Download Basic Electronics to Test Fixtures

Atomic Structure (a review)
Why can't we see an atom if we have a powerful enough microscope?
It's almost improper to say we see matter. What we see is reflected energy. When
you hit a bell, it resonates at a frequency depending on the properties of the bell. When
radiated energy (photons) hit an atom, the atom resonates at a frequency (or frequencies)
also. There is a narrow band of frequencies that we can see from about 400 nm to 700 nm
in wavelength. Below this frequency is Infrared, energy we can perceive as heat, and
radio waves. Above this frequency is Ultraviolet, and beyond.
When a wave of photons hit a group of atoms, the electrons are raised to higher
energy levels. Energy is stored in the electron at a higher energy level. Eventually the
electron acquires an excessive amount of energy than can be retained. The electron gives
off the excess amount of energy, which gets radiated out (as photons or phonons), and
falls back to the lower energy level it should be. This process is repeated endlessly. The
rate of absorption and radiation of energy gives the radiated energy a frequency. Often
that frequency falls into the spectrum of frequencies we see as visible light. Now that we
know how we see, let's see what there is to see.
An atom is made up of a cloud of electrons surrounding a nucleus. The nucleus is
made up of Protons and Neutrons, which are made up of quarks, which are made up of...
(We may never know the answer to this endless question). An atom so small, it is on the
edge of imagination. The nucleus of the atom is smaller, and electrons are smaller still.
The electrons are made of not much more than energy, themselves. They have almost no
mass, but a predictable amount of energy. The electrons are in motion around the
nucleus, traveling at about 300,000 km per second. When you take into consideration
how small the atom is, the electron is making a phenomenal number of revolutions per
second. The question of where is the atom at any given instant is meaningless because we
can't define an instant small enough to say when the electron is at any instant. The
Uncertainty Principle may as well be a poem.
Most books show a picture of an atom with the electron in close proximity to the
nucleus. A more realistic drawing would be difficult to put on a page. Speaking in nonspecifics, if the nucleus were the size of a quarter, the first electron shell would be about
a hundred meters away. Specifics would depend on Temperature, Pressure, and Gravity.
The orbit (not the best word to choose) of one electron around an atom is roughly
spherical, as with Hydrogen. In Helium, with two electrons in the first shell, the orbits of
the two electrons take on the shape of a fat ice cream cones, opposite on another. The
atom takes on a shape somewhat like an hourglass. This atom is constantly tumbling in a
random pattern, influenced by external forces. The nucleus also is doing a random tumble
with the protons rotating to chase the position of the electrons. In the next more complex
atom, Lithium, the electron shell has three electrons. Since the first shell can only have
two electrons, the third electron starts another shell. This orbit takes a toroid shape
around the middle of the hourglass. This shell is also influenced at any given instance by
where the electrons are in the inner shell, as well as external forces. As more complex
structures are formed, the shape takes on more complex configurations.
The atom is primarily nothing but empty space, made of unimaginably small
particles with relatively great distances between them.
These great distances are filled with absolutely nothing we can perceive as stable
1
matter or energy.
What is there to see?
Conductors, Insulators, and Semiconductors
Whether an atom is a good conductor of electricity depends on how many
electrons are in the outer shell of the atom (the Valence Shell). One, two, or even three
electrons in the valence shell make good conductors. The electrons are easily pulled away
by external force. If the outer shell has seven or eight electrons, it's hard to free an
electron. These elements make bad conductors, or good insulators. In between conductors
and insulators is a group of elements called Semiconductors. They are neither good
conductors, nor are they good insulators. Carbon, Silicon, and Germanium, are popular
semiconductor materials.
These statements are true whether we are talking about atoms or molecules. Even
a good conductor can make a bad conductor when included in a molecule. To put it in a
simple case, Iron is a good conductor. When formed into molecules with Oxygen (as in
Iron Oxide, or rust) it becomes a bad conductor. The valence electrons of the Iron are tied
up by the Oxygen, and are no longer available to conduct electricity. Likewise, a
semiconductor may be doped with another poor conductor to make a reasonably good
conductor.
Conduction
I have read some appalling stories about how electricity flows in basic electronics
books. I agree simplification is necessary, but it shouldn't be misleading. One such story
says that electrons are at rest (not moving, shown sleeping) until a voltage is applied, and
then suddenly take off at 186,000 miles per second. Another describes AC as electrons at
rest, slowly increasing in speed until they reach a maximum, then decrease in speed to
zero, then increasing slowly in the other direction to a maximum speed, finally slowing
down to zero again to make a complete cycle. That such things should ever be taught,
especially in the 1990's, is frightening.
Of course, that someone in the future may look back at my descriptions, and being
equally appalled by my words, is also a possibility. Nonetheless, I continue...
Electrons (at standard temperature, pressure and gravity) are never at rest. They
are constantly in motion from atom to atom, or molecule to molecule, at a speed of
300,000 km per second (or 186,000 miles per second, if you prefer), or closely at that
speed anyway. I don't have any instrument that would measure the difference. Since this
motion is random, there is no perceptible current.
When a voltage is applied to a conductor, the electrons of the conductor are both
repelled by the more negative potential, and attracted to the more positive potential. The
movement is almost instantaneous, although, it may take an individual electron a little
longer to move from point A to point B (at normal temperature, pressure and gravity,
anyway). At extremely low temperatures, super conduction becomes a factor, resistance
disappears, magnetism does weird things, but that's another story.
2
If we could imagine what is happening somewhere along the conductor at the
atomic level, we may see something like this:
As an electron feels the applied negative voltage, and the attracting positive
voltage, it is motivated to leave its present orbit around the nucleus, and move to an atom
closer to the more positive charge. This action leaves a hole (an absence of an electron) in
the atom it just left. The atom with the hole now has a more positive charge than it used
to, and attracts another electron from an atom closer to the more negative charge. This
tension between the free electrons and the positively charged nucleus is the source of that
quality we call voltage.
What we can imagine happening is electrons (negative charges) moving from
negative to positive, and holes (positive charges) moving from positive to negative.
If you take a narrow necked bottle filled with water, and pour it out, do you see
water coming down, or bubbles coming up? The same is true for electric current, with the
electrons flowing in one direction, and holes flowing in the opposite direction. The
concept of hole flow becomes important when we get to the study of semiconductors.
Electromagnetism
An electron has a magnetic field. As long as all the electrons are moving in a
random direction, the magnetic fields cancel one another out, and no perceptible field is
present. When a voltage pushes the flow of electrons in a unified direction, these
magnetic fields add to one another and a magnetic field is present around the wire.
If we wind the wire into a coil, these magnetic fields add to one another, and a
strong magnetic field develops around the coil. We can simulate a permanent magnet by
applying a DC voltage to the coil. The negative side of the coil, takes on a polarity equal
to the North Pole of a magnet. There is no notable difference between the magnetic field
of a permanent magnet, and that produced by a current through a coil.
The electromagnet has the advantage of being one we can control. We can use
this electromagnet phenomenon to make an electric motor, or an electric generator. Large
electromagnets are used to move cars and scrap steel around in junkyards. There are
countless uses for electromagnets.
Induction
An electromagnetic field crossing a conductor induces a voltage in the conductor.
Likewise, a wire crossing a magnetic field gets a voltage induced into it. As long as there
is a difference of motion between the wire and the magnetic field, there will be an
induced voltage in the wire. This happens at the component level, as well as at the atomic
level. At the atomic level, the electron moving from atom to atom creates a magnetic
field, which crosses the electron structure of neighboring atoms. The electrons of the
neighboring atom are affected by this magnetic field, and the electrons are motivated to
move also (but in the opposite direction as the electron that caused the magnetic field).
This induced current is an opposition to a change in incoming current, and this effect we
call inductance. Any conductor has an inductance of some kind (at standard temperature,
pressure, and gravity). In superconductor environments this world is upset, see BoseEinstein Condensate).
If we wind turns upon turns of wire around one another, this induction
characteristic is magnified, and we create strong magnetic fields. If we place another
3
winding of wire close to the original coil of wire, we get an induced current in the second
winding. This makes a transformer with primary and secondary windings.
Transformers
If we take one winding of wire, and wind a second winding around it, the
magnetic field produced by one winding will induce a voltage in the second winding. The
voltage induced in the second winding will depend on the ratio of windings. If the first
winding, we'll call it the primary, has the same number of turns as the second winding,
we'll call it the secondary, the voltage on the secondary will be equal to that of the
primary. (Neglecting any loss. In the classroom, all our transformers are perfect. In the
real world, transformers are less than 100 percent efficient. It depends on the design of
the transformer.)
If the secondary has more windings than the primary, we have a step-up
transformer. That is, the voltage on the secondary will be higher than the primary. If the
secondary has fewer windings than the primary, we have a step-down transformer. That
is, the voltage on the secondary will be lower than the primary. An isolation transformer
is designed to have the same voltage on the secondary and primary. In all cases, the
voltage on the secondary has no reference to ground. That is, it provides isolation from
ground on the primary circuit.
Please note than we cannot gain power in the transformer. If we step up voltage
on the secondary, we have less current available to draw. If we step down voltage, we
have more current available. Watts available in the primary (Volts times Amps) will
always be the same in the secondary. That is, the same maximum Watts available. How
much current we have flowing in the primary depends on how much current we have
flowing in the secondary, which depends on the characteristics of the load.
Step-down transformers will have fewer turns of larger wire on the secondary.
Fewer turns means lower voltage. The larger wire is to accommodate higher current.
So, how does drawing current from the secondary of a transformer result in more
current flowing through the primary? What’s the connection? Are the electrons in the
secondary linked to electrons in the primary by the magnetic field?
Yea! Right! And the universe is one infinitely inter-linked universal entity. Let’s
get out of the dark ages, shall we? The answer is Permeability.
When we pass an AC current through a coil, it creates a magnetic field around the
coil. The inductance (AC resistance) of the coil depends on the size of the wire, the
number of windings, and the nature of whatever the coil is wound around. Is it air, ferrite,
iron, lead? Each type of core material has a quality called permeability. If we bring a
piece of metal close to a coil, it effects the permeability of the coil just as changing the
type of core material would. This changes the inductance of the coil, which changes the
current through the coil. (Somewhat like a metal detector, right?) The secondary winding,
and its load, also affects the permeability of the primary winding. When we pull more
current from the secondary of a transformer, the permeability of the transformer changes,
which changes the inductance of the primary, which causes more current to flow.
4
Keep in mind, in a transformer, the magnetic field must be constantly moving.
Transformers work on AC, not DC. If we apply DC to a coil, we get an inductive
reactance only on the rising and falling edges of the signal. While the DC level is
constant, the only effect the coil has on the circuit is the resistance of the wire. When the
signal rises the magnetic field expands, and we have some degree of energy stored in the
magnetic field around the coil. When the DC level drops, the magnetic field collapses,
inducing a current flow in the wire in the opposite direction as the original signal. This
inductive kick can be a hazard to the components of the circuit if it is not taken into
consideration. Note the presence of protective diodes across coils, and transistors
designed to drive inductive loads.
Capacitance
A capacitor is two (or more) plates of conductive material, separated by an
insulator. In a capacitor, the insulator is called a dielectric. Since there is no actual
electrical contact between the two plates, it would seem that current would not be able to
flow through a capacitor. The electrostatic pressure caused by the voltage being applied
to the plates can cause electrons to be pushed off the more positive plate, resulting in a
charge between the capacitor plates. The closer the plates are together, or the larger the
plates, the higher the capacitance. The material of the dielectric also plays a role in
capacitance. Different materials have a different dielectric constant (k). Air and a vacuum
have a dielectric constant of 1 (the reference value by which all other materials are
compared). To make a short list:
Material
Air
Vacuum
Waxed Paper
Mica
Glass
Ceramic
Metal Oxides
Dielectric Constant
1
1
3.5
6
8
100+ (depending on structure and type)
(higher)
What this means is that, roughly speaking, a capacitor made of mica one
thousands of an inch thick would have six times as much capacitance as one of similar
size of air or a vacuum. Many other materials are popularly used. Tantalum and
Aluminum oxides are popular. Plastic films are good for making capacitors for audio
circuits, or RF applications, or where high reliability is required. Consult a parts
distributor's catalog for all the possibilities and applications.
I remember reading somebody's description of a capacitor's operation as saying
that the charge of a capacitor was stored in the distorted field of the electrons in the
dielectric. It would seem from that, a vacuum could not be used as a dielectric in a
capacitor. I don't think that is so.
Another story I have read is that a capacitor stores electrons. I can’t let that one
pass either. For every electron that goes into a capacitor, another electron leaves. The
number of electrons stays the same. Capacitors store an electrical charge (not electrons).
5
When a capacitor charges, the electron entering the negative side pushes an electron off
the positive side, storing a charge equal to one electron. But, the capacitor stores charges,
not electrons.
The charge is stored in the area of the dielectric, between the plates, but it is
improper to say the charge is stored in the material of the dielectric. This subject leaves a
need for a better explanation. It can't be stored in the material of the dielectric, because
even a vacuum may be used to make a capacitor. The material of the dielectric, as
described above, in deed, affects capacitance but there is something missing to this story.
Some would say that the charge is stored in the plates of the capacitor. This is a good
concept. We can make a capacitor without material for a dielectric, but we can't make one
without plates.
The best story I would repeat about capacitor operation concerns electrostatics.
The charge is stored in the electrostatic field, between the plates. All the formulas work,
and dielectric constants work into the formula.
To get up to date on capacitors, I remember my High School electronics teacher
talking about capacitors and how large a Farad was. In his world of paper and plastic
capacitors, a one Farad capacitor would "fill this whole room". Today, a one Farad
capacitor is about a half cubic inch, and is used on CPU boards in place of a battery to
keep power to CMOS RAM when power is removed. (High School was a long time ago
for me.)
Questions I've never answered about capacitors
Not being one of those to claim to know everything, or make up answers that are
incorrect, just to have an answer (I hate when people do that). "An educated person is
firmly aware of what he doesn't know," quote me on that one.
By what force does the capacitor actually transfer the energy from one plate to the
other? Some books describe it in the same terms they use for magnetism, but, to me,
magnetism is not at work here, is it? If we put two metal plates in a magnetic field, we
don't get them to take on a charge. I have never done any experiments concerning
capacitors in a magnetic field, and having it affect the capacitance, or charge the
capacitor.
Some books say it is electrostatic forces at work, as opposed to electromagnetic. I
have the same questions concerning this story. Never having done any experiments, I
couldn't support or deny the story. It just doesn't sound like a complete theory to me.
We can readily show that the capacitor does work, but all our stories are empirical
logic, not rational theories, supported by solid explanation of cause and effect.
Most of the explanation are only explanations-by-comparison, and say "it kind of
works, like ..." Don't give me this type of explanation. It sounds like "The principle of
Correspondence" from Hermetic philosophy. Such obsolete ways of thinking should have
been tossed aside long ago. If you don't really know what you are talking about, say so.
Don't make up a bunch of BS just to justify the letters behind your name, in hopes that
nobody realizes you don't know what you are talking about.
If anybody has a better explanation of how capacitors work, I'm seriously
interested in listening.
6
Voltage and Current
Voltage is the motivating force behind the flow of electrons. Current is a matter of
how many electrons are in motion passed a given point in a given time.
To keep up with a current trend (no pun intended) to avoid the term "current
flow" to describe the flow of electrical charges, I will try to avoid the use of the term
here, also. Current is already defined as the flow of electrical charges, so the term current
flow is like saying the flow of current flow.
Normally an atom has an equal number of electrons (with a negative charge), and
protons (with a positive charge). In a conductor, when an electron is pulled away, it
leaves a hole (an absence of an electron) and the forces between the electron structure
and the nucleus result in a positive charge on that atom. This force is the basis of that
quality we call voltage. This is a static force being exerted on the circuit at any given
point with reference to another point. The presence of this voltage is what causes current
to flow (whether you view current flow as positive or negative charges). Voltage doesn't
flow, current flows.
Voltage is measured in Volts, and given the symbol V, or sometimes E, or EMF
for ElectroMotive Force (literally, the force that motivates the electrons).
Current is measured in Amps, or Amperes, and is given the symbol A, or
sometimes I, for Intensity of current flow. This is a measure of how many charge carriers
(electrons or holes) pass a given point in the circuit in a given time.
Resistance
Resistance is the opposition to the flow of charge carriers (electrons or holes) in a
circuit. It is a matter of how good a conductor the material is, as well as temperature,
pressure and gravity. Temperature plays an important factor in resistance. Pressure and
gravity have an effect in extreme conditions. This statement is true for any place in this
paper that mentions STPG (Standard Temperature, Pressure and Gravity).
Resistance is measured in Ohms, and usually given the symbol R, or sometimes
Omega, from the Greek alphabet. Since this text will be converted to ASCII, I couldn't
give the actual symbol here; it wouldn't stay with the text.
Many texts still reference resistors as having tolerances of 20%, 10%, and 5%. I
haven't seen a 10% or 20% resistor since I saw a vacuum tube. Most resistors today are
5%, with 2% values coming into popularity at a reasonable price.
The color code follows in later text, but is not worth elaborate discussion. It is
worth getting to know. You can find it in any book on Basic Electronics. What is usually
omitted is the fact that this same color code scheme is used for capacitors, and even
diodes in some cases. Some manufacturers also use this same color code to refer to colorcoded wires. A Blue wire with a red stripe would be called wire 62, and would be unique
in that circuit. This makes troubleshooting a lot easier if you have to trace a wire down.
Another point not mentioned enough is that resistors don't come in all possible
values you can make up with the color code. 5% resistors only come in certain values.
7
OHM’S LAW
Many of the "Grand Old Men of Electricity" got their name tied to an aspect of
electronics to which they made contributions. The "Electromotive Force" of Volta's days
is now called Volts. The "Intensity of Electrical Current" of Ampere's days is now called
Amps. In a way it is a shame to getaway from descriptive names of these characteristics
of electricity and use the non-descriptive terms Volts and Amps. Having said that, we
will move the conversation along to Ohm's Law. Ohm states that:
I = E/R
In words, the formula states, “The current in a circuit is proportional to the
applied voltage and inversely proportional to the resistance.” What he was trying to say is
that current in a circuit increases with increasing applied voltage, and decreases with
increasing resistance. (My kids would say "well, duh!")
E is in Volts. E is for EMF (Electromotive Force); the force that motivates the electrons.
This is the electrical pressure in the circuit that pushes (-), or pulls (+) the electrons
through a circuit.
“I” is in Amps. This is the actual amount of current flowing through a circuit, or a part.
“I” for Intensity.
R is in Ohms. This is how much opposition to current flow a component has.
As long as we know two of these factors, we can find the other.
E=IxR
Amps times Ohms gives us Volts.
E
I = ---R
Volts divided by Ohms gives us Amps.
E
R = ---I
Volts divided by Amps gives us Ohms.
8
Power, measured in Watts, is an indication of how much heat a part, or a circuit, will give
off, or consume. This is strictly a mathematical computation, but does equate to other
units of heat or power in other sciences.
P=ExI
Volts times Amps gives us Watts.
P
E = ---I
Watts divided by Amps gives us Volts.
P
I = ---E
Watts divided by Volts gives us Amps.
9
Basic Circuits 1
The objective of this session is to get you familiar with the fundamentals of
reading a schematic, and applying Ohm’s Law. We will introduce you to a few schematic
symbols and build simple circuits.
Referring to schematic document number “BC 1”, these are the schematic
symbols of a battery, a switch, an incandescent lamp, and a resistor. Batteries and
switches you may already be familiar with, we’ve all handled both.
Resistors
Resistors are devices made of some member of the semiconductor family, usually
carbon, but not in all cases. We use resistors to tailor the amount of current we want to
flow in a circuit. The schematic symbol is, as shown in the drawing.
For basic electronics classes we use resistors to symbolize “any component, in
general”. All components have some degree of resistance, and can be substituted by a
resistor for the purpose of learning the basics of Ohm’s Law. For Ohm’s Law, it doesn’t
matter if the 100 Ohms in the circuit is a real resistor, a lamp, a heater, a motor, or what
ever. It is “some component” that has a certain resistance.
Later we will get around to using real parts and learn their characteristics. For
basic Ohm’s Law, we are only concerned with the resistance of these devices, and use
resistors to symbolize their presence in a circuit.
Flashlight
The top drawing is a schematic of a flashlight. We have a battery, a switch, and a
lamp. The other side of the lamp returns to the other side of the battery, completing our
circuit. We must have a complete circuit for current to flow.
We apply power from a 6 Volt battery. Our switch is either On (zero resistance
and allows current to flow), or it is Off (infinite resistance, preventing current from
flowing). (In the real world these extremes are not found, but this is basics, so our
components work perfectly and simple.)
When we turn the switch on electrons leave the negative side of the battery (the
smaller line), travel through the switch, through the light, and to the positive side of the
battery. When the switch is off no current can flow, and the light goes out.
We have a 6 Volt battery and a light that runs on 6 Volts, so our world is correct.
How much current do we have flowing? The manufacturer of the lamp states that at 6
Volts, the lamp should draw 0.20 Amps. Ohm’s Law states that E/I = R, so we have 6 V
divided by 0.20 Amps, or 30 Ohms.
10
Roll your own flashlight
Consider that you are McGyver, trapped in a room of electronic components and
needed a flashlight. Looking around you, you find a 9 Volt battery, a 6 Volt lamp (rated
at 0.20 Amps), and an assortment of resistors. If you try to run the lamp off of 9 Volts, it
will exceed its ratings and blow out. You need to limit the current through it to 0.20
Amps, from a 9 Volt source. What resistor do you need?
You are applying 9 V. You know the lamp will drop 6 V at 0.20 Amps. A quick
calculation (9V – 6V) shows that the resistor will have to drop 3 Volts across it. Ohm’s
Law, again, 3 V / 0.20 A = 15 ohms. We look through our drawers of resistors and pull
out a 15-Ohm resistor, and build our circuit.
11
What component do we use to select the amount of current we want to flow in a
circuit?
1)
What is the schematic symbol for a resistor?
2)
What is the schematic symbol for a battery?
3)
What is the schematic symbol for an incandescent lamp?
4)
What is the schematic symbol for a wire?
5)
What is the schematic symbol for a switch?
6)
In order for current to flow we must have _________________.
12
COMPONENTS
Resistors
The purpose of resistors is to determine the amount of current we want to flow in
a circuit. Resistors (perfect resistors, anyway) have the same resistance to AC or DC. If
you have had the chance to play with components, one of our basic exercises is a series
circuit made up of a resistor and an LED. The voltage drop across the LED is fairly
constant over a range of operating current. We select the resistor to get the current we
desire to flow through the LED.
A resistor is a semi conductive material, typically carbon or some metal oxide,
that is neither a good conductor, nor a good insulator.
Reactors (capacitors and inductors)
In contrast to resistors, there is a group of components called reactors. They
exhibit a different resistance to AC than to DC. These are basically capacitors and
inductors. Reactance is resistance to an AC signal. Capacitors have Capacitive Reactance.
Inductors have Inductive Reactance.
Resistors
Resistors are made of a semiconductor material, usually carbon, and are rated
according to how much resistance it has, in ohms. Leaded components are typically
cylindrical in shape, with leads coming out the ends (axial case). Colored bands show the
rating of the resistor, in ohms. The physical size of the resistor indicates its wattage.
The color code used for resistance is an industry standard, used for many other
components. This color code is worth getting to know.
The first three bands follow this system:
Black
Brown
Red
Orange
Yellow
0
1
2
3
4
Green
Blue
Purple
Gray
White
5
6
7
8
9
The first two colors are interpreted as numbers (significant digits). The third is the
multiplier (how many zeros should be added to come up with the resistance). For
instance, a resistor with yellow, violet, and red stripes would be 4700 ohms.
To get resistor values below 10 ohms, the third band may be Gold or Silver. A
Gold third band indicates a value between 1.0 and 9.1 ohms. A Silver third band indicates
a value between 0.1 and 0.91 ohms.
The forth band gives the percentage value:
Gold
Silver
(none)
5%
10% (seldom seen anymore)
20% (really old, you are not likely to ever see these)
13
Many schemes have been created to aid in remembering the color code.
Black
Brown
Red
Orange
Yellow
Green
Blue
(Bad)
(Boys)
(Race)
(Our)
(Young)
(Girls)
(But)
0
1
2
3
4
5
6
Violet
Gray
White
(Violet)
(Generally)
(Wins)
(the)
(Gold)
(and)
(Silver)
7
8
9
5%
10%
Not all possible values are represented. The pattern they follow is another thing worth
spending some time with, and getting to know. 5% values are 5% values, whether they are
resistance, capacitance, voltage, or anything else. The pattern formed is the same.
10
11
12
13
15
16
18
20
22
24
27
30
33
36
39
43
47
51
56
62
68
75
82
91
14
Surface mount resistors
Surface mount components are small rectangular, flat packages. The top surface is
usually black and the sides are white. The resistance is stated in numbers, as if they were
colored bands. A resistor of 4700 ohms would say “472”.
Surface mount resistors, being smaller packages, cannot dissipate the heat a larger
package can, and typically have ratings of 1/8th, 1/10th, or 1/16th of a Watt. These are a
ceramic substrate with the resistive material covering the top side. A coating covers the
resistive material, usually black. The ends are covered on three sides (top, end, and
bottom) with a metal cap that makes the connection to the board.
2% and 1% devices
On occasion you may find resistors with five colored bands, instead of four. The
fifth band will usually be Brown (1%), or Red (2%). These values have three significant
digits, followed by the multiplier, and Brown or Red. A resistor with Brown, White,
Brown, Red, Brown stripes would be 19,100 ohms, 1%.
Most resistors you will encounter in the gaming industry will be the 5% variety.
Another thing you may want to watch out for. On schematics, decimal points
often become blurred, or faded out. There is a trend in schematic creators to label resistor
values, avoiding decimal points. Instead of labeling a resistor as 1.2K ohms, it will say
1K2. In either case the resistor is 1200 ohms.
Construction
Most resistors today are “Carbon Film” construction. Physically, these are a
ceramic rod with carbon film coating it, and metal caps on each end to connect the leads
on. A plastic coating covers the resistor, and colored stripes show the rated value.
Higher wattage resistors are usually “Metal oxide” instead of carbon. If you
remember from earlier lessons, metal oxides are not good conductors, and the oxides may
be tailored to give a specific resistance.
Older resistors were “Carbon Composition” construction. These devices are a rod
of carbon with leads attached, and covered with a plastic (brown) coating. These are the
ancient devices that came in 20%, or 10%, and seldom 5%, ratings. Avoid these devices.
Carbon Composition devices offer no improvements, and have a few design flaws.
Just to give you an idea of how old these devices are, the last time I saw a 20%
resistor in a circuit, it was a hand wired chassis with vacuum tubes. The last time I bought
them, they were manufactured in 1971.
Carbon Composition devices change effective resistance at higher frequencies. At
high frequencies we get a phenomenon called “Skin Effect”. The current tends to stay
close to the surface of the device. For carbon composition devices, this means that the
whole resistor no longer affects the current.
Metal oxide, and metal film devices are a ceramic rod with all the resistor
material on the surface. Higher frequencies do not get this skin effect.
Failures in resistors
15
Other than suffering physical damage, the only way resistors fail is to burn up,
and open. Resistors do not short out. A given physical size case is rated at a certain
wattage, typically ¼ W. As you pull current through it, we get a voltage drop across the
resistor (I x R). When the voltage drop and current exceeds the rated wattage value (E x
I), the body can no longer dissipate the heat, and it gets hot. In extreme conditions the
resistor may get so hot it actually bursts into flame. Metal film devices are capable of
dissipating more heat for a given size package. Metal film resistor also are covered with a
colored ceramic coating, instead of plastic. These devices are flameproof. They are
guaranteed not to burst into flame. They may get hot enough to burn another nearby
component, but the resistor will not burst into flame.
Wire Wound resistors
Lower wattage resistors are typically 1/4 W. Some are 1/8 W, or 1/2 W, or 1 W.
Above 1 Watt, we need a design that can tolerate higher temperatures. The resistive
element is usually a metal wire that is not a good conductor, like tungsten, or a compound
like Nickel-chromium (nichrome). The case is ceramic material, round or square, to
tolerate the high heat that may be encountered. Usually these devices have their values
printed in numbers, or as a coded part number. Colors tend to change under heat, so
colored bands are avoided.
Resistance
Continuity is (theoretically) zero Ohms. No resistance. An open is (theoretically)
infinite resistance. Maximum resistance. The real world seldom actually sees either
extreme. Continuity is any resistance low enough to allow current to flow easily. An open
is a break in the circuit that allows no meaningful current to flow. Resistance is what the
real world sees as an opposition to current, measured in Ohms.
Wire has some resistance, usually measured in Ohms per Foot, and is usually
some number close to zero, a small fraction of one Ohm for most measurements. For long
runs, even this resistance can be meaningful.
16
Capacitors
A capacitor is two (or more) conductive plates separated by an insulator. They
have a very high resistance to DC, and a resistance to AC that changes with frequency.
Capacitors have less resistance as the frequency gets higher.
The forces at play here are the electrostatic charges that build up on the plates.
Even if there is no complete path through the capacitor we can still pass a change in
voltage through the capacitor. As we apply a voltage to one plate of a capacitor it builds
up a charge on that plate. The insulator between the plates is so thin that the electrostatic
field on one plate can be felt through the insulator and move affect electrons on the other
plate, leaving a charge between the plates.
For the purpose of capacitors (and batteries) this insulator is called a dielectric. A
capacitor can store a charge between its plates, just like a battery. This is another
characteristic of capacitors, the ability to store a charge.
Capacitance is measured in Farads. One farad is a relatively large value. Most
capacitors are rated in micro-Farads (millionths of a Farad). This value indicates the
capacitors ability to store a charge. A capacitors resistance to AC is called Capacitive
reactance, and changes with the applied frequency according to the formula:
Xc = 1/(6.28 x F x C)
Xc is Capacitive reactance, measured in Ohms
6.28 is actually 2 x pi (3.14159…)
F is the frequency, in Hertz
C is the capacitance, in Farads
Inductors
An inductor is basically a winding (or windings) of wire. Inductors have very
little resistance to DC, and a resistance to AC that also changes with frequency. Just
opposite that of capacitors, as frequency gets higher inductors have a higher resistance.
17
Light Emitting Diodes
w - wavelength of the light emitted, in nm (nano-meters), approximate values
V - Typical forward voltage of the junction
w
940
900
880
690
640
615
590
565
430
370
Color V
IR
1.3 to 1.7 (lowest frequency)
IR
1.2 to 1.6
IR
2.0 to 2.5
Red 2.2 to 3.0
Red 1.6 to 2.0
Orange 1.8 to 2.7
Yellow 2.2 to 3.0
Green 2.2 to 3.0
Blue 3.6 to 5.0
UV
(recently developed and available for sale)
The white 2-leaded LEDs are really blue LEDs with a phosphor surface that
glows white.
As with any diode, the forward conduction voltage depends on the chemistry of
the semiconductor material. All diodes emit light somewhat. The light comes from
electrons combining with holes in the junction, and releasing excess energy as radiation.
With proper construction and materials, we can design Light Emitting Diodes with
radiation in specific frequencies we can see. In doing so, the forward conduction voltage
raises to 1.2 V, to 3.0 V, or more. The reverse breakdown voltage also becomes
dangerously low (around 4 to 6 Volts), and needs to kept in consideration when
designing. In a Germanium diode, this forward voltage would be closer to 220 mV. In
Silicon, it may be 600 mV to 1 V. Most LEDs are single junction devices with unusual
forward voltages. Some of the materials used in LEDs are Indium Phosphide, Gallium
Arsenide, Gallium Arsenide Phosphide, Gallium Phosphide, Gallium Nitride, just to
name a few.
In contrast, your typical incandescent tungsten light covers 400 nm to 950 nm,
and is more yellow than white. It lacks somewhat in the UV range. A good white light
would cover from UV to IR. (Maybe a Mercury Vapor or Halogen bulb might be closer
to white.)
The sensitivity of CdS cells covers a range from 500 nm to 650 nm, and lacks
sensitivity in the IR range. I just thought I'd throw that in here. It didn't seem to fit
anywhere else.
18
Semiconductors
Semiconductors are materials that have (typically) four electrons in the valence
shell. As such they are neither good conductors, nor are they good insulators. We have
here the opportunity for creativity. Silicon, and Germanium, are among the more popular
semiconductors we use to make diodes and transistors. In their pure form they take on a
crystalline structure.
Pure silicon, or any other semiconductor, has a specific resistance. Depending on
purity that resistance may be in the tens of thousands to hundreds of thousands of ohms.
Impurities play a major role in the resistance of the crystal. Heat is also a factor. As the
crystal gets hot more current carriers (electrons or holes) are freed from their bonds,
allowing resistance to flow easier, lowering resistance.
If we deliberately add impurities to the semiconductor material (called doping),
we can control how conductive the material is. By doping the Silicon with some element
that has three valence electrons we make a silicon material that we call “positive-type, or
P-Type”, meaning it is capable of accepting an electron for each impurity atom that we
add. If we dope the Silicon with some element that has five valence electrons we make a
silicon material we call “negative-type, or N-Type”, meaning that it has an excess of
electrons. If we melt these two types together, the junction where they bond forms a
region that will allow the extra electrons in the N-type to pass through to the P-type if we
apply a voltage across the junction, negative to N-Type and positive to P-type. Resistance
drops down to hundreds, tens, or even a fraction of an ohm. If we reverse our voltage the
electrons are pulled away from the junction, toward the positive voltage, and the holes are
pulled away from the junction, toward the negative voltage, and our resistance of the
junction is in the millions of ohms.
This is our basic semiconductor device, called a Diode, meaning that it has two
electrodes. If we apply Forward Bias to the diode (Negative voltage to N-type and
Positive voltage to P-type) we conduct electricity. If we reverse the voltage (Reverse
Bias) we do not conduct electricity.
The terminal on the N-type side we call the Cathode, meaning something that
emits electrons. The terminal on the P-type we call the Anode, meaning something that
accepts electrons (as viewed from the inside of the device).
Our doping need only be in the parts-per-million to be effective. By changing the
doping material, doping percentage, size and structure of the junction, and material of the
substrate (in our example, the Silicon), we can have different results.
A closer look
When a diode is forward biased the electrons from the N-type material move
toward the junction, and holes from the P-type material move toward the junction. The
hole and the electron combine in the area of the junction, allowing current to flow. Our
supply of electrons and holes are supplied by our battery, or power supply, or whatever
our source of power is.
The combining of the electron and hole releases an electromagnet radiation of
some sort, usually in the frequency of heat, and in some cases, light. Silicon produces a
radiation in the infrared region. Gallium Arsenide, a semiconductive molecule, produces
radiation in the red region.
19
Before the junction can conduct we have to overcome the resistance of the
junction to conduct at all. This voltage and current we must apply makes a Threshold
level that must be attained before we see conduction. In Silicon we must have at least
0.60 volts, or so, before the junction will break into conduction. Germanium requires less,
about 0.40 volts. Gallium Arsenide requires about 1.20 volts.
Real devices
Diodes used in low level signals (below 100 mA, and below 100 V) are called
signal diodes, and we use them in various ways we will discover in circuits to be
introduced later.
Diodes designed to work at higher power levels we use to change AC to DC in
power supplies. This process of changing AC power to DC power is called rectification,
and diodes used in these type of circuits we call rectifiers.
Signal diodes and rectifiers both work in the same way, the only difference is
their physical size of the substrate and the case they must be in to dissipate the power.
Silicon signal diodes we see most often are of the part numbers 1N914, 1N4148,
and 1N4448. The “1N” at the beginning indicates that the device is a diode (one
junction). The numbers that follow have no special meaning that indicates characteristics.
A 1N914 made by any company has about the same characteristics. All will be made of
Silicon (and thus have a forward voltage of around 0.6 volts). All will carry about 100
mA and still be within a safe operating range. All will be able to withstand about 100
volts when reverse biased before the junction breaks down are the device self destructs.
Silicon Rectifiers we often find are numbered 1N400x series of numbers. No
matter who makes them, they will all pass 1 Amp of currently safely. All 1N4000’s will
have a breakdown voltage of 50 V. 1N4001’s have a breakdown of 100 V. 1N4002’s
have a breakdown of 200 V. The list continues as standard part numbers up to 1N4007
with a breakdown of 1000 volts.
1N540x devices are capable of operating at 3 Amps. 1N5400 has a breakdown
voltage of 50 V. 1N5401, 100 V. 1N5402, 200 V. And so on.
These part numbers are Industry standards, and are used in home computers,
microwaves, stereos, as well as slot machines both reel and video.
The cases are standard sizes also. You can’t tell much by looking at the case,
other than judging that is works within a certain power range. Larger cases are required at
higher current levels.
The DO-35 case is a standard size for signal
diodes. The case is usually clear, or white glass. The
banded end is the Cathode. !N914, 1N4148, and such
typically look like these.
20
The DO-41 and DO-15 cases are deices that are rated at
around 1 Amp, maximum, and less than 1,000 volts. 1N400x
devices look like these.
The DO-201AD size case is physically larger than DO-41
and are capable of handling up to 3 Amps and 1,000 Volts.
The R-6 size case is usually a device rated at 6 Amp, and
less than 1,000 volts.
The main difference between the DO-41 and DO-15
cases are not visible on the outside of the device. Between the
actual silicon diode substrate itself and the case is a layer of
insulation material called Passivation. In DO-15 devices this
passivation is plastic. In DO-41 devices this passivation is glass.
Glass passivated devices are capable of working at higher
operating temperatures than plastic passivated devices before
failure occurs. Typically plastic passivated devices can operate at
up to 125 degrees centigrade before failure. Glass passivated
devices are rated at 150 degrees centigrade. That is “ambient”
temperature. The temperature of the silicon substrate inside the
case, not the outside temperature.
In all these circumstances the silicon substrate inside is enclosed in a hard plastic
case. The part number is printed on the case. If you want to know the characteristics of
21
the device, you look up the part number in a reference book, or catalog. There are only a
few dozen different types of diodes in use in the gaming industry. It isn’t a difficult task
getting to know the major players by part number.
Part numbers
The manufacturer of the diode assigns an Industry Standard part number to the
device, like 1N4004. When IGT buys these devices they assign their own part number to
it (48402190). When Bally uses it they assign their part number (E-587-40). When Wells
Gardner uses t in their monitors they assign their number (066X0071-001). Distributors
assign their part number (900-2869 for Radio Shack). Happ Controls assigns them their
number, as does Aristocrat, Konami, or any other person who uses or distributes the
device.
In purchasing the parts for our shop we need not buy a different supply of
1N4004s from each vendor. We can shop around and buy a 1N4004 from whoever has it
at the best price. They may all, in the beginning, come from the same manufacturer. We
can be sure that all diodes marked 1N4004 will have the same characteristics, and be
compatible.
To make stocking even easier, we can realize that a device rated for a maximum
of 200 volts will work fine at all lower voltages as well. So why not just buy a stock of
1,000 volt rated devices and use them for 1N4000, 1N4001,1N4002, and so on?
This is what companies like Philips does in marketing their line of “Replacement
Parts”, like the ECG line of components. NTE Electronics does this also with their NTE
brand. They stock the components with the highest voltage rating and list it as a substitute
for all equal, or lesser, rated devices.
A word of warning here. SUBSTITUTE does not mean EQUAL. A 1N4007,
rated at 1,000 volts, will substitute for a 1N4001 rated at 100 volts. But the 1N4001 will
not substitute for the 1N4007.
The higher voltage device may be stocked and used for all lower voltage ratings.
In most cases the higher voltage rating may cost you a few cents more per diode. It is a
judgment call on the part of the person who purchases the components to choose what is
going to be stocked.
Prices can vary greatly. A 1N4004 from one vendor may cost you $0.05, or $1.50
from another vendor or distributor. Shop around. Be aware that a real bargain price may
be an older component with oxidized leads. When buying a component from a vendor
you don’t know, buy in small quantities first just to sample quality and promptness of
service. Any of the companies listed herein I would recommend. I can think of none off
hand that I would warn you against.
Purchasing in quantity brings the price down somewhat also when purchasing
from a distributor, like Happ Controls, or Radio Shack. Usually manufacturers (like IGT
or Bally) do not give discounts, and have higher prices than available from distributors.
Not many manufacturers want to get into being parts distributors. If they sell a batch of
100 diodes, they may short themselves on a production run. A major error to a
manufacturer. Personal experience is highly valuable. In many cases I can get better
prices on common parts from IGT than Radio Shack, for instance. Some things are more
available from Radio Shack.
22
Standard Part Numbers
As mentioned a specific component may be known by various part numbers from
different game manufacturers or gaming component distributors. Some game
manufacturers do not even give you a parts list or a tech manual. To combat this a
standard part number list is available at the Bench Techs forum at Delphi Forums.
www.forums.delphiforums.com/benchtech
This list is in constant development and will cross part numbers between various
gaming manufacturers and distributors, to Industry Standard part numbers. It also
provides an often educational description of what the part is.
Delphi forums is free for basic service, or advanced service and capabilities at a
modest fee. There is no charge for access to the Bench Techs forum, or for the parts cross
list. Typical size of the file is between 500K and 1 MB. It will fit on a floppy disk for
convenience. It is distributed as a Microsoft Excel file, but easily converts to text, if you
prefer. Don’t try to print it unless you have a lot of paper and time. It is typically between
2,000 and 3,000 lines long (in Excel) and VERY wide.
Suppliers mentioned:
Happ Controls
(Wholesale Electronics is a division of Happ Controls)
6870 S. Paradise Rd.
Las Vegas, NV 89119
(702) 891-9116
(702) 891-9117 fax
www.happcontrols.com
Radio Shack
(Get the Commercial catalog, not the store catalog)
AKA Tech America
PO Box 1981
Fort Worth, TX 76101-1981
(800) 877-0072
www.techam.com
Others:
Mouser Electronics
1000 N. Main St.
Mansfield, TX 76063-1511
(800) 346-6873
www.mouser.com
Allied Electronics
7410 Pebble Drive
23
Fort Worth, TX 76118
(800) 433-5700
www.alliedelec.com
24
Diode exercise
Our first exercise with semiconductors will be with diodes. We will use the same
breadboard we used with making previous circuits and hook up various devices,
analyzing what we observe in the process. From these observations we will get an
understanding of what proper behavior of the components is. Once we understand normal
behavior we will deliberately destroy the device and get a familiarity with what it takes to
destroy the device, and what characteristics it takes on when it fails.
We will choose from a variety of diode devices (Silicon, Germanium, and a Light
Emitting Diode, at least).
Select a diode to do the exercise with.
1N914
Silicon signal diode
1N270
Germanium signal diode
1N5819
Schottky diode
1N400x
1 Amp rectifier
Zener
Any Zener diode of a low voltage (below what ever VCC is used)
LED, red
LED, yellow
LED, green
LED, blue or white
Select a resistor within the safe limits of the diode. Reference a data book that contains
operating characteristics for the diode. If no data is available keep the maximum current
below 100 mA, but higher than 1 micro-Amp.
Make a simple series circuit using a diode with the cathode connected to ground and a
resistor connecting to a positive voltage, VCC (4.5 Volts to 12 volts).
25
Measure the voltage at VCC.
Measure the voltage across the diode.
Calculate the voltage across the resistor. (VCC – V diode). (This step is for an exercise in
Ohm’s Law. You could just measure the voltage across the resistor directly.)
Find the current through the diode. We can find this by finding the current through the
resistor (V / R = I). Since this is a simple series circuit, the current through the resistor
will be the same as the current through the diode.
Calculate the effective resistance of the diode at that current. (V diode / I = R).
Repeat the exercise using different resistors. Keep the current in the safe operating range
of the diode.
Graph the voltage across the diode at different currents.
Graph the effective resistance of the diode at different currents.
Repeat the whole exercise using different diodes.
What can we tell from the exercise?
Different types of diodes have a different pattern of forward voltage.
At different currents the voltage across the diode changes to some degree. These changes
reflect the changing resistance of the diode at various currents. Below a certain level the
change is major. Once the diode has sufficient operating current the voltage across the
26
diode stays fairly flat, but its resistance still changes as current through the diode
changes.
27
Bi-Polar Junction transistor exercise, NPN and PNP
Select a transistor for the exercise:
Pinout
Transistor
Type 1 2 3
2N2222A
NPN E B C
2N2907A
PNP E B C
2N3904
NPN E B C
2N3906
PNP E B C
2N4401
NPN E B C
2N4403
PNP E B C
PN2222
NPN E B C
PN2907
PNP E B C
(For other possibilities, other reference material may be required.)
Construct the above circuit using appropriate resistors.
28
Select a base resistor to give about 1 micro amp of base current. (VCC/ 1 micro amp)
Select a collector resistor to give about 100 mA, maximum, if the transistor should turn
all the way on. (VCC / 100 mA)
Connect the transistor up in a Common Emitter circuit (as shown).
Measure the voltage on the base.
Measure the voltage on the collector.
Find the voltage across the collector resistor. (Calculate or measure).
Calculate the collector current. (V R3 / R3 = I).
Calculate the gain of the transistor (IC / IB).
Calculate the effective resistance between the Emitter and the Collector of the transistor.
(V collector / I = R e-c).
Calculate the wattage being dissipated by the transistor. (V c-e x I).
Repeat the exercise using a different base resistor and the same collector resistor.
Repeat the whole exercise using a different collector resistor.
Note how gain changes under different base and collector currents.
Characteristics of the transistor:
Gain must be stated under a certain condition.
Voltage at the collector during saturation varies with different collector currents.
Wattage rating of the transistor can be exceeded even while within the safe operating
range of collector currents and voltages.
Transistor exercise
Select an NPN transistor for the exercise.
Select a base resistor to give about 1 micro amp of base current. (VCC/ 1 micro amp)
Select a collector resistor to give about 100 mA, maximum, if the transistor should turn
all the way on. (VCC / 100 mA)
29
Connect the transistor up in a Common Emitter circuit (as shown).
Measure the voltage on the base.
Measure the voltage on the collector.
Find the voltage across the collector resistor. (Calculate or measure).
Calculate the collector current. (V R3 / R3 = I).
Calculate the gain of the transistor (IC / IB).
Calculate the effective resistance between the Emitter and the Collector of the transistor.
(V collector / I = R e-c).
Calculate the wattage being dissipated by the transistor. (V c-e * I).
Repeat the exercise using a different base resistor and the same collector resistor.
Repeat the whole exercise using a different collector resistor.
Note how gain changes under different base and collector currents.
Characteristics of the transistor:
Gain must be stated under a certain condition.
Voltage at the collector during saturation varies with different collector currents.
Wattage rating of the transistor can be exceeded even while within the safe operating
range of collector currents and voltages.
30
SCR exercise
An SCR (Silicon Controlled Rectifier) is exactly what the name implies. A
rectifier (diode), made of silicon, that we can control with an input. SCRs are a member
of a class of components called Thyristors. They all have the characteristic that once
triggered on, they stay on until power is removed on the output. The SCR is a thyristor
that will pass current in only one direction (DC only).
An SCR is made up of two transistors, as shown below. When we draw current
from the base lead of the NPN transistor (called a Gate), we begin to turn the NPN
transistor on. The NPN transistor turning on starts drawing current through the emitter
and collector of it. This circuit just happens to include the base circuit of the PNP
transistor. As the NPN transistor turns on it turns on the PNP transistor, which starts to
conduct. The collector of the PNP transistor feeds back to the base of the NPN transistor.
As the PNP transistor conducts it turns on the NPN transistor even more.
All we have to do is send a pulse to the base of the NPN transistor, and the SCR
latches in the ON condition. It will stay on until we remove power between the Anode
(the emitter of the PNP side) or the Cathode (the emitter of the NPN side).
31
Construct the above circuit. Start with 0V out at the slider of the pot. As we
increase voltage at the pot the SCR will come on at some point. We want to note at what
voltage and current level the SCR turns on. After it is on, we will return the Gate current
we apply back to 0 V, and observe that the SCR stays on until we remove power at the
output.
1)
Monitoring the voltage at the slider of the pot, increase the voltage out by 0.10 V.
2)
Measure the voltage at the gate of the SCR.
3)
Calculate the gate current.
4)
Is the SCR ON?
5)
Repeat steps 1) through 4) until the SCR turns on.
6)
Note the voltage on the gate and the current through it when it first turned on.
7)
Return the voltage at the slider back to 0V.
8)
What is the voltage on the Gate?
9)
Did the SCR stay on?
10)
Remove one side of resistor R3.
11)
The SCR should turn off and stay off after R3 is reconnected.
32
Triac exercise
A Triac is another member of the Thyristor family. The Triac is essentially two
SCRs placed back to back, allowing the device to operate on AC. We have a Gate input,
common to both SCRs, and two Main terminals, MT1, and MT2.
While SCRs are used on logic boards to control DC devices, SCRs are used to
control AC devices. Operation is the same.
Like any other transistor, SCRs and Triacs may come in various packages. The
larger the package, the higher the power it can control. Larger cases allow for more heat
to be dissipated, meaning more current may be passed through the device, and higher
voltages may be tolerated.
Surface Mount Device Case styles
33
Typical cases of leaded components (not shown in proportional size).
34
Construct the circuit below, leaving the jumper shown to be attached in one of
three places. It may be attached to D1 to have Q1 pass only the positive sides of the AC
signal, to D2 to pass only the negative side of the AC signal, or directly to the gate to
pass both sides of the AC signal.
35
7400 Quad 2 input NAND gate
A following drawing shows the basic internal structure of the 7400, and how it
works. This is the basic Totem-Pole output, and basic TTL input. Schottky and Advanced
devices are more complex in order to reach higher speeds, and protect inputs against
static electricity.
When both inputs of a NAND gate are high, the output will be low. If either input
is low, the output will be forced high. This gate may be used as an active high input
NAND, or an active low input NOR.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Input 1A
Input 1B
Output 1
Input 2A
Input 2B
Output 2
GND
Output 3
Input 3A
Input 3B
Output 4
Input 4A
Input 4B
VCC
Below are some drawings of examples of the
circuit inside a basic 7400 gate. Be aware that the
drawing is simplified for clarity. The actual device
will have more components. These will be included
later on in the lessons. Right now, this is still basics.
The first following drawing is an equivalent of the 2-emitter’ed transistor, Q1, in
the next drawing. The AND-ing is done in this circuit. In the diode circuit, if either input
is low the diode will conduct to VCC through R4. This puts point where D3, D4, and D5
connect one diode voltage above ground. This keeps Q5 from turning on. With both
inputs high D4 and D5 are off and Q4 is allowed to turn on through D3 and R4.
Both inputs must be high to operate. This is the basic requirement for a AND
operation.
This circuit is equivalent to the 2-Emitter’ed transistor of the following circuit.
This is a Common-Base circuit. The Base lead is common to both the Input and the
Output. Pulling one of the emitter leads low turns Q1 on. Both inputs must be high to turn
Q1’s Emitter-Base junction off.
36
The output stage in the circuit below does the inverting of the signal to make the
AND operation a NAND gate. With two high in Q2 turns on, making it’s emitter more
positive, turning on Q3, giving a Low out. With either input low Q2 is off, making the
Collector more positive, turning Q4 on, giving a high out.
37
The examples below show how the 7400 may be used to drive an LED. Do not
use this output to drive other logic inputs also. These uses exceed the voltage limitations
for a valid High or Low output of the gate.
The following example shows how to make a basic S-R Latch using two NAND
gates. Note the inputs are active low inputs. A low at the Reset side resets the latch (Q
low, Q\ high). A low on the Set side sets the latch (Q high, Q\ low)
38
The following example shows the 7400 used in the Active Low 2-input NAND
operation. We may view the NAND gate as "All highs in gives a low out", or "Any low
in gives a high out". Both are descriptions of a NAND gate.
Most drawings are made using the symbol for a NAND gate that matches
operation of the circuit. This is not always true, but most manufacturers are thoughtful
enough to do so.
Now that we know the basics, lets get some hands-on experience with some real
devices. For this we need a breadboard with switches for inputs and LEDs to hook up and
monitor the signals. If you are using our standard Training Kit, you should have a 7400
device included. (7400, 74LS00, or some variation. Avoid using CMOS devices.
(74HC00, 74HCT00, or anything with a “C” in it is CMOS.)
39
The following circuit is an exercise in learning how the 7400 operates. The
switches will put in either Highs or Lows. When both inputs are high, the output should
be low. The LEDs will be on when that signal is low.
Construct the following circuit. Double check wiring before turning power on.
With either input low, the output should be high. Only with both inputs high
should the output go low.
40
BASIC DIGITAL LOGIC
If you’ve been keeping up with Slot Tech Magazine’s Basic Logic Course you should
have a good understanding of basic Gates and Latches. This article picks up from there
and continues on emphasizing Hands-on experience. We will review flip flops and
latches and build a training station to exercise these devices so we can watch them work
and build circuits using them. In my opinion, the best way to understand ICs is to sit
down and build a circuit using them. This gives you the base of understanding necessary
for troubleshooting them in a circuit.
This part of the course is not merely for academic purposes. The circuits we will use to
manipulate the Inputs of the ICs and monitor the Outputs of the ICs will be the same (or
similar) circuits we will use in designing test fixtures later on in the series. Now that you
know where we’ve been and where we’re going, let’s get started on where we are.
Gates and simple circuits can be tested with simple switch inputs. We need a resistor to
pull the input up to VCC when it isn’t pulled to ground through the switch. TTL outputs
are capable of driving enough current to power an LED. 74xxx, 74LSxxx, 74Asxxx,
anything that doesn’t have a “C” in it. Inclusion of the “C” in the part number means the
IC is actually CMOS technology, not TTL, and may only be able to drive a milliamp, or
so, of current. Let’s take a circuit to test basic gates with first.
Building the circuits
Figure 1 shows basic switch inputs and an LED output to exercise a 7400 2-Input NAND
Gate. As we close one of the switches it inputs a low to the input of the 7400 it is
connected to. If the output goes low the LED should light. If we remember the 7400, both
inputs must be high (switches off) before the output goes low.
Figure 1 shows basic switch inputs and an LED output to exercise a 7400 2-Input NAND
Gate. As we close one of the switches it inputs a low to the input of the 7400 it is
connected to. If the output goes low the LED should light. If we remember the 7400, both
inputs must be high (switches off) before the output goes low.
41
Our first decision is how to accomplish building the circuit. Radio Shack, and quite a few
other stores, sell small general purpose circuit boards such as Radio Shack’s 276-168,
and 276-150A. (See pictures with these titles.) These are circuit boards etched with a
general-purpose pattern. On either you will find rows of copper clad 0.300” apart.
Exactly the size of a DIP IC. You will also find long strings of holes running lengthwise
for power buses. These are one possibility for building the circuits we will use. For a few
more dollars you can get a “Huge IC Socket” type of breadboard. (See picture title
“Breadboard”.) The breadboard gives you the flexibility to reuse the board many times.
Since our “Basic Input Switches” and “Basic Output LEDs” will be repeatedly used, I
suggest building them on the circuit boards, and using the breadboard for the circuit
unique to the IC we are going to use in the exercise.
42
The picture above is Radio Shack part
276-150A. This board is suitable for
projects that have only one or two ICs
and a few discrete components. The
picture to the right of it is 276-168. This board is more suitable for a circuit with a few
ICs or more complex circuits.
For more flexibility, a breadboard can be used, as shown below. This is one big IC socket
that may be reused.
It is arranged in
rows of five dots.
Each row of five are
connected together.
If you wish to
connect two
components
together you just
plug one end into
one connector in
that row.
Powering the circuits
Of course we need to power these circuits somehow. We can use batteries or a power
supply. Most TTL and CMOS will work fine off of 4.5 Volts (three 1.5 V batteries). For
more safety, we can use a pack of four batteries, giving us 6 Volts, and include a series
diode. The diode drops the voltage down closer to 5 V, and prevents damage to the circuit
if we should ever connect the batteries in reverse order. Radio Shack sells battery packs
43
that will work just fine. If you would rather run off of wall power you can use a “wall
wart” power supply like the kind used to charge cell phones. I have a box full of various
kinds I bought from second hand stores for $1.00 each. Ideally, you want one that has a
regulated output of 5 Volts at a few hundred milliamps. If you find one that is
unregulated you can build a voltage regulator to bring the voltage down to 5 Volts.
Unregulated wall warts can be identified by measuring the output voltage and comparing
that to what the label says it should be. One that puts out 3.6 Volts and has a current
capability of 500 or 600 (or more) milliamps, may measure at up to 8 Volts with no load.
As you draw current from it the voltage drops down so that at the rated current you will
have the rated voltage. The picture “Power Supply” shows a 7.5 Volt at 800 mA
unregulated power supply made for these exercises.
44
45
The picture and schematic above give you an idea of the circuit required. Components are
not tightly regimented. The LEDs and their resistors are optional. C1 may be anything
from a few hundred microfarads to a few thousand microfarads, as long as the voltage
rating is higher than the unloaded voltage coming out of the “Wall Wart” supply. C2 also
may be anything from 1 microfarad to something less than one hundred microfarads. The
wall wart supply must be able to supply more than 7 or 8 Volts at a few hundred
milliamps (up to 18 Volts).
The 7805 will regulate the voltage coming out of the wall wart down to +5 Volts, as long
as the input is above 7 or 8 volts, but not higher than 24 volts.
Input Circuit
What you use for switches is more a matter of prerogative. Any kind of switch at will do.
The cheapest way to go may be a DIP switch. For exercising simple Gates switches are
adequate as inputs. When we get to Latches we need cleaner square waves than we can
get out of switches alone. When a mechanical switch opens and closes the contacts wipe
on one another and bounce. If we could look closely at the signal on an oscilloscope we
would see bounces in the signal over milliseconds. For anything besides gates we cannot
accept such dirty signals. In order to get clean square waves we can build a
“Debouncing” circuit using 7400 gates to build an S-R Latch, or the 74279 is four S-R
latches built into one case.
46
The above would be a suitable circuit for exercising basic gates. The circuit below will be
more appropriate for latches and more complex circuits.
A circuit like this provides a clean (glitch-free) signal for latches, counters, and such.
47
48
Output circuits
From TTL outputs we can get up to 25 mA of output current. From 74xx we can get 16
mA easily. From 74LSxxx we can get a little less, but as long as we are not driving other
logic devices that are sensitive to the low level output voltage, we can still drive an LED
directly. For these ICs the “Four LED” circuit will work fine.
49
CMOS devices may only have drive currents of a milliamp or so. For these we need an
LED driver that will work on a milliamp or less and drive an LED requiring 10’s of
milliamps. For these we need a “Low Current LED Driver”.
50
All of these examples were designed for minimal cost. For the cost of lunch you can get
started building these circuits. For a more reliable circuit you can use better switches and
mount the whole thing in boxes. This is the way we will build the test fixture examples
that follow. How you proceed to build your depends on your budget as well as your long
term intentions.
Using the setup to test a J-K Flip Flop (74107)
The 74017 is a popular device. They are easy to get hold of and not expensive. You can
get the part from Radio Shack, or most any other distributor, or it is IGT part number
32015590. We will exercise the 74107 one section at a time. There are two in a package.
We need four (clean) inputs and two outputs.
If we look at the Truth Table for a 74107 we find it to be like this:
J
K
Clk
Reset Q
Q-not
1
0
/
1
1
0
With J high, K low, reset high, on the
positive edge of the clock pulse the flip flop should set (Q high, Q-not low).
0
1
/
1
0
1
With J low, K high, reset high, on the
positive edge of the clock pulse the flip flop should clear (Q low, Q-not high).
1
1
/
1
T
T
With J and K high, reset high, on the
positive edge of the clock pulse the flip flop should toggle. If it was set it will clear. If it
was clear it will set.
0
0
/
1
(nc) (nc) With J and K low, reset high, on the positive
edge of the clock pulse the flip flop should not change. If it was set it should stay set. If it
was clear it should stay clear.
x
x
x
0
0
1
With reset low the flip-flop will clear. All
synchronous (clock related) operations are inoperative.
51
To test a 74107 we need four inputs from switches and two outputs. Since we have a
“Clock” input we need a glitch-free signal from our switch circuits. We can exercise one
section of the 74107 at a time.
From testing ICs to testing boards
The procedure for testing an IC, as we have just done, is little different if we were to test
a board. As long as we know what the board or assembly should do we can exercise it for
testing and troubleshooting. In following articles we will use these same, or similar,
circuits to test simpler boards, and more on to more complicated assemblies. Among
these we can do Coin Comparators of all types, Meter boards, Hoppers and Hopper
Control Boards… any board or assembly that only requires a few simple inputs and
outputs.
52
Above is an example of a basic, general purpose, four-input and four-output test setup. In
order to use the setup to test a certain assembly, all we have to do is make a cable to
interface to the assembly to be tested.
53
In the schematic above, J2 is the jack coming out of the test setup. J1 is the hopper
connector. This cable looks, as shown below.
By making different cables we can test different types of hoppers, VFD Display
Assemblies, Coin Comparators… most anything with four inputs and four outputs.
54
55