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Class D Switching Power Amplifiers and Power D/A conversion 

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7th June 2007, 07:30 PM  #1 
diyAudio Member
Join Date: Jan 2007
Location: Tarn

Air core making
I have been experimenting with making an air core inductor for my Amp6, winding 0,71 mm wire around a 6mm bronze rod (only as a tool) made 25 turns in 4 layers. I mesured 8uH. I need 10uH.
Does anybody know what lenght in how many turns would lead to 10uH saving me ending up with an giant amount of cores of all possible values?? 
7th June 2007, 07:46 PM  #2 
diyAudio Member
Join Date: Nov 2002
Location: currently in China

Hi Theon,
You might already get past 10uH , the metal rod is making your measurement inaccurate. You should try using PVC pipe instead. Hartono 
7th June 2007, 08:04 PM  #3 
diyAudio Member
Join Date: Jan 2007
Location: Tarn

Hi Hartono,
I'm using the rod only as a tool to make the turns. I'll take the out the rod when I'm done. I have a nylon 6mm rod though, wil try that. 
7th June 2007, 08:18 PM  #4 
diyAudio Member
Join Date: Nov 2002
Location: currently in China

Hi Theon,
If you can measure the coil without the metal rod, you will get more accurate measurement. Good luck ! Hartono 
7th June 2007, 08:20 PM  #5 
Did it Himself
diyAudio Member

Inductance increases by the square of turns. i.e. increase turns by 10% and you get a 100% increase in inductance. Try it, my memory is a little rusty but I think that's right.
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7th June 2007, 08:37 PM  #6 
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Join Date: Jan 2005
Location: Pennsylvania

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7th June 2007, 09:19 PM  #7 
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Here is the calculator that I use:
http://www.lalena.com/Audio/Calculator/Inductor/ "Coil Diameter" is the inside diameter. "Coil Length" is how tall the coil will be. It's all in inches and feet, so you will have to convert, sorry. Your 0.71mm wire is closest to 21AWG. (It's probably 22 gauge British wire) Your 6mm core is about 0.24" I chose 1.5" for coil length. You can try different coil lengths to get the shape you want. Here is a sample result using your wire, core and value. Inductance 0.01 mH DC Resistance 0.09 Ohms Wire Gauge 21 AWG Wire Diameter 28.5 mils (1 mil = .001 in) Coil Length 1.5 in Coil Inner Diameter 0.24 in Coil Outer Diameter 0.35 in Average Turn Diameter 0.27 in Wire Length 6.75 feet Copper Weight 0.02 pounds Turns 97 Level 1.84 Turns/Level 52.63 This calculator works well for me. Making 10uH (0.01mH) coils is a little tricky, it's the bottom range of my inductance meter. But I double check by measuring several mass produced coils. Then the air cores are made all the same value. Also check his help page for the calculator: http://www.lalena.com/Audio/Calculat...ctor/Help.aspx The formula is there, as well as a chart of wire sizes. Here is a nice calculator to help find wire sizes. http://www.efunda.com/designstandard...re_forward.cfm
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7th June 2007, 10:51 PM  #8 
diyAudio Member
Join Date: Jan 2007
Location: Tarn

Perfect guys... thanks a lot!!

8th June 2007, 11:53 PM  #9 
diyAudio Member
Join Date: Jun 2004
Location: Edmonton area, Alberta

I may not be thinking clearly, so correct me if you can. But it seems to me that if you are making a simple straight coil, tightly wound, as you add turns the length of the coil will increase at the same rate as the increase in turns. Inductance will increase in mere direct proportion with, not with square of, turns.

9th June 2007, 03:42 AM  #10 
diyAudio Moderator Emeritus
Join Date: Jan 2005
Location: Pennsylvania

Tim is right that it isn't only dependent on the number of turns because, as he said, the length of the coil will have to increase if you add turns.
To figure out how many more turns you'd need to achieve a specific inductance you will have to crunch some numbers. If the diameter of the wire you use to wind the coil is small compared to the length of the coil the "inductance increases by the square of turns" estimation should get you close.
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