Combining air core inductors in series

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gabkw

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Joined 2007
#1
I know this is verboten in speaker crossover design (i.e., here for one example: Resources - Crossover Faqs), but why?

I was taught basically that:

Inductors in Series

When inductance are not coupled (far enough apart to not influence each other) and connected in series the total inductance is the sum of the individual inductances.

LT = L1 + L2 + L3 + … + LN

So if I take my LCR meter and measure two 16AWG aircore inductors of 0.5mH ea and connect them in series, my meter reads 1mH. They are not inductively coupled. Why would this combination perform any differently in a speaker crossover than compared to a single 1mH 16AWG aircore inductor?

gabkw
 
#2
gabkw said:
So if I take my LCR meter and measure two 16AWG aircore inductors of 0.5mH ea and connect them in series, my meter reads 1mH.
They are not inductively coupled. Why would this combination perform any differently in a speaker crossover than compared to a single
1mH 16AWG aircore inductor?
Click to expand...

It won't work any differently, although the self resonant frequency will be higher.
 
#3
In a less than perfect situation, I've just suggested that somebody do this exact thing so it's not exactly verboten.

Besides the problem of interference if they are too close (which is easily overcome and not any different than the positioning of other coils in the xo), you may end up with larger series resistance from the 2 coils together than 1 larger coil of the same value and generally you want to keep the series resistance as low as possible on a woofer. I believe that alters the TS parameters of the driver which means that your box tuning may be slightly altered without you realizing it, but don't quote me on that last one.
 
#4
Well nothing wrong in doing that except you end up using more copper. Using a 1" long form on a 1" core and using #16 gage wire you need .53lbs of wire for that 1mh inductor. But you need .37lbs for each of the .5mh inductors still using a 1" x 1" form so you end up using .74 lb of #16 wire. And the resistance of the 1mh coil will be .28ohms where as with your pair of .5mh coils it will be .19 X 2 or .38 ohms.
 
#5
Is there any way of overcoming the problem of interference if inductors are too close together other than separating them and by how much should they be separated?

I have heard suggestions that the orientation of inductors is an issue, can anyone explain please?
 
#6
surv1v0r said:
Is there any way of overcoming the problem of interference if inductors are too close together other than separating them and by how much should they be separated? I have heard suggestions that the orientation of inductors is an issue, can anyone explain please?
Click to expand...

Yes, keep clear of the ends, and turn them 90 degrees apart for minimum coupling.
 
#7
surv1v0r said:
Is there any way of overcoming the problem of interference if inductors are too close together other than separating them and by how much should they be separated?

I have heard suggestions that the orientation of inductors is an issue, can anyone explain please?
Click to expand...

Thanks to all for the replies.

Here is a link I ran across that might help visualize inductor layouts:

Placement of coils in crossover networks

gabkw
 
#10
The only actual difference as long as the two are not mutually coupled will be higher DCR which is not necessarily a problem, just make sure to include the DCR in any modeling you do. (Cause is 41% more wire when on the same bobbin versus 100% more wire on a separate bobbin)
 
#11
There was another thread around concerning coil inerference where I did measure two coils in different layouts and posted the results. Anyways, placement of coils would affect their cross-talk as well as their inductance. The inductance fluctuated fro two coils of 0.56mH in the range of 1.1mH to 1.24mH depending on their orientation. Distancewise, for as long as you have 10cm between them, for values up to 1mH I couldn`t measure any crosstalk, that`s for air cores.

If they`re spaced properly, the only difference I could find is you`d end up with higher DC resistance compared to a single coil of the same cross-section and value, which was interesting.
 
#12
I'm still at a loss as to why? someone would use 2 or more coils instead of one.
Unless they happened to have said coils on hand??
Certainly having to pay for multiple coils would eliminate that configuration really quickly 🙂
 
#14
Yup, the joy of diy and ending up with boxes full of inductors!!! ... and caps. Seem to be oddly short on resistors but hey.

You can also parallel inductors, great for lower DCR especially if like me you have a number of cheap high value inductors laying around, easier to combine 3 out of 6 low cost inductors to prototype a desired value than order a new inductor every time... reminds me of the days when I thought all you needed was a speakers impedance and an online calculator to make an xo! Oh the joys of ignorance... or was it arrogance?!

Oh, and I couldn't find anywhere on that p.e. link that said dont use inductors in series????
 
#15
nannoo said:
Oh, and I couldn't find anywhere on that p.e. link that said dont use inductors in series????
Click to expand...

Near the bottom:

What if I Can't Find the Exact Value That I Need?

If you cannot find a value that is within 10% or so of your desired value, there are a couple of things that can be done. Here are some suggestions for dealing with the various components.

Capacitors: Combine two capacitors by connecting them in parallel. Using them this way you can simply add the two values together to get the total equivalent capacitance.

Resistors: Connect two resistors in series to provide a total resistance that is equivalent to the values of both added together. The wattage rating on both resistors must be high enough to meet the design requirements.

Inductors: For the most part, you do not want to use multiple inductors if at all possible. One suggested method for getting a custom value is to buy an oversized inductor, and then unwind some coils until you get to your target value. The drawback to this method is that some type of inductance meter must be used to determine when the target is reached. See the component selection guide for information on working with inductors.
 
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