Does "shielded" power inductor still susceptible to external magnetic field ? - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: February 20, 2014, 09:00:30 am
content: To be exact, we're talking about power inductor used in power switching circuit.I guess most of the times the purpose of the shield is to reduce the magnetic interference to "other" components, CMIIW.Reason, planning to use a reed relay switch that will be placed really close to the inductor in a very tight and space limited construction, and worry the moving magnet for the reed relay will affect badly to the power inductor.Ok, why reed relay, the project is to build a totally sealed, water proof led flash light head, with an external switch that is made from a strong moving magnet outside of the flash light body.

author: EEVblog
author_rank: Unknown
timestamp: February 20, 2014, 10:27:16 am
content: You can get reed relays with mu-metal shielding. Picking have them among others.They are designed to be packed close to other relays in a matrix so they don't effect each other.http://www.meder.com/fileadmin/meder/pdf/en/Technical_Documents/Reducing_Magnetic_Interaction.pdfhttp://pickeringrelay.com/pdfs/reed_relaymate_web-1.pdf

author: uwezi
author_rank: Unknown
timestamp: February 20, 2014, 10:50:41 am
content: If it is apowerinductor than any stray field from a nearby reed relay will be negligible... Yes, the dc component will to a small extent magnetize the core, but probably not more than the magnetic field of the earth would do anyway.

author: SeanB
author_rank: Super Contributor
timestamp: February 20, 2014, 06:06:06 pm
content: I would be more worried about the reed relay being operated at the mains frequency from stray magnetic fields from the inductor. Less if it is a toroid, but a lot more stray field if it is an EI core or any variant of it. Those cores are not known for having a good containment of their magnetic fields. Reed relays will happily operate at 100/120Hz with a field that is big enough to operate them.

author: BravoV
author_rank: Super Contributor
timestamp: February 21, 2014, 12:20:18 am
content: I guess I worded it wrongly, its not the weak stray magnetic field from the reed relay that I worry about, but the strong NIB magnet that is placed really close to the inductor. Its a shielded toroidal inductor BTW.

author: ConKbot
author_rank: Super Contributor
timestamp: February 21, 2014, 02:51:58 am
content: While we can speculate a bunch, if you have a circuit in hand, hook it up to a current limited power supply and use a resistor or dummy load instead of a LED. Put the magnet near it and see if your input current shoots up because the inductor saturates.The shielding will help keep the stray field from getting into the center core of the inductor, but put enough magnetic field there and you will eventually saturate the core, and the inductance will drop a lot.Better circuits (with inductor current limiting) may just get a ton of noise and ripple on the output, or the output may just drop out. However stuff with a simple feedback loop from a voltage divider could fail in interesting ways. I'd expect the mosfet on a simple boost converter would suffer the most. A buck converter could just bounce/oscillate between 0 and Vin if the inductance drops enough and there isnt any protection.However, given how fast magnetic fields fall off, if it does bother it testing the bare board, it could be ok once you have the driver board in a flashlight and it cant get closer then a few cm.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: February 21, 2014, 07:15:47 pm
content: In general, magnetic shielding should isolate internal and external fields. If not much is getting out, not much is getting in.Furthermore, for most geometric configurations of your reed magnetic and torrid, the internal and external magnetic fields will be nearly perpendicular to one another, so small additions of external flux will tend to have an even smaller total (magnitude) contribution towards saturation by way of vector addition.Do you know the material of inductor's core?