Hi all
I'am having probs finding some 470mH inductors(Toko 10RBH) for the attached circuit
so i was thinking of making some, i have some axial ferrite rod with leads at each
end they are about 10mm long and about 3.5mm dia, but i don't know what size of wire to
use, i have tried to make some but not having much luck.

I do have a home made LC meter but not to sure if it is any good at reading mH's, it is ok
reading μH inductors.

Anyone able to help me with what wire size and amount of turns to get to 470mH ??

Regards
Howard

 

Blimey! Those are some LARGE inductors!

Looks to me like you've done up a bandpass filter calculating the values by hand.

Why don't we re-visit your design, and see if the inductor size can be made a bit more reasonable. Othewise, you'll need a big lorry to deliver them.

What's the 3db points for your filter, or the center and width?
You want a Butterworth design, right?

You wouldn't want to use ferrite rod for this; it would not likely have much of an AL value, as it would likely be for tuning RF.

You'd want to use toroidal cores with very high AL values; like those you can salvage from electronic appliances (DVD players, satellite TV tuners, etc that've gone naff) - they are on the mains power wiring right inside the enclosure. They are used for keeping their own noise in, and other noise out.

Have a look at this Wiki entry: http://en.wikipedia.org/wiki/Balun
Halfway down the page on the left.

You really want to use a toroid because practically none of the energy escapes, and they're also more resistant to influence by outside sources than a rod would be.

 

Just a quick calculation; with a toroid that has an AL value of 2000, it would require 485 turns of wire (1 pass through the center of the toroid = 1 turn) to achieve 470mH.

[eta]
An Amidon FT-240-W ferrite toroid with 185 turns of AWG-23 (or smaller) wire would measure about 470mH; 10.3 yards of wire, the AL of the toroid is 13,690. However, those toroids are $30/ea on the Amidon website, not including shipping.

Amidon's W-material page: https://www.amidoncorp.com/items/22

Above calculated using the Mini Ring Core Calculator, freeware, available here: http://www.dl5swb.de/html/mini_ring_core_calculator.htm

 

It is UK based firm named "Sowter Audio Transformers" http://www.sowter.co.uk/index.php They may be able to help.

 

@SgtWookie
The do not have to be so big. You remember perhaps this discussion http://forum.allaboutcircuits.com/showthread.php?t=25947
This kind of inductor was used in audio equipment (equalizer stage). But they have been replaced with the gyrator and is not very common in production longer (if at all)

 

Thanks, I do remember that thread now that you mention it, but it's been awhile...

 

Hi

This is not my circuit!, but i was gonna use it for the audio filter in my Rx.

What i need is some audio filters for SSB, CW and FM(NBFM), to make
the audio sound better, and to drag the signals out of the noise, a poor man's
DSP lol

Howard

 

Hi
What kind frequencies do want to block out. I am asking because inductors are very frequency and material dependent. An inductor working well in the audio range may not work so well at higher frequencies.

 

Hello,

A quick search at mouser came up with this inductor:
http://nl.mouser.com/ProductDetail/...GAEpiMZZMueR%2blcXtRMwUgpeW7wajslwbI62ynMKgo=

Bertus

 

Hi all,
I design magnetics (transformers and inductors) for a living as part of my job designing Power Electronics products. You do not need a toroid, and for audio, I don't recommend ferrite material. You'll want to use a good grade of grain-oriented magnetic steel for the highest permeability (implies the lowest number of turns for a given size) and lowest losses for a given excitation. The problem with using a toroid of that sort is that you can't easily put a gap in it as the toroidal core is formed by winding a metalic tape. What I recommend as the best material for audio inductors would be an MPP core. These are toroidal, but the difference is that they have a distributed gap in them and this is often needed to keep the core from saturating on high peaks and low frequencies. If you used an M6 steel E-I core, you can gap the center leg and this is very effective as well and would have low loss. This would only be preferable to the MPP Toroid if you need high inductance, which I guess you do. As it does leak magnetic flux that fringes around the gap, you would want to shield around the gap with a magnetic material (any magnetic steel foil will do). This way, you determine the Al value (inductance constant per turns squared) and the permeability (the two go hand-in-hand) by the gap. For audio, it's likely a small gap of about 5 to 30 mils will do in either an E-I core or a C-Core (also called cut cores). As for the wire size, as a rough generality, you want to use a current density of about 600 circular mils per amp (see any magnet wire chart for awg size versus copper area in circular mils). You'll also have to compute the total window area the wire will use based on its diameter and the number of turms and make sure the core window cross-sectional area has enough room for it. The magnetic steels come in 3 main types: Selectron-useable up to 18kiloGauss, 80% Nickel-20% Iron steel useable up to about 5.5 to 6 kiloGauss, and 50% Nickel-80% Iron steel useable up to about 10-12 kiloGauss. For a given size, the maximum flux-density in kiloGauss is an inverse measure of the number of turns required for a given inductance (not including using a gap). The 80% Ni-Ir has a very low magnetization current, as does the Silectron steel. For some basic equations, and some very good tutorials on basic and intermediate magnetics design, I highly recommend the Texas Instruments Unitrode Power Supply Seminar books, done over several years, all free and in PDF format. Only a few of the articles are on magnetics design, but they are very thorough and accurate. You have to just download the seminar books and look thru the table of contents to see which ones have the magnetics tutorials in them.
Regards,
Kamran Kazem