Winfield said:

[email protected] wrote...

Michael A. Terrell wrote:

What power level and frequency range?

less than 1A (probably less than half that); 20KHz; 40 KHz;

60 KHz; 80 KHz -- a different inductor for each F (I presume

an adjustable one couldn't cover that entire range...)

What about a tapped inductor? Would that suit your needs?

It's not adjustable, but the inductance is selectable via

a rotary switch - or by moving an alligator clip. (But you'll

have to change your frequency - see below)

Frequency: - at those low frequencies LC is not your best

choice. To begin with, you would need a MUCH greater

capacitance to make it workable, and even then you'd neet

a LOT of inductance - unless my math is off. I figured a

4,000 uH inductor and a 15,000 pF cap would get you around

20.5 kHz [It's late, I'm tired, so you 'll have to check

my math: freq = 1/(2*pi*sqrt(LC))] Those values are not

practical.

4mH and 15nF are certainly practical with the appropriate

technologies. I see that Jameco's data sheet shows their

150uH toroid inductor has 51 turns.

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!

Rewinding it for 4mH

requires only 51 * sqrt (4000/150) = 263 turns, which isn't

hard by hand. The toroid core constant is A_L = 57.7 nH/t^2

and I can derive a formula t = (1/f) sqrt(25.33E6 / C A_L)

for the taps that DaveC will need as he winds. Using a

0.015uF cap we get t = 5.41E6 / f and so this table:

80kHz 67.6 turns

60kHz 90.2

40kHz 135.2

20kHz 270.5

The 1/f turn-to-frequency relationship is cute. Of course

DaveC can't do fractional turns, but I'm sure the nearest

whole turn value will be close enough.

Thanks,

- Win

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.