Cross-Over Network;
Air Core Inductor Calculator

The following application will generate a design, if you tell it what the inductance needs to be (in milli-Henries - mH).

Please fill in the following Information.

       Inductor mH:  (milli-Henries)

When you make the form, it should allow you to create an inductor that has this general shape. It may be possible to find pre-made forms, but you really don't need one once the wire has been wrapped and secured.

You'll need to create a form (this could be made of plastic or wood; what ever you have available) to wind your inductor in. You'll need to be able to have the correctly sized opening to wind the core onto (the program gives that as Radius - multiply this by 2 to get the diameter), and you'll need to be able to brace the form to keep the windings together at the right width. Once wound, use plastic/nylon Wire Ties, or electrical tape, to hold the inductor together. These can become quite warm when in use. The DC Resistance is an indicator of how much power these will soak up - the lower the DC Resistance the better. For example, if you create a 10 mH inductor using 26 gauge wire, its DC Resistance is 7.89 Ohms - which is approximately the same as the 8 ohm impedance of the Woofer - this implies that the Inductor will absorb an equal amount of power as the Woofer will. If you try to pump 200 Watts into this, the Inductor will dissipate 50 percent (or 100 watts) and will most likely fail in short time. Using 14 gauge wire changes the DC Resistance to .86 Ohms, and instead, the Inductor absorbs only approximately 10 percent of the power (or 20 watts). It still gets warm, but it won't fail catastrophically.

Use transformer wire that has a lacquer finish. Do not use wire that has plastic sheathing around it - this is not appropriate for winding these Inductors.

The program rounds up or down to the next full turn.

For assistance in determining cross-over points, See: First Order Crossover Networks and Second Order Crossover Networks

You may need a source of this type of wire (it needs to be solid wire that has a thin coating that insolates it electrically). This is normally called 'magnet wire' or 'bell wire'. For your reference, here are the first three sites that came up when I looked for 'magnet wire 18 gauge' in a search engine (I have no association with these at all):

• http://www.parawire.com/18gaugemagnetwire.shtml
• http://www.powerwerx.com/magnet-wire.html
• http://www.elexp.com/cbl_mg30.htm

I often buy my wire at electronic surplus houses - you can get exceptional deals there if you are lucky.

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Problems? Comments? Contact Jens Moller.