Estimating an inductor's value by reverse-engineering (need help with maths!)

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diesel_88

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Joined 2007
I'm repairing some speakers (off-the-shelf), one of which has a melted inductor. Its surviving twin is, of course, unmarked, so I dissected it to calculate the approximate value (I don't have access to proper measurement tools).

But I'm not very sure of myself, being unfamiliar with the units, even using an online calculator. Also, not sure how to handle the 2-layer construction. Can someone run a quick calculation to back me up pretty please?

Details:

- Number of turns: 152-ish total, in 2 layers
- Iron core diameter: .25 inches
- Overall coil (outer) diameter: .38 inches
- Length of coil: 1.2 inches
- Wire gauge: 28-ish

Also, where's a good place to get a pair of inductors like this?

Here's a picture of the surviving twin before dissection.

Thanks much!
 
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diesel_88

Member
Joined 2007
HED ("a division of Cerwin-Vega")

One speaker claims to be a model U-6 and the other claims to be a W-10, but all components appear identical except for how the plate is marked. They were freebies; trying to get them functional so I have music in my garage shop. :)

The melted inductor is on the ground leg of the tweeter's 2nd-order filter. Woofer has a 1st order with a much beefier inductor.
 
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dogshome

Member
Joined 2010
I bought a very cheap multimeter from Maplin, um er, maybe, um 30 years ago :eek:

She reads capacitance and inductance very well. It might be MORE than 30 years thinking about it. Oh dear. How much use has this meter had? Lots. How many electronic projects? How many sites?........

They still do very cheap meters like this, just don't use on 3 phase or 600VDC supplies (or swap the leads out first).



FYI :D
 
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diesel_88

Member
Joined 2007
lcsaszar said:
You don't need to know the inductance for restoring it. Take some magnet wire of identical AWG, wound same number of turns on the original former and there you go.
Click to expand...

I like how you think. 28 AWG magnet wire is pretty cheap, but the damaged inductor is a melted, misshapen, and distorted mess, so I'll need to start from scratch.

Would the unthreaded shank of a long 1/4" bolt be an acceptable substitute for a ferrite core? I'd think it would be in the ballpark. But if I go ahead and make two and replace both, the speakers would sound the same. Even if the inductance were off, I probably would never know the difference.

Science, yeah! I mean...engineering, yeah!

Thanks!
 
JMFahey

JMFahey

Member
Joined 2009
diesel_88 said:
I like how you think. 28 AWG magnet wire is pretty cheap, but the damaged inductor is a melted, misshapen, and distorted mess, so I'll need to start from scratch.
Click to expand...

I bet the plastic former is molten and twisted, copy it with a piece of cardboard with 2 cardboard disks glued to ends; hot glue is acceptable..
Once you wind same diameter, layers and turns as the old one, around same ferrite core, you have the functionally same inductor.

You do not even need to buy wire, since you need very little and minimum buy weight will be way too much, plus postage, just visit some local motor rewinder, ask him for a little wire (a couple meters would be enough, just do the Math, and "accidentally forget" a sixpack or a bottle of wine on his counter.
It assures next time you´ll be more than welcome. :cool:
 
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