If I wire two 22 micro henry inductors in parallel, do I still have 22 micro henries? Or do I have 11 micro henries?
Thank you.
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You'd have 11uH for two 22uH ones in parallel.
Originally Posted by gunrunnerjohn:
You'd have 11uH for two 22uH ones in parallel.
Well, 11uH plus or minus, depending upon the mutual coupling between the inductors. The mutual coupling is caused by the leakage field around one inductor that gets into the windings of the nearby inductor. Usually this is not much of a problem for trains. For more sensitive applications, the interaction can be minimized by spacing the inductors apart or orienting them at right angles to each other.
Well, yes, I assumed minimal coupling. You'd have to have them pretty close to get any significant shift in value. 
In layman's talk, are you saying one inductor will become a load to the other if the inductances are not equal?
No.
The simplest inductors are just coils of wire wound around a cylindrical form. Visualize a 1/2 watt resistor with some magnet wire wound around the outside and connected at the ends.
The magnetic field produced by an AC signal in the coil of wire produces lines of magnetic flux that flow through the "barrel" of the coil, out at the ends, and then loops around on the outside to the other end - just like the classic picture of flux around a bar magnet.
If you have two of these inductors near each other, this external part of the field may bump into the other inductor and interact with the other inductor's lines of flux. Changing the flux also changes the inductance. This interacting component is called "mutual inductance".
Some inductors have a shield around the inductor to minimize external effects. Torroidal cores keep almost all of the flux trapped inside the inductor.
The obvious approach to connecting two small cylindrical inductors in parallel would be to place them side by side and twist the leads together. My point all along has been that there will be a slight change in the composite inductance value because of the mutual inductance that is created by this proximity. If you needed an accurate inductance value, this should be taken into consideration. If you just need an inductor to isolate a DCS signal from a capacitor in a lighted car, it is probably not a problem.
I hope this explanation helps.
Of course, if you use a single inductor, all of this is moot.
Dale. I understand. Thanks for the lesson, and I would never have given this effect consideration. If I put two MTH Z750 Transformers next to each other, is mutual coupling something I should give consideration?
No effect that you can measure putting two 60hz transformers next to each other. First off, most of the magnetic field is contained within the core.
As John mentioned, there isn't any significant coupling between two train power transformers.
I hope I don't offend folks by turning what seems like a simple question into an opportunity to explore the topic more completely. This Forum is a great place for all of us to learn new things. I know I find it to be a valuable resource for stretching my knowledge.
Just as a final note on the parallel inductor topic:
I mentioned interaction of two cylindrical inductors placed side by side. Note that the interaction changes if you reverse the interconnections between the two inductors. In one case the effective value will be slightly higher than the calculated parallel value, and for the other interconnection the effective value will be less. In other words, the mutual coupling has a "polarity sign".
I remember that for series inductors the formula is L1 + L2 +/- 2M, where M is the mutual inductance. I don't remember an easy formula for parallel connection with the M included. Does anyone else have that handy?
Here's a good page on the topic, including the formula for mutual inductance computation.
Inductors in Parallel Circuits
I think the trick is figuring out how much mutual inductance there is between the two inductors.
