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Old 10th June 2012, 06:29 PM   #1
akis is offline akis  United Kingdom
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Default Inductance calculator

I am trying to work out the inductance of a coil wound around a ferrite core.

I am using the formula: L = u0 * u1 * N^2 * A / d

where u0 is permeability of space ( 4 * pi * 10^-7)
u1 is permeability of ferrite ( EPCOS N97 = 2300)
N is turns ( 2.5 )
A is cross sectional area = 1.767 cm^2
l is coil length = 5 mm

The formula says 638 uH but I am measuring 9uH.

If I wind about 12 turns and on a slightly larger diameter I measure 739 uH.

Is there a way to be able to calculate the inductance without having to wind up and then measure?


PS. I realise that the wire is wound on a former and there is a gap between that and the ferrite - may be that has something to do with it ?
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Old 10th June 2012, 06:42 PM   #2
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I find this useful at times.

Coil Calculator - Geotech Forums

It will tell you the approx thickness too.

Your inductance may vary depending on the frequency you are testing with.
Use the intended frequency or something close to it.

If you are using a multimeter for the test, it might not be real accurate but gets you in the ballpark.
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Old 10th June 2012, 06:52 PM   #3
akis is offline akis  United Kingdom
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Ahhh I have just gone to test my ebay, chinese hand-held inductance meter - it seems it puts out a semi-square wave at .. 200Hz.

My application uses 200 KHz.
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Old 10th June 2012, 07:28 PM   #4
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There's a useful lab saying, "test it the way you use it". It's especially important with ferrite transformers and inductors because there are so many types, optimized for different frequency bands and for different purposes. Sometimes, if you can't test at the frequencies and power levels anticipated, the calculations will be more accurate than the tests!
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Old 10th June 2012, 07:36 PM   #5
ODougbo is offline ODougbo  United States
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There are a few ways to measure, probably best to buy a meter.

However, just used a amp-load-generator-volt meter to wind a pair, it seems very accurate (but messy) pic on #12

2 way crossover design help
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Old 10th June 2012, 09:06 PM   #6
akis is offline akis  United Kingdom
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I have just ordered a better meter on ebay. I am also trying to think how I'd make my own measurement kit using my tone generator to produce any frequency I'd want.
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Old 11th June 2012, 07:27 AM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by akis View Post
I am trying to work out the inductance of a coil wound around a ferrite core.

I am using the formula: L = u0 * u1 * N^2 * A / d

where u0 is permeability of space ( 4 * pi * 10^-7)
u1 is permeability of ferrite ( EPCOS N97 = 2300)
N is turns ( 2.5 )
A is cross sectional area = 1.767 cm^2
l is coil length = 5 mm

The formula says 638 uH but I am measuring 9uH.

If I wind about 12 turns and on a slightly larger diameter I measure 739 uH.

Is there a way to be able to calculate the inductance without having to wind up and then measure?


PS. I realise that the wire is wound on a former and there is a gap between that and the ferrite - may be that has something to do with it ?
What shape is your ferrite core? Closed magnetic circuit like a U-core, open like a cylinder? Dimensional quantities, datasheet of the core?
Form and dimensions of the winding?
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Old 11th June 2012, 08:25 PM   #8
akis is offline akis  United Kingdom
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I use EPCOS RM10 and RM12 cores and bobbins. I have ordered a RM14 (bigger) to try and get better regulation. But my problem is the inaccuracy of my inductance meter. I have ordered a better one and will test again.
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Old 12th June 2012, 12:19 PM   #9
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by akis View Post
I use EPCOS RM10 and RM12 cores and bobbins. I have ordered a RM14 (bigger) to try and get better regulation. But my problem is the inaccuracy of my inductance meter. I have ordered a better one and will test again.
Then you do not use the correct formula.

In the case of a ready-made core, the manufacturer gives you directly the AL, in nanohenry/turn˛. All you have to do is to multiply this figure by the number of turns squared to get your inductance.

For example, the ungapped RM12 in N97 material has AL=5300.
http://www.farnell.com/datasheets/486640.pdf
Thus, with 2.5 turn, your inductance should be 2.5˛*5300=33125nH, or 33.125µH.
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Old 21st June 2012, 09:16 AM   #10
akis is offline akis  United Kingdom
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My inductance meter uses frequencies up to 800 KHz. Does the secondary coil affect the inductance of the primary? If I short the secondary coil I read different values. I think I read somewhere this is called "leaked inductance"?
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