I want to place two inductors next to each other on my PCB, will the fields interact in a way that would adversely affect each other?

If it's important they are 1mH each, with a 8V peak to peak, 800KHz signal running through them.

  • 1
    Do you have exact part numbers? In general, you'll have mutual inductance between them, but that's actually going to work depends on the exact part and orientation. There are also shielded inductors specifically designed to be placed close together on a PCB that could help. Also what current do you expect to have going through them? – AndrejaKo Dec 7 '12 at 16:32
  • It's the 8RHB: datasheet.elcodis.com/pdf/16/17/161743/r622ly-1r5m.pdf. There's not a lot of current, maybe a few mA? – Nate Dec 7 '12 at 16:37
  • The largest inductor on that datasheet is only 1mh, not 500mh. – Brian Drummond Dec 7 '12 at 16:44
  • Sorry, they are 1mH. Bad typo – Nate Dec 7 '12 at 16:47
  • What do you call an adverse effect? I would get 2 coils, drive one from a sig gen (or knock up an Arduino program or 555 circuit according to what I had) and connect the other to a scope. If you call what you see "adverse", try shielded coils or toroids. – Brian Drummond Dec 7 '12 at 16:54
up vote 5 down vote accepted

While the inductors you have selected appear to have ferrite cores, the ferrite does not provide a complete magnetic path for the coils. Therefore, there will be a significant external field, with the possibility of "adverse coupling" among multiple coils.

If this is going to be a problem in your application, consider shopping for "shielded" inductors, which do have a complete magenetic path in ferrite, minimizing the external field.

Another way to minimize coupling is to orient the inductors' axes at right angles to each other wherever possible.

A definitive answer is not possible since that depends on specific layout and routing around the parts. But, here are some things to think about that will help you make a more informed decision.

There are different types of inductors, broadly, unshielded and shielded.

  • unshielded are compact and inexpensive, but the fields are not contained in the inductor. These may or may not have a core, RF types often don't.

  • shielded inductors require a little more volume and are more expensive, but the fields are contained inside the magnetic material. They are best to use if there are sensitive circuits in close proximity to the inductor.

Interference from magnetic fields is a near field thing, which falls off by \$\frac{1}{R^3}\$.
Voltage gets induced in surrounding circuits through the mutual inductance between the inductor and those circuits around it. High impedance circuits are most susceptible to magnetic coupling.

The induced voltage is ~ \$I M \omega\$. So, you can reduce the induced voltage by lowering the current in the inductor (I), or lowering the coupling to reduce the mutual inductance (M). Coupling can be reduced by adding space around the inductor, or shielding the inductor.

For frequencies below about 50KHz or 100KHz shielding needs to be by a magnetic material. For frequencies above 100KHz or 200KHz the shielding can be by conductor since eddy currents induced in the conductor will generate an opposing magnetic field.

If you use an unshielded inductor you need to leave adequate space around it, especially if there are higher currents in the inductor or high impedance circuits around it. If you have multiple boards, like a stack or card rack, that space needs to be considered too.

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