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.
Transformers:
In a previous section (Switching
Power Supplies),
I touched on transformers. You should remember
that a transformer has a primary winding and a
secondary winding wrapped around a core. In car
audio, the core is usually a donut shaped toroid.
A transformer can be designed to step voltage up
or down, and/or isolate. We discussed isolation
in a previous section. This section will deal
with stepping the voltage up or down.
Winding Ratio:
Remember that the transformer
windings are enamel coated magnet wire which is
wrapped around the core (as seen below). The
number of windings is determined by the number of
times that a piece of wire makes a complete turn
around the core. The primary winding is the winding which is driven (in car audio amplifiers, it's
usually driven by transistors). The secondary winding is the output winding. The secondary is driven by
the magnetic field that the primary induces in
the core. A transformer with a ratio of 1:1 will not
cause a voltage increase or
decrease (disregarding small losses) from the
primary to the secondary (as measured across each
of the individual windings). If the ratio is 1:2
(primary:secondary), the voltage across the
secondary will be twice the voltage across the
primary. A ratio of 1:3 will result in a
secondary voltage 3 times as high as the voltage
on the primary. Of course all of this applies to
a transformer which is very lightly or not loaded
(minimal current flow).
When current is drawn from the secondary winding,
there may (will) be a voltage drop and therefore
a primary to secondary voltage ratio which may
not match the winding ratio exactly. This loss of
voltage is primarily due to the less than 100%
efficiency of the magnetic coupling of the
primary and the secondary windings through the
core and also some copper (resistance) losses.
Remember that the primary and the secondary
windings are not generally electrically connected
together. This means that all of the power
transfer between the primary and secondary is
transferred (magnetically) through the core. The
transformer below is similar to one that you
would find in a small car audio amplifier. The
winding ratio is 1:2. The different colors are
each half of the center tapped primary and
secondary. Notice that there are twice as many
secondary windings as there are primary windings.
The schematic symbol shows how the windings
relate to each other. The center tap of the
primary (red) is connected to the battery. The
center tap of the secondary (black) is connected
to ground.
As a side note:
The power driven into the
primary will equal the power produced at the
output of the secondary (if we ignore wire and
core loss). If we have a 'step-up' transformer
with a 1:2 ratio being driven with 24 volts A.C.,
the secondary output voltage (disregarding
losses) will be 48 volts. If we load the
secondary so that 5 amps of current is flowing
through the secondary windings, the power output
is P=I*E; P=5*48; P=240 watts. Since the power
driven into the primary equals the power out of
the secondary, we know that the power being
driven into the primary is 240 watts. If we use
the formula I=P/E, we see that the I=240/24; I=10
amps. If we were stepping the voltage down, the
current flowing through the primary would be less
than the current being drawn through the
secondary windings.
Advanced Info:
When designing a transformer
you have to calculate the number of primary
windings so that the transformer will operate
properly/efficiently. There are a few different
variables that have to be taken into account.
Ac:
Ac is
the effective cross sectional core area. This
number is supplied by the core manufacturer.
B:
Flux density (B) is expressed
in gauss. If the flux density is too high the
core will saturate (effectively disappear from
the magnetic circuit - very bad). Generally, in
car audio amplifier switching power supplies
operating at or below 35.000hz, the flux density
is kept at or below 2000 gauss. Some cores will
operate at higher flux density for frequencies
below 35,000hz but 2000 gauss is a good
conservative number. For higher frequencies, you
have to design for lower flux density to prevent
excess core heating. The following chart shows
the approximate maximum flux density for a given
frequency. For a more precise value for a given
core material, refer to the core manufacturer.
Primary Voltage:
The primary voltage for a
push-pull system is double the primary input
voltage. For car amplifier switching power
supplies, the input voltage is 12VDC. This means
that the total primary voltage is 24 volts. If we
use 13.5 volts as the input voltage, the primary
voltage would be 27 volts.
Operating Frequency:
The operating (oscillation)
frequency is simply the frequency at which the
primary is driven. Generally between 25,000hz and
100,000hz in car audio amplifiers.
Primary Turns:
The number of primary turns
returned by the calculator is the total number of
turns on the primary side of the transformer. Of
course, with a push pull system, the number of
turns on each half of the primary must be equal.
If the output says that you need 13 turns, you'd
round up to 14 turns and each half of the primary
would have 7 turns. From the previous diagram,
you'd have 7 orange turns and 7 green turns on
the core.
Skin Effect:
When wire
(specifically, solid copper conductors as are
used for transformers) is used to carry DC
current, the entire cross sectional area of
copper carries the current equally. When wire is
used for AC current, the current is carried
differently. At low frequencies, the current flow
is not significantly affected by the skin effect.
As you get into the higher frequencies (as those
used to drive a transformer in a switch mode
power supply), the current flow is carried
disproportionately by the outer area of the
copper wire (especially for large single solid
conductors). This is called the skin effect. If,
for example, you are using 14g wire at 100khz,
the wire will not be able to carry the same
amount of current as it could if it were passing
DC. If your calculations told you that you needed
to have ~4120 circular mils, you'd have a few
choices. You could use 1 strand of 14g wire, 3
strands of 17g or 6 strands of 20g. All would
have the same current carrying capacity if you
were using it in a DC circuit but... If you were
using it for AC, the 14g would only be suitable
for frequencies below ~6000hz. Above that
frequency, the voltage losses and power
dissipation may be unacceptable (it would still
work above 6000hz but not efficiently). The
maximum frequency that you'd want to use with the
17g would be about 11,000hz. For 22,000hz the 6
strands of 20g would be a good choice.
As you can see in the
following demo, the current flow is AC (because
the direction of flow changes at regular
intervals). You can also see that the electrons
on the outside of the conductor change direction
instantaneously. The electrons deeper in the
conductor take a little longer to get going. When
the frequency is high, the electrons deep in the
conductor don't precisely follow the flow of the
outer electrons.
The following shows how the area
of current carrying copper (the copper colored area) diminishes
as frequency increases. The up/dn buttons control frequency. If
you have a large diameter solid round conductor, the conductor
will appear to be increasingly more hollow at higher frequencies.
At high frequencies, the wire will act more like copper tubing
than a solid copper wire.
Notes:
This is the maximum diameter
solid wire that will have 100% current density (100% of
the wire will have current flowing through it).
This is the maximum cross
sectional area of round wire that will have 100% current
density.
This is the maximum wire
gauge that will assure ~100% current density at the
frequency entered in the calculator above. If you're
selecting the conductors based on copper area (in
circular mils), you need to use enough strands of this
gauge wire to add up to the desired number of circular
mils.
When using large gauge wire
for high frequencies, you have loss due to 'AC
resistance'. When using a number of smaller strands to
equal the desired/required number of circular mils, you
reduce the loss. If each strand has a radius that's less
than or equal to one skin depth, you will virtually
eliminate the loss at high frequencies. In effect, the
resistance to AC (at or below the frequency entered
above) and DC will be the same.
Number of strands of
calculated wire gauge (3) to equal the wire gauge that was entered
in the input section of the calculator.
If
you find a problem with this page or feel that some part
of it needs clarification, E-mail
me.
