We are developing a small consumer electronics product consisting of a small 4-layer PCB along with a few PDM microphones and BLE chip/ an antenna, as well as a LiPo battery. We are locating a small but pretty strong magnet next to these components in the enclosure. It is an N52 (Surface Field: 3032 Gauss, \$B_{r_{max}}\$: 14,800 Gauss, \$B_{H_{max}}\$: 52 MGOe). Are there any long term effects of the magnet which may be detrimental to the battery or PCB electronics?

  • \$\begingroup\$ One of those "probably not" questions that still needs an authoritative answer. I suggest you post details about the microphones, they could use magnets. \$\endgroup\$
    – pipe
    Mar 18 '19 at 13:20
  • 1
    \$\begingroup\$ The magnet may affect the antenna, simply by being a conductive object in its vicinity, thereby altering the radiation pattern. If much smaller than the wavelength, the effect will be as a capacitor; if the magnet is larger, than it may act as a reflector. In either case, this has nothing to do with a static magnetic field. \$\endgroup\$ Mar 18 '19 at 21:30
  1. LiPo Battery - The battery will be fine. A good way to verify this is to see if your magnet is attracted to the battery at all. There can be some weird variations on LiPo chemistry, but as far as I know, there aren't any ferromagnetic materials in most, if not all, LiPo cells. High strength (even N52 grade) magnets are frequently used to attach LiPo batteries in hobbyist RC products, and many other consumer products.

  2. Magnetic Cores - I know you didn't mention this, but this is really the only thing you need to worry about. Static magnetic fields generally won't impact how the circuit itself behaves (ignoring magnetic cores, of course), but static magnetic fields will impact any bulk magnetic material used in the circuit. This includes any inductors that don't use an air core and ferrite beads.

    All magnetic core materials can saturate. With inductors, this is usually specified as saturation current, which is the current through the inductor that, if you exceed, will cause a rapid drop in inductance. Measuring it as current is just for our convenience, and what is really occurring during saturation is that the magnetic field in the core has reached saturation.

    Any nearby static magnetic fields will cause some degree of magnetization in the core material, and any current or ripple current through the inductor will be another field superimposed on the core material. Depending on the exact properties and geometry of the core, this can potentially lower the saturation of the core well below spec, or increase losses in the core.

    So this is very much worth considering, especially if you are using any power inductors (like in a buck or boost converter) that you're running near saturation.

    That said, the saturation field for power ferrites is usually in the range of 300mT (conservatively). So that is already equal to the surface field of your magnet. Considering that magnetic fields fall off at distance cubed (as opposed to squared), so you can be extremely close to a magnet and experience almost negligable field strength from it.

    Case in point: assuming your magnet is an N52 square 10mm x 10mm x 2.5mm (which would yield a surface field of about 3000 gauss, give or take. Unless the dimensions are different by a huge amount, numbers will be similar), the static magnetic field just 2mm away from the center, in the direction of the poles, will be ~2140 gauss or 214mT. 5mm, and it is down to 993 gauss. This is low enough that I would not be concerned.

So the short answer to if it will affect your circuit is: maybe. But probably not, especially if you put even the tiniest amount of distance between the magnet and inductors that might present on board or inside the bluetooth module.


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