O.K here we go. This first bit is for those who have no idea at all. I have tried to explain what an inductor does and how to caculate the size required. If any of the more knowledgable peeps out there spot a flaw or think something can be worded better (including speelin mistakes) don't hesitate to let me know asap.
The next post to come (assuming it doesn't go all pear shaped) will be about the physical attributes of the inductor and how to calculate height, depth, width, No. turns and wire length. The post after that will encompass how to physically make it, making a bobbin, winding the copper and finnishing off.
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How do inductors work?
The very basics, when current flows through a coil a magnetic flux is generated
proportionate to the current flow. When the current changes direction inside the coil
the magnetic field must also change. How easy it is for this current to alter the
already established magnetic field depends on the ammount of inductance and speed of
current change. The more inductance you have the harder it is to alter the current flow. If you have a very small inductor (say 300 micro Henries) it will start to oppose current that changes
at or faster than 4.24Khz. What this means is that it will choke any current that
alternates above 4240Hz because (to put it in laymens terms) that speed of change
is too fast to alter the magnetic flux. However currents that alternate slower,
that is any below 4240Hz will have sufficient time alter the flux and thus don't
get opposed by the inductor. Another analogy that can be given for the inductor is water, the faster you try to change directions in water the harder it becomes.
I normally have a lot of trouble with communication so if the above doesn't make sense
I will include a few links that might help explain it better.
Wiki Howstuffworks although it isn't all that crash hot.
Now the maths:
lets start with how I worked out that a 300 mH inductor would filter at 4240Hz,
L = (0.159*Z)/F
or
F = (0.159*Z)/L
If L = Henries
F = Frequency
Z = impedance of circuit (in our case the speakerdriver) in ohms
then
(0.159*8)/0.0003 = 4240 Hz
Another example:
Assuming we have a speaker with an impedance of 6 Ohms and we want to cross at 3500Hz.
then: (0.159*6)/3500 = .000272 Henries or 272 micro henries.
thats all for now.
And if this takes off maybe we could work on an XO design tutorial and sticky it for all to learn
