q: magnetic fields and conductor isolation

I’ve gotten to the point in my project where I’m cutting holes in my project box for cabling. I’m realizing I have some questions about noise, insulation, isolation, magnetic field intensity.
Specifically, what measures (if any) do I need to separate/insulate the high current carrying conductors from the analog signal carrying conductors?

My project includes…
6 x 1amp @ 12V solenoids switched by MOSFETs
2 x 2amp @ 12V heater circuits
multiple 5 V signals passed in/out between Arduino and other digital logic devices and sensors (analog).

I’ve calculated the magnetic field intensity for some of the higher current conductors (ground sink for the solenoids in particular) at a distance of 1mm (est. of AWG 20 w insulation):

B = mu_not / (2*pi) * I/d = 1.2E-3 Tesla

mu_not = magnetic field constant = 1.26E-6
I = 6A
d = 0.001m

*ref http://www.fizika.org/skripte/of-prakt/4_3_05.pdf

This is unfamiliar territory to me. Is this a correct calculation? Is this magnitude of magnetic field small/tiny/medium/other? I need to consider what voltage levels could be coupled onto adjacent conductors from this magnetic field expanding and collapsing while the Solenoids are switched.

My thoughts: I found reference that the earth’s magnetic field is on the order of 10^-5 Tesla. This magnetic field is 100 X larger @ 1mm from the conductor. To me this sounds negligible.

Your thoughts:


Have you considered trying to measure the induced current? You do have an Arduino with an A/D converter. Unfortunately, if you guess wrong about scaling the voltage to be measured you could damage the Arduino.

The at1280 DAC isn't fast enough for that. Field collapse will be extremely quick. What I probably need is a 20MHz scope (which I don't have).

I do appreciate the suggestion though.


Is this a solution looking for a problem. ;)


I would worry less about magnetic fields of the conductors causing problems, and more about "noise" induced on the common wires. Almost ALL of the circuit protection schemes I've seen have been about radiated RF-frequency stuff and conducted noise in power supply traces, rather than currents induced by magnetic fields...

Which doesn't mean that it's not a good idea to keep your high-current wires and traces as far as possible from high-impedence digital or analog circuits...

Power-trace noise is another concern, but for those issues I have measures in place.

Maybe the reason I can't easily find any info on this is because it is not a typical concern in elec design...

I am not sure why you are worried about this. The only problem with magnetic fields is that they induce noise voltages in adjacent conductors. This can be a problem in designs with chokes or transformers but is seldom a problem with single conductors due to the small fields involved. If you do want to shield against a magnetic field then you can use mu-metal. http://en.wikipedia.org/wiki/Mu-metal This is a metal alloy designed to do this and you can get it in laminated foil sheets that can be cut with scissors.

Oh good, Grumpy_Mike is here! I was hoping you'd spot this thread.

I am not sure why you are worried about this.

I'm not sure either. I was hoping it would not be an issue. I'll take my cue from "seldom". Thanks for the link.

I was hoping you'd spot this thread.

In general I don't do weekends. ;)