Inductor

The simplest method to make the large value of a 3 Henry inductor is to make a very large coil of wire. There are many web based calculators available that can permit you to figure out how large of a coil you will need. You can also place a ferromagnetic material inside this coil to reduce the number of turns needed to make this inductance. But you will now have to know more of the signals this inductor will see to be sure that the frequency will still be in the ferromagnetic material's range and that the flux density will not saturate any part of the core material.

You cannot make an inductor (L) from just a resistor and capacitor (RC) combination. You can make an RC act like an RL in making either a high pass or low pass filter but not just the inductor impedance. With active circuitry of a few operational amplifiers one can make an input impedance of an inductor with a general impedance converter and several Rs and a C. But as the link shows, this is not a simple circuit. It is used very often though in filter designs that require very large stable values of C or L.

Thanks for the link...must go deeper there.

However, i have been using this formula ...

Do you think this is also OK?

i will check out both tomorrow (two shots of single malt is not conducive to inductance )

The classic formula for the inductance of a tightly wound solenoid is L=μ(N²/l)S. Now L is the inductance in Henry. μ is the permeability of the space inside the solenoid. N is the number of turns. l is the length of the solenoid. Lastly, S is the cross sectional area of the loops.

This is from Fawwaz T. Ulaby's book "Fundamentals of Applied Electromagnetics". But I'm sure you'll find this elsewhere.

I obtained the photo from this website, which has a nice discussion of gaps in cores, why you want one, and how to think about the subject.

http://info.ee.surrey.ac.uk/Workshop/advice/coils/gap/

I agree that a 3H inductor is a very large inductance. I also agree that making a 3H air core inductor will be a large, probably impractical component. As you pointed out, it all depends on how the OP intends to use this enormous inductance. The OP may have been just wondering if a 3H inductance was a practical value. When and if the OP gets back to us, we cannot offer any further guidance.

Ahh how stupid of me. Never looked at the 3H number. Thanks for the tip, GA to you.

IEC 60947-5 Annex B gives the details of the iron-cored reactor to be used as a test load for DC relays which can work on highly inductive circuits (L/R ≈100ms) Here is an image from the standard. Obviously, this is of American origin when you see that the mm dimensions are in fact inches x 25.4. So i am sure many CR4 experts will be familiar with the US standard where this appears .. i look forward to learning more on this subject.

Inductors are more efficient at producing required inductances than a regular solenoid as they are smaller in size and require fewer turns. The amount of magnetic flux that escapes outside the core is minimum, thus radiating less electromagnetic interference to nearby circuits. There is no air gap, very low electromagnetic radiation, no noise, and a safe design with superior quality wiring, thus making our inductors the safest, and Miracle Aerospace one of the most dependable air core inductor manufacturers in India.

Please describe in detail (with specific numbers) what you are trying to do.

Why? Because designing a 3 [H] inductor requires MORE information.
LC, LR, and RC, filters need to be properly scaled to the circuit and load parameters.

nominal current=
nominal voltage=
driving circuit impedance=
load impedance=
maximum allowed power dissipation=
passband and/or stopband requirements=

When these are properly defined, you can properly design your filter.