a fixed resistance load ( a resistor ) across the output of the Plate, we will notice that when the Plate Current goes High, the Plate Voltage goes Low. As the Plate Current goes Low, the Plate Voltage goes High. ( Ohms Law applies ... E=IR ).

So if we compare, when the GRID Voltage swings High.... the PLATE Current swings Low, and by virtue of a fixed output resistance and Ohm's law, the PLATE Voltage also swings Low. The Voltage at the Plate swings OPPOSITE the voltage of the Grid. The Output of the Plate will look like a mirror image of the Input to the Grid.
 
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Dangerous Curves

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As discussed in the previous lesson, a Characteristic Curve is found by applying several different voltage levels, and measuring plate voltages vs. plate current. We note that in a diode, if we go below a certain plate voltage, ( in this case 0 volts ) no plate current flows. The minimum point at which the tube no longer operates is called the CUTOFF POINT . Above a certain plate voltage, additional plate voltage has very little effect in increasing the plate current. The maximum point where raising the plate voltage no longer increases current is called the SATURATION POINT .

In reality, there are two different factors involved in the control of the amplitude ( or level ) of the plate current that flows in a diode. These are the filament voltage ( sometimes called the heater voltage ), and the plate voltage.

Remember that the cathode must be HEATED into thermionic emission. The temperature of the cathode must be high enough to "boil" the electrons from its surface. It stands to reason, that the higher the temperature we heat the cathode to, the more electrons will be "boiled off". Much like raising the temperature of a pan of water causes it to boil away into steam faster.


ADVANCED CONCEPT

There does come a point though, where we can boil the electrons off no faster. As we raise the voltage on the heater, it will actually begin to slow down the movement of electrons toward the plate, and begin drawing them toward the heater itself. The underlying reason for this is the space charge itself. As the electrons get boiled off, it causes the overall electrical charge of the cloud of electrons to grow.

At some point, the cloud of electrons reaches a high enough negative charge that it repels any new electrons being boiled off, and they return back to the filament - or have to overcome so much of an opposing force from the cloud that they simply never leave it at all. Therefore, when setting up a tube for operation, you want to make sure that you don't set the filament voltage of the tube so high that it will cause this effect, as it will reduce the efficiency, as well as the life expectancy of the tube. Most tubes are operated at standard values of 6 or 12 Volts.

It is typically considered good practice to begin with a new tube somewhat under that (say 5.0 or 5.5 volts assuming a 6 volt tube) and run it that way until the tube gets older and begins to soften. Often, if a tube gets weak and doesn't output its rated power anymore, you can boost, say a 6 Volt filament to run as high as 7.5 or even 8 Volts. This may extend the life of the tube for a while, but you must watch the plate current. If plate current begins to drop off, then you have the filament voltage set too high. For longest life of a tube, the rule of thumb is to run the tube at the lowest filament voltage that will make the current necessary for proper operation of the device. This maximizes the life expectancy of the tube.

Now the question might arise in one's mind, "Why on earth would someone go through all that effort to keep the $10 tube of a guitar amplifier a few more months?" The answer is that they wouldn't. However - If we are talking about a $30,000-$60,000 final output tube of a broadcast transmitter - the picture changes quite a bit. I've taken tubes that were rated to last 5-7 years, and extended their lives to 10-15 years by this method. And if you can squeeze 3 more years out of a $60,000 tube... you've saved the company thousands of dollars. Companies tend to be grateful for that and reward you with a higher paycheck.

END OF ADVANCED CONCEPT



Assuming that the filament voltage of the tube is set, and that we are running our tube into a fixed resistance load, we can increase the plate current by increasing the plate voltage. Normally when plotting (or drawing) an operational curve for a tube, we assume that the filament is held constant, and the plate voltage is raised. As the plate voltage rises, so does the plate current. We do note, however, that there is a minimum and maximum point, at which the curve is no longer linear ( in a straight line ). We call the minimum point the "lower knee" and the maximum point the "upper knee" of the curve. The saturation point occurs at the beginning of the upper knee, while the cut-off point occurs at the beginning of the lower knee. Under normal conditions, we usually operate the tube within the linear portion of the curve.

Later, we will discuss the characteristic curves of various other components. Each type of component has a slightly different curve, which dictates how the component will operate under different conditions. Understanding these curves will give you a more thorough knowledge of how the component works, and insight as to what it will do when it fails.
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Vacuum Tubes: A Historical (Hysterical ?) Overview

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The vacuum tube, in its very primitive form, evolved from the light bulb. Invented by Thomas A Edison in 1883, the incandescent lamp, had 3 basic necessities to operate:
(refer to fig. 1.1)

The Envelope
The Filament
The Vacuum


figure 1.1


Many of life's failures, are people who did not realise how close they were to success when they gave up... ... ... Thomas Edison. The envelope is basically a sealed container, a box or jar so to speak, which completely surrounds (envelopes) whatever is inside. The first envelopes were made of glass, however, there was no written law that they must be made of glass. In fact, many modern tubes have metal and/or ceramic envelopes.

The filament, otherwise known as the heater, was the basis of the light bulb. The idea was that if a high enough electrical current flows through a coil of wire, it generates light (and heat). Edison's object, was to create a thin enough piece of wire, that even a very low current could generate a great amount of light. The problem was that he kept burning up the filaments. They would work for a matter of seconds, then die out. He experimented with many different filament materials. Finally he found a metal material that would last - tungsten. Most modern filaments are made up of a thoriated tungsten material.

The vacuum was added along the way, as an attempt to keep the filament from burning out. It was logical, that in order for fire to exist, you must have oxygen. Edison assumed that if all the oxygen were removed from the envelope, by creating a vacuum, the filaments would stop burning up. It helped, but was not the total solution to the problem.


He did find, however, that if a filament were energized within a vacuum, that after time, a "shadow" would be left on the inside of the glass, which resembled the shape of the filament. He surmised from this, that within a vacuum, particles (we now call them electrons) were emitted around the wire, forming a cloud, or SPACE CHARGE.
(refer to fig. 1.2).

This effect became known as the EDISON EFFECT, which is the basic operating theory behind all vacuum tubes.
fig 1.2


During his experimentation on the electric light bulb, Edison found that many metallic substances will emit electrons when heated to incandescence. In a light bulb, these emitted electrons become waste, as they serve no useful purpose. The vacuum tube is, however designed to make use of these emitted electrons. Edison experimented by placing a second ELEMENT, or ELECTRODE within the vacuum along with the filament, but not touching it. He then connected an ammeter to the second element, and attatched the other lead of the ammeter to the positive terminal of the battery. He found that when doing this, current would flow through the ammeter. The second element is called the PLATE
(refer to fig. 1.3).
fig 1.3

The emitted cloud of electrons, bearing a negative charge, is attracted to the positively charged plate. It flows through the vacuum toward the plate and is collected upon its surface. This action was monitored and proven by use of the ammeter. But what happens if the plate is connected to the negative side of the battery? Edison discovered that when this is done, NO current flows through the ammeter. So electricity flows, within the vacuum, in one direction only - from Negative to Positive. This was in direct contradiction to Benjamin Franklin's conventional theory, that electricity, being a fluid (much like water), flowed from positive (a full glass) to negative (an empty glass).

Edison further reasoned that since, with the polarity reversed, the negative particles of electricity didn't flow from the plate to the filament, that there must be some outside force causing the electrons to leave the filament. He discovered that while he was working with a heated filament, the plate was not heated. The heat of the filament caused the electrons to be "boiled" off, and freed from the solid matter of the filament into the surrounding vacuum. Once the electrons were freed from the confines of the solid matter, they could be attracted to any positively charged source within the vacuum.

This is known as THERMIONIC EMISSION, which is the process of the electrons being forced out of the solid metal via thermal agitation.

fig 1.3


This picture of J. Ambrose Fleming was borrowed from elsewhere on the internet.  It will be replaced shortly. This is the basic concept of the FLEMING VALVE invented by J. Ambrose Fleming in 1904. It was noted that since electricity flowed within a vacuum tube in one direction only - from the filament to the positively charged plate, it was as if there was a 'one way valve' placed in the circuit. By this method, a direct current (DC) charge, formerly only available by chemical production through a battery, could now be converted from an alternating current (AC) source. This outstanding development was called RECTIFICATION and the Fleming Valve was known as a DIODE (two element) RECTIFIER. It wouldn't be until 44 years later that the crew at Bell Labs would recreate this effect using semiconductor materials.

fig 1.4


This picture of Lee Deforest was borrowed from elsewhere on the internet.  It will be replaced shortly. The AUDION came about when Lee DeForest, In 1906, added a 3rd element between the two. This third element, a control grid, allowed one to electronically control the output of the tube based directly upon the input. Along with the ability to control the output, came the ability to AMPLIFY the output as well. A small signal could be injected at the input of the tube, resulting in a very large signal at the output of the tube. Electronics was about to take on a whole new role in life, as radio as we know it would now soon be born. The term AUDION was later replaced by the term TRIODE, as the tube has 3 elements within the vacuum.


Later improvements included the adding of 2 more elements, the supressor and accelerator grid, which allowed higher frequency operation, increased stability, and eliminated unwanted oscillation. The 4 element tube was called a TETRODE and the 5 element tube was called a PENTODE.

The biggest problem in tube design came when trying to reach higher power levels, at higher frequencies. The higher the frequency, the tighter the tolerances became.






In an effort to overcome this problem, the BEAM POWER TUBE was developed. This tube was special, in that it FOCUSED a BEAM of electrons, rather than simply creating a cloud of electrons boiled off the cathode. This illustration was adapted from an old (circa 1955) RCA (c) Recieving Tube manual

The beam is focused by applying a sufficiently high negative potential to repel the electrons being boiled off the cathode. At the same time the highly positive plate is attracting the negatively charged electrons.

This focused beam of electrons places more energy directly on the plate, eliminating losses, and allowing for better heat distribution.

This is a Harris 5 cavity Klystron Transmitter ( Steamer Type ) at a TV station I used to be Chief Engineer at.

Even today, in the age of the semiconductor, we can not do without tubes. This is why I insist that we still study them. They are still (as of the year 2000) used in Televisions, Computer Monitors, Microwave Ovens, Medical Equipment, Radar, Transmitters, and many other phases of high tech electronics.

We use some tubes, such as the big red ones pictured above, that are as large as a man. There is also a new wave of "nanotube" technology which might be worth riding. The point is, that tubes are not dead, nor will they be for quite some time, and should be taught as a viable technology.

As stated before, this is only meant to be a historical overview. Now we will get into the detailed theory of how each of these tubes operate. 

Tube Theory - "ODE" to Electronics


The "Ode to Electronics" would have to be the vacuum tube. This is because you will learn lots of "odes" in tube theory: Electrodes, Diodes, Triodes, Tetrodes, and Pentodes, just to name a few.

Before we go too far, we'll have to learn what an electrode is:

An Electrode is a conductor which permits current to flow. Remember Frankenstein's Laboratory? It had the two round balls with the high voltage applied, and you saw the electricity arcing across from one ball to the other? These were electrodes, and they allowed current to flow from one ball to the other through the air. Electrodes are always used in pairs. You could say that the big alligator clamps on your automobiles jumper cables are electrodes, as are the little pads the doctor puts on your chest when he hooks you up to an EKG. Although I'd rather that the doctor use the little pads than the jumper cables!

In tubes, the electrodes are elements within the vacuum which emit, collect or control the flow of current within the tube.

The simplest of tubes has only 2 elements:

The Cathode, and the Anode.


 


Looking at the pictures above, the picture on the left is a graphical representation of a "diode" or 2 element tube. It has a CATHODE (K), which emits electrons, and an ANODE (A), otherwise known as a PLATE (P), which collects the emitted electrons, allowing current to flow.

The picture on the right is a schematic representation of the same. The circle represents the glass envelope, and the elements are contained within. The Cathode shown here looks like an inverted "V", and the Plate looks like, well, a flat "plate" of metal.

What you see in the schematic on the right above is actually two circuits combined. We will now break these two circuits down, to simplify and show what is happening in each circuit.

Keep in mind that there are multiple theories on the flow of electricity. We will use the electron theory (negative to positive) first to describe how current flows through the tube. However, whenever describing how power gets to the tube, it may sometimes be easier to think in terms of conventional flow from positive to negative.

In the schematic to the left, arrows show the flow of electrons from the negative battery terminal, through the cathode of the tube, and back to the positive terminal of the battery to complete the circuit. The cathode gets red hot and glows. It gets so hot, that some of the electrons are thermionically emitted into the vacuum space directly around it, as shown by the little specks in the picture.

The Cathode, in this case, is directly heated by the high flow of electron current through it. An electrode which is directly heated in this manner is also called a heater or filament. Not all filaments or heaters are cathodes, and not all cathodes are heaters or filaments.

When a cathode is DIRECTLY heated, as this one is, then it is a heater / filament / cathode. Otherwise, tubes may have separate heaters & cathodes, in which case we say the cathode is INDIRECTLY heated.
Below are illustrations of both types of cathodes, so that you may understand them better.



As you can see from the schematic diagram, the circuit operates in the same manner, except the heater is separate from the cathode. This is done for several reasons, including increased reliability and longer life, less interference between signal and power supply stages, lower resistance, higher frequencies can be obtained, and ease of design.

Understanding the left side of the schematic gives us some insight to what happens when we apply voltage to the plate.

By applying a positive voltage to the plate, a current can be observed on the Current Meter ( also called an Ammeter ), indicating that the circuit is complete between the two terminals of the battery. So by the indication of current flow on the Ammeter, we can assume that there is a complete loop formed via the plate! The electrons leave the negative side of the battery, and are emitted by the cathode of the tube. They are then collected by the anode (plate) and returned to the positive side of the battery. Edison noted that if the polarity of this battery is reversed, so that a negative voltage is applied to the plate, no current flowed. It was surmised from this, that current only flows in one direction within a vacuum - from negative to positive.

Assuming you are craft and oriented, you could actually use the knowledge in this page to begin building your own tubes. Let's face it - the first tubes were ALL hand made. That being said, I do not send you to other people's pages often, but this one may be of interest to those of you wanting to try your hand at building your own tubes. The man's name is Claude Pailliard, and his website is primarily in Italian. However, all the necessary info/parameters can be found there, and at the bottom of his Main Page you can find a video of him actually producing tubes in his home lab. Very interesting.

Tube Theory - The Diode


  


Both of the schematics above show the operation of a diode tube. Now let's study a little about the theory of its use.

While not the only use for a diode, the most common use is that of RECTIFICATION . Rectification is just a big fancy word for changing Alternating Current into Direct Current. A RECTIFIER turns AC into DC, and a diode is an excellent rectifier.

This is because it only allows electrical current to pass through the plate circuit in one direction.

Let us examine what happens when we apply an alternating current to the plate circuit.

We replace the battery in the plate circuit with an AC generator. The AC generator creates electrical voltage which swings from a positive to a negative potential each cycle. When the generator's terminal on the cathode side swings negative, the the one on the plate side swings positive. This energises the tube such that the cathode is negative and the plate is positive. The electrons floating about in the electron cloud are repelled by the negative cathode. At the same time, they are attracted to the positive potential of the plate. For this reason, they travel through the vacuum to the plate. These electrons then go through the Ammeter, making it read current flow, and continue onward to the load resistor. Finally they reach the most positive point in the circuit - the positive terminal of the battery. They are drawn to the positive, much like a South pole magnet is drawn to a North pole magnet, and they will move toward each other until they meet.

When, however, the AC generator reverses (alternates) its polarity, such that the generator's cathode side terminal swings positive, and the plate side swings negative - look what
happens !!

With the cathode being positive, the electron cloud collapses, and no electrons are present to cross over to the plate. Because the plate is not heated to thermonic emission, it does not radiate electrons, and so will not allow current to flow. Current flow stops at the plate.

Therefore, in a vacuum tube, ELECTRICITY CAN ONLY FLOW IN ONE DIRECTION

If we draw a graph, indicating voltages fed to the cathode vs current flow monitored at the plate, we will see a pattern. Electricity only flows on the positive cycle of the AC waveform. While the input swings both positive and negative, the output fluctuates from 0 volts to some positive number of volts. In effect, the input is Alternating between positive and negative (AC) but the output is positive only (DC). We say that the output of the tube has been rectified. It is, however pulses of DC, and for most general purposes, useless until we clean it up. This IS however, the basis of EVERY POWER SUPPLY in every piece of electronic equipment you own.

The Characteristic Curve of the diode is found by applying several different voltage levels, and measuring plate voltages vs. plate current. We note that below a certain plate voltage, ( in this case 0 volts ) no plate current flows. The minimum point at which the tube no longer operates is called the CUTOFF POINT . Above a certain plate voltage, additional plate voltage has very little effect in increasing the plate current. The maximum point where raising the plate voltage no longer increases current is called the SATURATION POINT .


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