Combining Inductors

[1100xxben]

Re: Combining Inductors

But whats wrong about photo7/ pdf#8 "never do this "?
stacked coils (non-inverted) are > 1.31
That's an increase of .2
- with the price of copper, why not do it (at least in this case, and as long as you can measure)?

same for photo6 / pdf4 "very bad" > 1.17
still an increase.
Free lunch?

The whole reason for that PDF is to show the amount of mutual coupling between two inductors and what NOT to do with two inductors in two different circuits. Hence, you should not place two inductors from 2 different circuits as shown in #8.

If you place two inductors in series electrically (but mechanically isolated), they will add their inductance. If you place them in series electrically and put them together such as #8, then you also receive mutual coupling with their fields, so you attain more than A+B inductance. Because of the bobbin, you will have a small gap in the "middle" of your inductor which will cause some loss in the mutual coupling. If you had a single wind with 2x the wire (and the optimal physical size), you'd end up with a higher inductance than the two separates wired together and coupled together. You'd typically use less copper with a single inductor than trying to gang multiples together. So, no free lunch. Coupling together is just convenient if you already have some smaller inductors and need a larger one.

[PWR RYD]

Re: Combining Inductors

Cost is also a factor. As an example an 18 ga Jantzen 0.5 mH is $6.12 so two in series would be $12.24. A single 1.00 mH is only $8.23.

Two of the above coils in series would have a total DCR of 0.68 ohms. The single 1.00 mH only has 0.48 ohms.

[thekorvers]

Re: Combining Inductors

The whole reason for that PDF is to show the amount of mutual coupling between two inductors and what NOT to do with two inductors in two different circuits. Hence, you should not place two inductors from 2 different circuits as shown in #8.

If you place two inductors in series electrically (but mechanically isolated), they will add their inductance. If you place them in series electrically and put them together such as #8, then you also receive mutual coupling with their fields, so you attain more than A+B inductance. Because of the bobbin, you will have a small gap in the "middle" of your inductor which will cause some loss in the mutual coupling. If you had a single wind with 2x the wire (and the optimal physical size), you'd end up with a higher inductance than the two separates wired together and coupled together. You'd typically use less copper with a single inductor than trying to gang multiples together. So, no free lunch. Coupling together is just convenient if you already have some smaller inductors and need a larger one.

You hit the nail on the head!

[jeff_free69]

Re: Combining Inductors

You hit the nail on the head!

Yes this is limited to a very special case.
a) same circuit
b) you happen to already have said inductor$ and would like to avoid buying more .
Also handy to know if you have least have 1 of them and can get away with buying a smaller one and tack it on.