Combining Inductors

[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.

[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!

[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.

[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.

[jeff_free69]

Re: Combining Inductors

This is the defacto guide...

Nice!
But while it nicely demonstrates the relative effect, one thing I'm missing is the actual measurements of the 2 individual coils.
Are they .56 by label or is that their actual measurement on this rig?

If they are actual .56 than the theoretical combination would be 1.12 and many of the measurements shown are more than that.
8" apart they are >1.14
4" > 1.13
(not such a big dif)

photo # 5 / pdf 6 (the generally acceptable "tire rolling over donut")
> 1.14
(same as 8" apart, so I guess this is ideal)

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?
OR are there unpalatable side effects?

[jhollander]

Re: Combining Inductors

Yeah, I would think the ideal alignment would be to have them aligned on-axis with their windings in the same direction.

This is the defacto guide...

[john trials]

Re: Combining Inductors

If you've measured to within 2%, you're good. Just keep other inductors away.

Now, try stacking them on top of each other with their centers aligned and see how much it changes, just as an experiment.

Yeah, I would think the ideal alignment would be to have them aligned on-axis with their windings in the same direction.

[6thplanet]

Re: Combining Inductors

How ever they are laying when measured is how you'll have to have them when utilizing them. If you have a way to measure then you can prove this to yourself by stacking/moving them/inverting them, it all effects the mH out come.

[jeff_free69]

Re: Combining Inductors

You got blisters on your fingers from improperly mounted inductors? Seriously? Who was stupid enough to do that in the presence of current potential high enough to cause burns - and more importantly, who was idiotic enough to ensure that happened - and even MORE importantly - who was stupid enough to get burned?

I'm pretty sure he was just quoting John Lennon, and probably just exaggerating for emphasis...

I was thinking about this last night....
if they were they same physical dimension and you were able to stack them in a way that the windings were the same direction and continue from the bottom of one to the top of another...
I wonder if you could get away with it since its almost like one with a discontinuity in the middle

In any event I'm temporarily patching different ones together for testing, but I keep them 6-8 inches apart. (I'm not worried about economizing space on the final layout)

[johnnyrichards]

Re: Combining Inductors

Iron cores get mad at each other if they get close. (Ask me...) They can get hotter than a $2 pistol, so don't do that! "I got blisters on me fingers!"

You got blisters on your fingers from improperly mounted inductors? Seriously? Who was stupid enough to do that in the presence of current potential high enough to cause burns - and more importantly, who was idiotic enough to ensure that happened - and even MORE importantly - who was stupid enough to get burned?

[Whitneyville1]

Re: Combining Inductors

Iron cores get mad at each other if they get close. (Ask me...) They can get hotter than a $2 pistol, so don't do that! "I got blisters on me fingers!"

[jeff_free69]

Re: Combining Inductors

If you've measured to within 2%, you're good. Just keep other inductors away.

Now, try stacking them on top of each other with their centers aligned and see how much it changes, just as an experiment.

Interesting question...
I was playing with 2 hefty inductors the other day and noticed when moving them together (like stacked donuts) you could feel the physical resistance, like magnets.
Perhaps its best to be prudent and avoid any possible interaction by using the "tire rolling over a donut" orientation.


The other thing you should check is the total resistance. If they're small gauge it could easily add up to over 1 ohm, which could have an impact.

[jhollander]

Re: Combining Inductors

If you can measure inductance, you can reach your target inductance by taking the wire from your smaller inductor and wrapping on to the larger inductor. It should take less wire in total.

[Pete Schumacher]

Re: Combining Inductors

That is what I am thinking as well; just wanted to know what others think. I believe cross-talk is another concern, but that should only come into play when the inductor is in close proximity to an inductor from another circuit. In this case, the two combined inductors should be acting as one, at least in theory.

If you've measured to within 2%, you're good. Just keep other inductors away.

Now, try stacking them on top of each other with their centers aligned and see how much it changes, just as an experiment.

[fernandov]

Re: Combining Inductors

This is a good question. My answer is: I don't think so. If measured under those conditions and the total inductance is acceptable, you should be good to go. I have not seen any information or testing done contradicting my thesis.

That is what I am thinking as well; just wanted to know what others think. I believe cross-talk is another concern, but that should only come into play when the inductor is in close proximity to an inductor from another circuit. In this case, the two combined inductors should be acting as one, at least in theory.