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Making inductors adjustable?

That's great info - I didn't know Jameco had data sheets at all. You sent me scampering to the web site, and lo and behold there it is under Tech Doc when you search on the choke. Thanks!

I think the OP rejected toroids a couple of posts back, but maybe he'll re-consider. Still, it's a no go. He's got a 1500 pF average load, and he can't resonate that with an inductor to the frequencies he wants. He'd need something like 40 Henry to get his 20 kHz. I was trying to point out that he would have to have a much higher load capacitance - at least ten times higher, and even then he is in the realm of large inductances. I should have inquired if he could place a parallel cap across the specified load.

If he's starting from scratch where the load capacitance can be whatever he wants to make it, then he can go with far more practical values. (I wouldn't call 263 turns practical - but that's not really relevant.) I fiddled around with the values and came up with a .22uF cap and the following more practical (i.e. less winding, fewer turns) inductances:

~80 kHz 18 uH (79.946 kHz) ~60 kHz 32 uH (59.959 kHz) ~40 kHz 72 uH (39.973 kHz) ~20 kHz 288 uH (19.986 kHz) ~20 kHz 272 uh (20.566 kHz) !!!!!!!!!

272 = 150 + 72 + 32 + 18 !!!!!!!

The 150 uH inductor can be purchased, and series connected to a wound, tapped 72 uH - or the 72 uH can be wound for

88 uH, tapped for 18, 32, 72, and series connected with the 150 uH - all to save on how many turns he has to wind.

If it were me starting from scratch and just experimenting, I'd buy off the shelf inductors and pick capacitors to get the frequency, perhaps using a variable cap in parallel with the series cap.

Reply to
ehsjr
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your timing is perfect. Run, don't walk to your nearest magazine stand and get a copy of the December QST ( ham radio magazine ).. There's an article for a home built condensor microphone in this issue.

Simple to build. Works great.. Have fun

- jim

snipped-for-privacy@bellatlantic.net wrote:

Reply to
James W

Yes - L in Henries, C in Farads

This might be helpful if you haven't been exposed to it before:

1 picofarad (pF) = .000000000001 farads (10^-12) 1 nanofarad (nF) = .000000001 farads (10^-9) 1 microfarad (uF) = .000001 farads (10^-6) 1 micromicrofarad (uuF) = 1 picofarad 1000 picofarads = 1 nanofarad 1000 nanofarads = 1 microfarad 1 "mickeymike" = 1 micromicrofarad

Same is true for Henries

You may see notations like 10E6 - that E means raised to the negative power, so for example, 10E6 = 10^-6

Reply to
ehsjr

On Sat, 6 Dec 2003 21:52:56 -0800, James W wrote (in message ):

Can I presume that this post was mistakenly put here?

Reply to
DaveC

I think you might have a decimal point in the wrong place.

By my calculations, 1500 pf will resonate with 42mH at about 20 kHz.

A perfectly practical set of values for audio work, you can probably buy the inductor pre-wound.

Not sure what the OPs actual project is, but at, as you said, at those values I might be inclined to adjust the frequency with a variable compression capacitor, as surplus they are likely to be less expensive and more available than a variable inductor.

Or have I completely missed something here?

Thanks in advance.

Reply to
BFoelsch

Hi

I missed what the app was. For some apps changing the coil Q works: this can be done with a swinging metal plate, swing it towards or away from your inductor.

Regards, NT

Reply to
N. Thornton

I hate it when that happens! :-) You're right - thanks for spotting it.

Reply to
ehsjr

Dunno what your space constraints are, but my old GR Impedance Measuring Box does it with an air-cored inductor swivelling inside another one. IIRC, I have had it resonate at 10 kHz and 80 kHz, but it's been a looonnnggg time. :) The variable cap was considerably larger than 1500 pf, tho. (also IIRC). I have toyed with toroids (pun intended); if you can reasonably add a DC component, you can vary the inductance by varying the current. May be more than you wanted to fool with, tho.

Reply to
als

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