Mismatch the transformer core AL and the result inductance

[havazli]

Hi there! I'm aiming to wind a output transformer for DIY ribbon Microphone,
To find the AL of an unknown transformer, I wound 100 turns and than measured the inductance.
100 turns gave me 87 mH. Which mean that the transformer core AL is 8700 (87*100). According to
the formula, (L / AL = N^2, and N^2=L/AL) for 3mH (3000000nH) (3000000/8700=344.8
Square root of 344 is 18.5. So for 3mH need 18.5 turns. But these 18.5 turns gave me only 0.03mH (30uH).
Do I miss something. Where is my mistake?
Appreciations!

 

[ruffrecords]

The correct formula is L = AL * N^2

Inductance is proportional to turns squared. So your AL is 87mH/(100 x 100)

Cheers

Ian

 

[havazli]

The correct formula is L = AL * N^2

Inductance is proportional to turns squared. So your AL is 87mH/(100 x 100)

Cheers

Ian

Thank you for the answer Jan!
Exactly! The math is AL=87/(100*100)=87/10000=0,0087. AL is 0,0087.
So 3mH=0,0087*N^2 and N^2=3/0,0087=344,8 Square root of 344,8 is equal to 18.5.
Which mean that for 3mH primary I have to wound 18.5 or roghly 19 turns.
Whell I did it but it gives me only 0.03 mH (30uH)

 

[CJ]

What kind of core?
How big?
How much voltage you apply?
What frequency?

The Al formula has a couple of different versions depending on what units you use.

 

[ruffrecords]

Hi there! I'm aiming to wind a output transformer for DIY ribbon Microphone,
To find the AL of an unknown transformer, I wound 100 turns and than measured the inductance.
100 turns gave me 87 mH. Which mean that the transformer core AL is 8700 (87*100). According to
the formula, (L / AL = N^2, and N^2=L/AL) for 3mH (3000000nH) (3000000/8700=344.8
Square root of 344 is 18.5. So for 3mH need 18.5 turns. But these 18.5 turns gave me only 0.03mH (30uH).
Do I miss something. Where is my mistake?
Appreciations!

The math looks right. 20 turns is one fifth of 100 turns so the inductance should be 1/25 of the inductance of 100 tuns which will be 87/25 = 3.48mH.

I would suggest you wind on 100 turns with a tap ar 20 turns. and measure both inductances.

Cheers

Ian

 

[havazli]

What kind of core?
How big?
How much voltage you apply?
What frequency?

The Al formula has a couple of different versions depending on what units you use.

Hi CJ!
It's a small transformer, salvaged from Japanese Solid State Hi-Fi Monacor SA_616 amplifier, used as a phase splitter. I was aiming to use it as a ribbon microphone trasformer and the applied volage is 0.1mv ( I guess), May be I overrated that small guy... Did I? Here are the photos.

 

[havazli]

The math looks right. 20 turns is one fifth of 100 turns so the inductance should be 1/25 of the inductance of 100 tuns which will be 87/25 = 3.48mH.

I would suggest you wind on 100 turns with a tap ar 20 turns. and measure both inductances.

Cheers

Ian

Hi Jan,
I guess I have to try that...

 

[gyraf]

..perhaps you have a shorted turn?

Do your DC resistance match the expected?

 

[havazli]

..perhaps you have a shorted turn?

Do your DC resistance match the expected?

Hi Gyraf,
The coil is 19 turns of 0.4mm wire bifiliar and the DC resistance is 0.2 Ohms which is within expected value limits.
By the way does it matter which coil is wounded first? The coil in question is wound first.

 

[KLDVOX]

The correct formula is L = AL * N^2

Inductance is proportional to turns squared. So your AL is 87mH/(100 x 100)

Cheers

Ian

im always impressed with your clear answers to questions i then see 100 people posting after your nice replies.

 

[CJ]

That looks like regular steel

I believe you want a hi nickel core to bring your inductance up.

 

[KLDVOX]

thanks for the info

 

[havazli]

That looks like regular steel

I believe you want a hi nickel core to bring your inductance up.

Yes CJ, it's a regular steel.
Because of his AL, I thought it'll do the job, but you're right. I guess I need a nickel core.
Thank you all!