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Registered Member #1731
Joined: Thu Oct 02 2008, 02:22PM
Location: Indiana
Posts: 52
Can anyone give me some pointers to how I can calculate the fabrication of the 4.7mH inductor (L3) here: (In other words, if I know the dimensions of the ferrite, and the wire gauge, how many turns/layers will it take to create a suitable inductor?)
I ordered some pre-made from Digikey but when they arrived they were odd dual-coil and only 4.0mH.
I have a bunch of ferrites (linear and toroid) and a whole lot of magnet wire from 18 up to 40 AWG.
The only thing I don't know is the permeability of the cores* - apart from they came from AM radios and PC power supplies.
I did some Googling but only turned up air core calculators.
All formulae gratefully received.
Steve
*I understand that it is technically impossible to accurately calculate the inductance without knowing this - but if there is a practical assumption I can make then that would help.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
The relative permeability of a ferrite core can range between roughly 16 and over 640 according to Wikipedia, so guess 100 for example but you can be off by a factor of 6 in either direction which isn't super useful. Roughly you can assume that the inductance is that of an air coil times the relative permeability. If you keep googling you will find non- air-cored inductor calculators, they are out there, such as this hyperphysics one.
You could quickly estimate the permeability of your core by winding an inductor and then measuring the inductance, plugging the value into the formula to solve for permeability. But it's hard to predict precise values for inductance, especially for hand-made coils, so I'd recommend to just guess and check as you go. Measure the inductance and if you're halfway there then add about twice as many turns.
I'd also wonder if it matters if you have 4mH or 4.7mH. I don't fully understand the linked schematic, but I'm guessing L3 is an RF choke (don't quote me on that :) ), in which case should still work even with quite a bit of variation in the specific value of L3.
Registered Member #1731
Joined: Thu Oct 02 2008, 02:22PM
Location: Indiana
Posts: 52
That's a good idea - I wish I'd thought of that.
I'll give it a try and see what numbers come up.
I currently have problems with the circuit so I'm slowly rebuilding it - the 4.0mH coil was one of the things I wanted to do better the second time around.
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Need to wind a 4.7mH inductor in the smallest possible space
