Inductor calculations with ferrite cores. ESR meter for testing?

[fastline]

First off, I would like to ask if the primary reason to use ferrite in an inductor is to simply amplify the inductance, thus reducing size and wire length requirements?

All I seem to be able to find is calculations for air cores. I was curious about using my ESR meter for testing but not sure if that can give an accurate value without additional hardware.

 

[dr pepper]

Yes ferrite increases the inductance for a given size and amount of wire.
Not sure what you mean about your esr meter, such meters usuallyapply around 50kc to a load, usually a capacitor, and then compare the volatge dropped to a resistance using a bridge connection, the ferrite trans used in the circuit is fairly irrelative, I've built a couple of these with any old trans pulled from scrap gear, both worked fine.

 

[fastline]

It just seems there is a lot of variables in winding an inductor, particularly with a ferrite core, and I have no way to calculate the values or test for them. I am trying to get around that issue right now.

 

[Sangoma]

Do a search for "toroid turns" or "toroid winding data"

Another reason to use them is the fields are more contained, and inductance is less affected by nearby objects.

You can get a cheap L/C meter on ebay for less than £20 usually, and they are quite accurate enough for general use.

 

[fernando_g]

Will the inductor carry DC current?
If so, to prevent saturation it will require an air gap...or use a powder-iron core instead.

 

[MrAl]

Hello,

What are you trying to do?

If you want to test ESR for an inductor you have to use DC. For a capacitor you use AC.

The permeability of the core affects the inductance by multiplying it over the value it would have with just air. So you could get 100 or more times the inductance with a core rather than just plain air.

When you look at core data sheets you usually find values that help calculate the inductance like AH values. These values are for cores with 1000 turns not to be confused with inductance per 1000 turns.

 

[Nigel Goodwin]

Not sure what you mean about your esr meter, such meters usuallyapply around 50kc to a load.

It's 100KHz, because that's the value used in manufacturers specifications - although I suspect 50KHz would make little difference.

 

[dr pepper]

The osc in my tester is 50kc but that might well be doubled by the circuit

 

[Nigel Goodwin]

It just seems there is a lot of variables in winding an inductor**broken link removed**

There is, it's an art as much as a science

Single layer air-cored coils are pretty straight forward, but once past that it gets more complicated.

I would suggest probably the best way is to do a sample number of turns (multilayer - if you need multilayer?) on the core you're using, and then measure the resulting inductance - simple calculation will give you the correct number of turns from that.

 

[dr pepper]

Dont worry about it, hobbyists have found wound components difficult for decades, I still think a lot of designs are only calculated near enough and then tweaked by hand to get right.

 

[MrAl]

Hi,


Yes it appears difficult at first, because there are a lot of variables. Temperature and DC bias are typical influences, as well as the material variation itself. But it's also been tried and done for ages now, so it's not impossible. It's a matter of knowing how to apply the theory and what differences from ideal to expect and how to effectively deal with them.

Two typical formulas for inductance are (length dimensions are in centimeters):

L=1.26*10^-8*N*N*u*A/(gap*u+c) {L in Henries}

and

L=N*N*AL*1e-6 {L in mH}

It's obvious that AL is equal to:
AL=1.26*10^-2*u*A/(gap*u+c)

but this AL value is usually given on the manufacturers data sheet for the core.

For example, if we have a core with AL=1000 (check data sheet for actual core and units) and with 45 turns we have:
L=45*45*1000/1000000=2.025 mH

However we must make sure that the AL for a given manufacterers core is in the same units as above. They usually give a formula for inductance along with their version of what AL is for their cores. In this example AL was in units of nH/N^2 or in words, nanohenries per number of turns squared. Sometimes they misstate this parameter too, stating it as "inductance per 1000 turns" but that's not really what it is

DC bias affects the core substantially. That's because the bias pushes the operating point up along the BH curve, and the BH curve flattens out more and more the higher we get, and the flatter the BH curve is the lower the permeability and the lower the permeability the lower the inductance. Since this is so dependent on the actual BH curve for the material, it's best to review the curve for that material to get a good idea how the inductance will change when we apply a DC bias. Note that the more turns on a core the more a DC bias affects the core, and the higher the AL value the more DC bias affects it too, bringing it more into saturation and thus lower inductance.

There are a few other problems that come up too such as do all the turns fit inside the toroid window, and surface area vs heating. Here's a drawing of a toroid core center window that is entirely filled up with wire. If we needed more turns, we'd have to go to a larger core.

 

[RCinFLA]

Al's got most of the info right. Probably the biggest mistake hobbist (or inexperienced) makes is not understanding the saturation or flux handling capability of the core they are using for the application they intend.

Al is also correct is asking what you are trying to do. Design for a flyback transformer/inductor is quite different then designing a power coupling transformer. The first has to store energy in the resultant inductance while the later is tightly coupling magnetic flux between windings for power transfer. The power coupling transformer core does not have to store energy and gapping would degrade performance with detrimental leakage inductance.

Gapping a core does not improve its maximum flux density handling capability. It only stretches out the B/H curve horizontally allowing a higher MMF force to a given B flux level. This allows more energy to be stored in the magnetic field, again, needed for a flyback design The stretching of the B/H curve also give the appearance of 'soft' saturation as the flux density increases toward maximum levels. This causes the inductance to drop with any DC bias offset or with greater AC excitation levels. Both these levels need to be specified before you can make a statement on inductance value.

Read some of the papers at following link. I think it can get you up to speed on toroid design pretty quickly. Gapping design is more involved and because it is hard for a hobbist to grind ferrite to an accurate gap dimension in a Ferrite core I would steer you away from that type of design. Yes, you can just put spacers between cores but that technique creates even more flux leakage.

https://www.cwsbytemark.com/index.php?main_page=page&id=117

 

[MrAl]

Hi,

Nice link there RC.

Yes unfortunately gapping a core for anyone without very very expensive equipment presents a significant challenge to accomplish successfully.

I've done some extensive experimentation in this area in the past so i can offer some advice to the hobbyist...

There are a few different approaches:

The first is to buy a core that already has two parts, such as Pot Core cores. These have two halves that you can use a spacer to achieve a gap. This is probably the most sane way to do it.

The second is more adventurous. Enter, the 'cracked' core. This is where you take a toroid core and put it inside say a clean rag, then insert in a vise and squeeze the core (across the circular part not across the flats) slightly until it cracks. It usually cracks into four pieces. You can then experiment with various glues to glue it back together and what you end up with is a core with a small but significant air gap, which changes it's characteristic quite a bit even with thin glue like Super Glue as the gap here appears four times within the magnetic path.

The third is interesting too. Using a Dremel tool with a diamond cut off wheel, you can cut through a toroid like it is butter. The idea here is to slice it into two halves where you can then use an insulating material to achieve a gap. The gap enters twice into the magnetic path as usual. Here we also would see a significant change in inductance and DC handling.
You might also try a single gap, but it is unlikely that the gap will be small enough unless it is a somewhat larger toroid.

The first above is tried and proven, the last two are for experimenting with so be sure not to use your only core for these last two. Make sure you have others on hand.

As RC points out, the gap reduces the inductance and it also stretches out the BH curve horizontally. What this means is that it takes more current through the winding to reach the flat portion of the BH curve and therefore loose inductance. This is especially useful in DC power supplies.
The gap is not entirely unique for achieving this goal however, as a lower permeability core also allows more DC current through the winding before saturation.

As the attachment shows, adding a gap has benefits and drawbacks so a gap is only used when necessary.

 

[dr pepper]

I use a very simple test jig and my 'scope to check for saturation, I'm sure its a naff way of doing it, but it works for me, it also tells you the al factor and a few others.
Designing ferrites at least for one offs and experiments a flux density of 0.1 Teslas seems to be a good starting point.

 

[Nigel Goodwin]

I use a very simple test jig and my 'scope to check for saturation, I'm sure its a naff way of doing it, but it works for me, it also tells you the al factor and a few others.
Designing ferrites at least for one offs and experiments a flux density of 0.1 Teslas seems to be a good starting point.

I wouldn't worry about it been 'naff' - testing methods often are VERY simple and crude, but that's all that's required, and it's what's effective.

Often it's just a question of applying a little thought to the problem, and you can 'invent' a testing method.

 

[dr pepper]

Thats really what I meant, compared to lots of really expensive gear its naff.
But it works for one offs in the 'shop.
Lots of expensive gear is something you'd get if you were producing zillions of these things where every gram of copper or ferrite counts, and every mm2 space is important, and of course if you messed up you'd have highly paid lawyers on yer back.

 

[Bob Scott]

I am surprised that nobody has mentioned the inductance formula for closed core (non-gapped) inductors. It would seem to be very close to the knowledge that the OP is searching for:

L = 0.4*pi*N^2*mu*Ae/Le*10^-8
Henries

It is accurate enough that you can check the posted Al value for accuracy.

 

[MrAl]

I am surprised that nobody has mentioned the inductance formula for closed core (non-gapped) inductors. It would seem to be very close to the knowledge that the OP is searching for:

L = 0.4*pi*N^2*mu*Ae/Le*10^-8
Henries

It is accurate enough that you can check the posted Al value for accuracy.

Hi Bob,


Maybe because we didnt need it

If you take the formula i gave (or any other formula for that matter) and set the gap equal to zero, you get the formula for an inductor with no gap. Simple right?

You can double check to make sure i posted the right formula though.