Ferrite Inductor Tolerance

I don't know, but given the brand, size, form and material, the manufacturers site would probably have that info. I have Ferroxcube and Amidon catalogs if you are using those let me know. Mike

An ungapped ferrite core could be all over the place. 25% wouldn't surprise me. They will vary with temperature, too. You can buy gapped pot cores in tolerances around 2-5%, I think. Or use a pot core with a slug adjuster if you need 1% or better. See the datasheets.

Powder-type cores can be bought with better tolerances.

The people who wind inductors commercially get the exact number of turns every time.

John

The ones I am thinking of are the VTB series by Carnhill.

Cheers

Ian

That depends on how much you are willing to pay and whether it is a higher power core. 5% to 20% are common tolerance values, sometimes with inductors from China I spec 30% when it has to be really low cost.

It seems to me there are quite a few factors that could affect the actual inductance achieved and perhaps the least of them is the accuracy in counting the number of turns. I would expect there to be some tolerance in the Al value of the ferrite, that its exact dimensions would have an effect along with how neatly or otherwise the turns are wound. I have absolutely no idea if these are the major factors nor of the likely size of the actual major factors affecting the actual inductance. I am just trying to get a feel for the likely tolerance of ready made inductors.

The reason I ask is am am designing some passive audio filters and I know exactly what tolerance of resistance and capacitance I can obtain but not a clue about inductance. It is no good me using 1% capacitors and resistors if inductors normally fail to achieve 5%.

Cheers

Ian

You can get 5% catalog inductors from Delevan, Miller, TDK and several others. If it needs to be more precise then you'd be off to boutique lines, meaning $$$.

Example:

But sit down before looking at prices for the F grade (1%), they can run well north of $10 a piece in small qties and several months leadtime. Hardly anyone does this stuff for audio anymore since DSPs became cheap. Last time I designed an analog wideband audio phase shifter was ... ... oh, about 20 years ago :-)

If it absolutely has to be analog there's opamp gyrators, maybe that could work? Sure would be cheaper.

I'd expect more like tens of percents. Keep in mind that the inductance also varies with current. Thats why you always see air core inductors in loudspeaker filters. If you need accurate inductors you better to use a gyrator circuit.

Nobody needs to be that precise. Nobody here anyway.

Ahm, I have been. But that was in the RF world and done with active laser trim. Sometimes down to 0.25%.

Laser trim of an inductor? Small form factor maybe. Anything in the power realm has no need of that level of precision and there are few ferrite formulations that can hold any such tolerance, even merely sitting on the shelf.

We buy 2% inductors and 1% capacitors to make LC clock oscillators in our digital delay generators.

John

Active filters make more sense at audio frequencies. Inductors are big and expensive and have rotten Qs down there, so you have to do a predistorted filter design if you want any frequency response accuracy. PITA.

John

Well, Ian wrote that his app isn't power but precision audio. Different thing.

I did get nice results with discarded 88mH toroid inductors from Missy Bell though, built lots of filter with those back at college. Until one sunny day I looked in the box ... all gone :-(

But you are right, if there is power available active is the way to go. Or DSP :-)

Once you get above 100uH though prices can go through the roof.

Just the same... once you get up there, tight tolerance is of little importance. 5% is fine.

You want precision? You buy 500 cheap, 5% coils, and use in-house matching and culling techniques to get the matched set that you desire at a far far cheaper overall cost.

Those devices seem to be in the sub milliHenry range, the one I need are in the humfreds of milliHenries.

Cheers

ian

Fractional henry? In 1% sizes? With Q more than 1? Ya, they call those op-amps. You don't have any choice now...

Tim

How do you deduce that it's of little importance? Got a spy camera in every enterprise, worldwide? :-)

Environmentally and financially not very friendly unless you can sell the excess for a reasonable price. Also, I found that when inductors were at minus 15%-20% then, usually, the whole series was. In fact sometimes over months. So no dice there, I would not sign the ECO for that.