Please
'Boom' Responsibly As
most of you have noticed, the noise ordinances have
become much tougher lately. Most of this is due to
idiots, yes IDIOTS, who drive through residential areas
with their windows down while their system is playing at
full power. To make things worse, the music they listen
to has all sorts of foul language that's not suitable for
small children, (who may be playing outside). There are
even a few people, who are even beyond idiot status, that
play their systems at full power through residential
areas after 10:00 PM (when many people go to bed). I
don't believe that this type of behavior is good for the
industry. If the fines get too stiff, people will stop
buying large systems. If this happens, more people will
get out of car audio (who wants a mediocre system).
People get interested in things because they're exciting.
A deck and four 6.5" speakers are not going to
interest many of the younger car audio enthusiasts. If
car audio enthusiasts keep annoying more and more people,
the fines will keep getting tougher. All of this will
only reduce interest in the equipment that fuels the
industry. If you want to listen to your system at full
volume, get out on the highway where there's little
chance of bothering anyone. When you get to a red light,
turn it down. If the only thing attractive about you is
your 'system', you have some work to do. Bottom line...
Think about what you're doing. Think about other people.
It's not the end of the world if you have to turn the
volume down for a little while.
Diodes:
A diode is an electronic
component that, in general, will pass current in
only one direction (there are a few exceptions
like zener and current regulator diodes). They
are used in virtually every piece of electronic
equipment. In head units, they are virtually
always used across the power input terminals to
protect the head unit in case of reverse polarity
(hooking the power wires up backwards). In
amplifiers, they are used as rectifiers to
convert AC to DC. In a large percentage of audio
equipment, Zener diodes are used as voltage
regulators. In alarm systems, rectifier diodes
are commonly used to isolate 2 trigger sources.
For a general purpose rectifier
diode... when the voltage on the anode is more
positive than the voltage on the cathode, current
will flow through the diode. If the voltage is
reversed, making the cathode more positive, then
current will not flow through a rectifier diode
(unless the peak reverse voltage rating is
exceeded).
When voltage is applied to a
diode and current is flowing through the diode,
there will be approximately a .6 volt drop across
the diode. In this first diagram, I've included
the voltmeter so that you could see how the
voltage indicators represent voltage. I'll use
the indicators only in the rest of the diagrams.
The green rectangular device is a current
limiting resistor. It's needed to prevent
excessive current flow through the diode when
forward voltage is applied.
When the voltage on the cathode
is greater than the voltage on the anode, current
will not flow through the diode.
A picture of a couple of common
diode packages.
SPECIAL PURPOSE DIODES
Before we discuss any specific
types of special diodes, we need to show how
voltage across a diode and current flow through a
diode are related. The following graph shows
voltage on the x-axis and current flow on the
y-axis. As you can see, for a forward biased
diode, as the voltage reaches ~0.6 volts the
current flow starts to increase significantly.
Before the voltage reaches ~0.6 volts, there is
virtually no current flow. Above ~0.6v there is
virtually no resistance to the flow of current.
The same thing happens as the reverse voltage
approaches the reverse breakdown voltage. If you
push the 'sweep voltage' button, the voltage will
sweep from the greatest negative value to the
greatest positive voltage.
ZENER DIODES:
Zener diodes are generally used
for voltage regulation. The diodes are used with
reverse polarity when compared to their rectifier
counterparts (you hook them up backwards to make
them work properly). All diodes have a point at
which they will conduct current when sufficient
reverse voltage is applied. Most diodes are
damaged when the reverse voltage reaches the
breakdown (or avalanche) voltage. This is
primarily due to the lack of any current limiting
resistor. Zener diode circuits have a current
limiting resistor in series with the diode as
part of their design. In the diagram below, you
can see how the positive terminal of the battery
is connected to the resistor. The other end of
the resistor is connected to the cathode of the
zener. The other end of the zener, the anode, is
connected to ground. If the zener diode is a 5.1
volt zener, the voltage on the cathode of the
zener will be very close to 5.1 volts. The
voltage is going to be close (but not usually
exactly) the rated zener voltage. You can
sometimes get the voltage very close to its rated
zener voltage by varying the value of the
resistor. This changes the current flow through
the diode. If you look at the curve, you can see
that a change in current (near the breakdown
voltage) corresponds to a small change in the
breakdown voltage. This type of circuit is good
for use as a voltage reference but it is not very
good to supply regulated voltage to circuits that
draw a large amount of current.
Diode analogy:
In the diagram below, you see a
zener diode connected to a voltage source with a
current limiting resistor. Just to the right of
the electronic circuit, you see a rubber band
(cyan) which is analogous to the resistor and a
piece of nylon string (green) which is analogous
to the zener diode. In the mechanical version of
the resistor/zener, you see the piece of looped
string and rubber band lying on the 'ground'
because no voltage (force) is applied to them.
When you 'apply voltage', you see a string (a 5
volt zener) being pulled up to 5 volts (inches)
by the rubber band. There is no resistance to the
change in voltage until the string (zener)
reaches 5 volts. Any voltage between 0 and 5
volts will be unregulated and will fluctuate with
the force (voltage) exerted on the rubber band.
As the simulation ends, you see that the force on
the rubber band is significantly more than it was
when the voltage was near 5 volts. Even as one
end of the rubber band is at 12 inches (volts),
the piece of string (zener) is holding the bottom
of the rubber band to 5 inches (volts). The
rubber band (resistor) keeps from breaking the
string when the voltage exceeds 5 volts (inches).
If there were no rubber band and instead the
force (which we will consider to be infinitely
strong) moved past 5 inches (volts), the string
would be instantly broken. If the resistor has
insufficient resistance or the rubber band would
pull up too high (higher than 12 volts/inches),
the string/zener may still be destroyed.
Now for maximum confusion, what
if we have a secondary load on the system. The
load would act as a second rubberband pulling
down on the first rubber band. A higher current
load would pull down harder. To keep the
regulated voltage constant, you'd have to make
sure that the top rubber band could apply enough
force to over come the downward force of the load
and still be able to hold pull up to 5 inches
(volts). If the downward force from the load was
too great and the pull up resistor (rubber band)
isn't stiff enough, the voltage will become
unregulated. As long as the pull up resistor is
stiff enough to pull up the load to 5 volts, you
will have a 5 volt regulated source.
Current boost regulator:
The diagram below shows a
circuit which will increase the available current
output at a regulated voltage. The current
supplied to the electronic device will pass (for
the most part) through the transistor. This
allows the regulator to supply current to a much
tougher (lower resistance) load. Please keep in
mind that you will lose approximately 0.5-0.7
volt through the transistor. If you need a
regulated voltage of approximately 15 volts,
you'd use a 16 volt zener. This would actually
give you a regulated voltage of approximately
15.4 volts. If you need tighter/more accurate
regulation, you'd have to use a more complex
regulator circuit. More complex regulators are
shown on the op amp
page.
Calculating resistor value:
For a basic voltage reference,
choose a resistor that will allow about 1/4 to
1/2 of the zener's allowable current to flow.
Since Zener's are rated by the
amount of power that they can dissipate and you
have a 1 watt 5 volt zener diode, use the formula
P=I*E.
You know that the power rating
is 1 watt and the voltage is 5 volts (it's a 5
volt zener) so... with a little manipulation of
the formula we have:
I=P/E
I=1/5
I=.20
amps This is the most current that the zener can
safely pass.
Now bearing in mind that we are
only using the zener as a reference, we will only
pass enough current to make the zener function
properly. About 25% of the max current should
work well. 25% of .2 is .05 amps. I will assume
that we're using an unregulated supply voltage of
13.5 volts (the 12 volt battery). The difference
between 5 volts and 13.5 volts is 8.5 volts. The
resistor will have to drop 8.5 volts at .05 amps
to properly limit the current. If we use the
formula E=I*R we have:
R=E/I
R=8.5/.05
R=170
ohms or the closest available value.
Then we have to find the power
dissipation in the resistor. We can use the
formula P=E^2/R:
P=8.5^2/170
P=72.25/170
P=.425
watts
This means that the resistor
needs to have a resistance of 170 ohms and must
be rated to dissipate 1/2 watt of power or more.
A resistor rated at less than .425 watts will die
a horrible, painful death.
These values will generally
work well with the current boost regulator. If
the load is taken off of the resistor/zener
junction, the voltage regulation will not hold if
the current draw is more than .05 amps.
All of these formulae can be
found on the Ohm's Law page.
You should remember:
1.Current will only flow in one direction through a diode.
2.When current does flow through a diode, the voltage on the
anode will be approximately .6 volts higher than the voltage on
the cathode.
If you find a problem
with this page or feel that some part of it needs
clarification, E-mail
me.
