strange behavior of multi tap inductors

during last 2 months I spend time to try to DIY a multi tap inductor like steffen done it for his projects. My goal was to obtain 2 segment of 250 mH.
(one between pin 1 and 2, and one between pin 3 and 4) straping pin 2 and 3 I expected to obtain 500mH, right ? wrong! I have obtained 1H.
How it is possible? the answar is in the relation of the number of the turn;
in this case I have made about 400 turn to get 250 mH for a total of 800 turn for 2 segments. So I have understand that the numberr of the total turn of the segment does multiply the series inductance, for about 1/2 times every 100 Turn. So , to know what value I 'll obtain seriesing 2 segment I have to do the followig formula:
N-1 + N-2 /2 = M
Were "N-1" is the number of Turns of the 1st seg.
"N-2" is the number of Turns of the 2nd seg.
"2" is dividend of the 2 series wound
and "M" is the multiple factor
400+400/2= 400; you have to multiple the "L" sum of the 2 seg. 4 times

Ones obtained "M" you have to do math as follow:
L-1 + L-2 /2 * M= LM
were
"L-1" is the inductance of the 1st seg
"L-2" is the inductance of the 2nd seg
"2" is dividend of the 2 series wound
"M" is the multiple factor
"LM" is the value of inductance I will Find

250 mh + 250 mh /2 *4 = 1000 mH

this formula comes true only if we use more then 400 turn for the 2 seg.
M value of 0.5; 1; 1.5; are trascurable
I really don't know what's they behave itself in a toroidal wound, or in a differente shape inductor, mine is a square with E ferrite core,All this thing does metter I think.

this is what I have found till know, but I'm sure there's mutch to know about, (maby some square root, or ? here and there) I didn't found nothig about it, even in Audio cyclopedia if there's guru over there, pleas comment it.

MIK

When connecting inductors in series, the formula is - like resistors - L=L1+L2..

UNLESS..

..the inductors are on same core, sharing magnetic field, thus having mutual inductance.

In this case L=L1+L2+Mutual_L1+Mutual_L2, whitch adds up to roughly L1+L1+L2+L2 ...

Jakob E.

thank you Jakob, does anybody know how to compute L mutual ?
anyway it seems to me stange L mutual is double the total value. :shock:

http://www.qsl.net/wa7zcz/area2/page13.html

[quote author="mik"]anyway it seems to me stange L mutual is double the total value. :shock:[/quote]

Why? it makes perfect sense when you look at the formula. Inductance (L) of a coil of wire is proportional to the square of the number of turns ... Increase n by a factor of 2 and inductance increases by a factor of 2^2 (plus/minus some error caused by the length of the coil changing when you increase n).

Peace,
Al.

[quote author="mik"]thank you Jakob, does anybody know how to compute L mutual ?
anyway it seems to me stange L mutual is double the total value. :shock:[/quote]
Only if two of equal value... the important concept to grasp is that inductance increases as the square of the multiple of the original number of turns.

For example, if you have 200mH + 200mH on the same core, the total is not 400mH, but 800mH. (since 2 x the number of turns = 2 x 2 x the original inductance)

If you have 200mH + 200mH + 200mH, the answer is not 2 x 3, it's 3 x 3 times the original inductance.

Simply put, if you double the original number of turns on the same core, you get four times the inductance. If you triple the turns, you get nine times the original inductance. If you increase it ten-fold, you get a hundred times the original inductance.

Keith

thank you.... I love this place :green:

Yes, N^2.
But, what frequency and level did you measure the inductance at? It will be different in the actual circuit.