BASIC
ELECTRONICS COURSE
Page 15
INDEX
In this section we are going to talk
about waveforms and show how a rising and falling voltage produces a
"curvy" waveform very similar to a sinewave. A sinewave is called a
natural wave because things like bouncing balls and pendulums produce this type
of waveform when in motion.
The animation below shows how a
bouncing ball produces a curve when the rising and falling motion is combined
with forward movement. Move the mouse over the "Move" button and the ball will
shift right. The speed can be increased by moving the mouse over the Move button a few
times without touching the Come back button. To reduce the speed the mouse is
moved over the Come Back button without touching the Move button. The
"Come back" button returns the ball to the starting position.
Watch the shadow of the ball and you will see different waveforms being traced
out.
We mentioned above that capacitors do not pass DC but they DO PASS "AC signals."
An AC signal is any waveform that rises and falls.
The waveform you are producing with the mouse is called an oscillating
waveform and is very similar to the 240v (110v) electricity entering your house.
This is called an AC waveform or "AC
signal."
An AC waveform takes the shape of a sine wave and is shown in the diagram below:
The next
"interactive feature" shows how a
waveform is produced when a quantity such as voltage, rises and falls. Click
on "Pickup Tracer on new page"
and move it up and down a
few centremetres. The "0's" from the pointer will produce a graph very
similar to the sinewave above.
This shows that oscillating voltages (rising and
falling voltages) produce waveforms and these are passed to
components in a circuit.
READING
A GRAPH
The next "interactive feature" shows how the voltage
and current rise and fall in three different components:
(a) a resistor, (b) a capacitor and (c) a
coil.
This feature shows how to READ
A GRAPH.
At the moment, there are only two things to observe in the
"animation." Watch how the voltage (blue) dot moves
over the graph - from left to right. This is how you read a
graph. You mentally move from left to right across the waveform,
and the graph takes you UP for positive voltages and DOWN for
negative voltages.
If there are two graphs, with one graph directly above the
other, the two dots move evenly up and down and across the page.
But if one graph is drawn slightly to the left or right, the two
dots move at noticeably different speeds at different
times.
At the moment we are not going into any more complexity except
to see how two dots move across differently positioned
graphs.
Now, back to the main part of the course:
Some components "accept" a waveform and
amplify it (transistors), others attenuate (reduce) it (resistors) while others pass it to the next
stage of a circuit (capacitors).
Without getting too technical, the capacitor
passes the AC signal to the next stage of a circuit but blocks the DC component
of the signal. That's why they are so valuable. You will see what we mean in a
moment. Firstly the production of the waveform, then an animation of a signal
passing through a capacitor.
HOW
A SIGNAL PASSES THROUGH A CAPACITOR
The
animation above shows a signal passing through a capacitor. In a previous question
we showed how an uncharged capacitor connected between a battery and LED, caused
the LED to flash briefly while the capacitor was charging, then remain
non-illuminated. This shows how a capacitor passes a current when it is charging.
In the animation above, when the voltage on the left-hand side of the capacitor
is rising, the capacitor "thinks" it is being charged, and a current
flows. This causes the voltage on the right-hand side to rise and
trace out a path very similar to the input voltage. The result is the waveform
on the right-hand side is very similar to the input waveform and we can say
the voltage is PASSING THROUGH THE CAPACITOR.
When the voltage on the left-hand side of the capacitor falls, the capacitor
"thinks" it is discharging and and the charge on its plates
"flows out" and the voltage on the right-hand side follows the
left-hand side. Question 52: A rising
and falling voltage is called an "__ voltage."
Ans
AC voltage Question 53: An AC voltage
(passes/doesn't pass) through a capacitor.
Ans:
AC passes through a capacitor Question 54: Draw the symbol for
a capacitor:
Ans:
refer to previous page Question
55: A capacitor (blocks/doesn't block) DC.
Ans:
It blocks DC WAVEFORMS
Before
we can talk about signals entering a capacitor, we have to understand how
signals are represented in technical books. The only way you can represent them
is by a graph. This is called a GRAPH OF THE WAVEFORM. A graph shows the way the
signal has risen and fallen over a period of time. To read this graph (and every
graph) you have to imagine having a pointer as shown in the animation below. The
pointer gradually moves across the graph and where the graph touches the
pointer, the amplitude is read off the graph. In the example below, the voltage
rises to a peak then falls to a low, rises to a peak and then falls. The voltage
being represented by the graph has a DC component as well as an AC
component. We can see this by the fact that the voltage does not go below the 0v
axis (x-axis) and you could say it represented a voltage of 7v, with a rise to
10v, and a fall to 3v.
We
will now take this one step further and show how the voltage enters a capacitor
and emerges with the same wave-shape but with the DC component removed. This
is what we mean when we say a "Capacitor blocks DC but passes AC." Take the case of
the signal above (a signal is any type of varying waveform). Suppose the signal rises to
10v and drops to 3v. The frequency of the signal is not important. The animation
below shows the result:
The shape of the signal is not altered
by the capacitor. Whatever goes into the capacitor comes out. However the DC component (the
height of the waveform above the X-axis) is removed and the signal emerges as a positive and negative waveform based around the x-axis.
Without an animation, this important concept could not be demonstrated. As
the pointer moves across the graph from left to right, the rise in voltage (that
is: the rise in voltage from the starting point of the graph) is passed to the
capacitor and the output lead of the capacitor delivers this rise to whatever
component is connected to the capacitor.
Using an animation, we can also show
the concepts of how a resistor works:
These are simplified
interpretations of how components work but the amazing part of being a circuit
designer is the ability to "see" a circuit working in your
"mind's eye" before actually putting it together. You can also
generate the ability to diagnose a fault in a circuit by bringing up a mental
picture of the circuit diagram and working out the fault.
Mental pictures of "components in
operation" is an essential part of understanding electronics. You will need
all your ability to understand how a component works. It is the
transistor.
THE
TRANSISTOR
The transistor is truly an amazing
device. The way the story goes, one of the inventors worked out the theory for
for its operation on his way to the university, in a train.
Lacking paper to write down his notes, he wrote the equation on the front page
of the morning's paper! - so started the design of the first transistor.
Early transistors fitted only about 10 to a thimble. We can now fit over
1,000,000 in the thimble - all connected together in a working circuit!
If you are going to work with a
component, it's very handy to know how it works. It makes designing so much
faster and easier.
The transistor is basically an amplifying device. It has three leads and when
one "unit" of electricity enters one of the leads, 100
"units" of electricity flows through the transistor via the other two
leads. The unit of electricity can be the microamp or milliamp. These are
sub-multiples of the AMP. The Amp is CURRENT and is the flow of electricity. The
transistor is called a current amplifying device.
Transistors come in all shapes and
sizes but they all work on the same principle. The different sizes of transistor generally indicates the amount of current
they will pass. The diagram below shows some transistor types.
All transistors have three leads: BASE,
EMITTER AND COLLECTOR. A transistor is an amplifying device. It can have an
amplification factor of 10 or 20 or as high as hundreds and even thousands of
times. For instance, if you allow one microamp of current to flow into the base,
the transistor will allow 100 microamps of current to flow In the
collector-emitter circuit. This means the transistor has a gain of 100.
The names on each line of the symbol of
a transistor are always the same but the pinout of a transistor can be
different. We have shown the standard "cbe" (c = collector, b = base,
e = emitter) when the face of the transistor is UP (the type-number is printed
on the face) but the pinout maybe "ecb" or "bce" and you
have to consult the manufacturer's pinout guide for identification.
An animation is the best way to
"see" how a transistor works. When a small current is fed into the
base, a LARGE current will flow between the collector and emitter terminals. The
animation is not entirely accurate. You have to understand the collector-emitter
current flows THE INSTANT current flows into the base.
A transistor has to be connected to the
power rails with a RESISTOR on the base lead so the appropriate current flows in
the base for the type of circuit you are constructing.
On the next page we will combine the
transistor and resistor to produce a simple circuit and show how a very small
amount of electricity is needed into the base and the transistor will allow a
higher current to flow between the collector and emitter leads.
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