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Old 15th July 2005, 02:51 PM   #1
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Default Reuse or rewind choke

I have three chokes salvaged from speaker crossovers. They are small (fits in the palm of the hand). They appear to be air gapped (which surprised me) but I haven't torn them appart to be sure. I doubt that I will ever use them for their intended purpose so I am trying to determine if I can put them to use elsewhere.

I imagine that the inductance is much to low to be used as plate chokes or power supply filter chokes so I am wondering whether the iron could be rewound for use as plate chokes for para feed, PS chokes, phase inverter transformer, or possibly even a small OPT. Anyone reused such small iron for anything useful?

mike
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Old 15th July 2005, 05:28 PM   #2
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> chokes salvaged from speaker crossovers. I imagine that the inductance is much to low to be used as plate chokes or power supply filter chokes

Well, think about it. These chokes are something like 8 ohms at 1KHz. A plate choke needs to be like 5K at 50Hz. So just counting on thumbs, the inductance is (50/1000)*(8/5000)= 0.05*0.0016= 0.00008 or 1/12,500 of what you need.

Inductance goes as square of number of turns. To get 10,000 times more inductance, put 100 times as many turns on. Obviously the original gauge won't fit, use wire of 1/10th the diameter.

> appear to be air gapped (which surprised me)

The inductance/voltage curve of a plain iron core is very bent. At very low signal levels you may have 1mH, 10mH at medium level, 20mH at high level, and then suddenly dropping to 0.0020mH as the iron saturates. For wall-power work, fine. For small-signal transformers, you get the zero-signal inductance good enough, and things just get better from there. But for setting a 1.0KHz crossover filter, these 10:1 changes of inductance may throw your actual crossover from 300Hz to 3KHz depending on signal level. You could over-size the iron to always work in very-small-flux range. Actually, instead of buying that much iron, spend the money on more copper and an air-core. But the near-universal commercial answer is to use enough iron to improve the inductance of an air-core, but leave enough air in the circuit that the iron's inductance shift is overwhelmed by the air.

If they are transformer iron (not ferrite): yeah, you can probably re-wind them. Finding small-gauge magnet wire may be hard, and your best bet may be places that will also sell you iron-cores. I imagine the value of a crossover choke core is $2-$10, while the wire may be $5 or more with minimum order of $50.

And winding small-gauge wire is frustrating. You have a Loooooong length to put on, and every few feet it snarls, kinks, breaks. Think about 3,000 turns, 500 feet of wire, very-very thin.
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