Can Someone Explain the Apparent Logical Inconsistencies of the JBL Tapped Inductor?

[toddalin]

I have a pair of 3115 crossovers that use a tapped inductor. The schematic shows the inductor to be 3.85 mH across the blue and black, with three taps, red, white and green, along the way.

I opened the crossover and cut the blue and black leads away from the circuit so I could measure the inductances. The R/W/G go directly to the HF +, so nowhere with no speaker and are disconnected.

With one probe on the blue wire, I read 0.90mH to the red, 1.29 mH to the white, 1.78 mH to the green and 3.63 mH to the black. The black should be the 3.85 mH and 6% is within tolerances. No problem.

OK, now the fun starts!

I would think that if I put a probe on black and a probe on green the value should be 1.85 mH (i.e., 3.63 - 1.78), but is 0.35 mH :confused:

Black to white should be 2.34 (i.e., 3.63 - 1.29), but is 0.63 mH. :blink:

Black to red should be 2.73 mH (i.e., 3.63 - 0.90), but is 0.98 mH.:banghead:

And...,

Red to white should be 0.39 mH (i.e., 1.29 - 0.90), but is 0.06 mH :blink:

Red to green should be 0.88 mH (i.e., 1.78 - 0.90), but is 0.17 mH :frantic:

White to green should be 0.49 mH (i.e., 1.78 - 1.29), but is 0.03 mH :barf:

http://www.jblproservice.com/pdf/Network%20Schematics/3115%20Network.pdf
http://www.jblproservice.com/pdf/Network%20Schematics/3115%20Network.pdf

[toddalin]

Think I figured it out. http://www.audiokarma.org/forums/styles/default/xenforo/smilies/naughty.gif

It would make sense that the blue wire is closest to the core and each successive tap gets further away with the black wire being the outtermost feed. With less influence from the core, the windings produce fewer mH per turn, though the number of turns may or may not be consistent between the various taps.

So for example if each tap consisted of 20 windings, the 20 windings between the core and first tap would produce more mH than the 20 windings between the last tap and outter feed. http://www.audiokarma.org/forums/styles/default/xenforo/smilies/scratch2.gif

[turnitdown]

What happens to the DCR and is it consistent/predictable?

[ivica]

Think I figured it out. http://www.audiokarma.org/forums/styles/default/xenforo/smilies/naughty.gif

It would make sense that the blue wire is closest to the core and each successive tap gets further away with the black wire being the outtermost feed. With less influence from the core, the windings produce fewer mH per turn, though the number of turns may or may not be consistent between the various taps.

So for example if each tap consisted of 20 windings, the 20 windings between the core and first tap would produce more mH than the 20 windings between the last tap and outter feed. http://www.audiokarma.org/forums/styles/default/xenforo/smilies/scratch2.gif






Hi Tod,

From the attached fro,ul for air-core multilayered coil it can be seen that all coil dimension have influence in its inductance.
So for iron-core coils I can imagine that it would be the case too, but the core dimensions would have an influence too.

http://www.electronics-tutorials.ws/inductor/mutual-inductance.html

http://www.66pacific.com/calculators/coil-inductance-calculator.aspx


regards
ivica

[Ruediger]

What kind of measurement gear did you use?

Ruediger

[mmaki]

What you measure seems normal. I don't know the exact science behind it but I have wound enough inductors to know that inductance doesn't increase linearly with number of turns or wire length.

ie it does not require twice the wire length to wind a 2mh inductor as it does a 1 mh. It requires much less, so if you wound a 2mh inductor tapped at 1mh you would find the second half has much less windings and would not read near 1mh on its own.

[toddalin]

What kind of measurement gear did you use?

Ruediger

Multimeter with inductance. (Uni-T UT70A)https://toolboom.com/nfs/product/819451/image/480/digital-multimeter-uni-t-ut70a.jpg

[Ian Mackenzie]

Gents,

This maybe of assistance:

http://www.audioheritage.org/vbulletin/showthread.php?5376-Tapped-Autotransformers
http://www.audioheritage.org/vbulletin/showthread.php?846-autotransformer&pp=15

[ivica]

What you measure seems normal. I don't know the exact science behind it but I have wound enough inductors to know that inductance doesn't increase linearly with number of turns or wire length.

ie it does not require twice the wire length to wind a 2mh inductor as it does a 1 mh. It requires much less, so if you wound a 2mh inductor tapped at 1mh you would find the second half has much less windings and would not read near 1mh on its own.

Hi mmaki,

L=k*N*N, where N is the number of turns

regards
ivica

[toddalin]

Here is an interesting observation.

I ordered six, 16 gauge, 2.0 mH inductors for the woofer. A single order 6.0 mH inductor would cross the 2241s over at 210 Hz.

When the three coils are connected, they produce 6.0 mH. But stack the three together, and you've got to get them near perfectly aligned, and they total 8.0 mH bringing the crossover point down to 160 Hz.