Pages: [1]   Go Down
 Author Topic: q: magnetic fields and conductor isolation  (Read 348 times) 0 Members and 1 Guest are viewing this topic.
Offline
Full Member
Karma: 1
Posts: 242
Engineer
 « on: December 12, 2009, 02:42:25 pm » Bigger Smaller Reset

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.

-MDH
 Logged

Global Moderator
Dallas
Online
Shannon Member
Karma: 119
Posts: 10170
 « Reply #1 on: December 12, 2009, 04:14:46 pm » Bigger Smaller Reset

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.
 Logged

Offline
Full Member
Karma: 1
Posts: 242
Engineer
 « Reply #2 on: December 12, 2009, 04:58:57 pm » Bigger Smaller Reset

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.

-MDH
 Logged

Left Coast, CA (USA)
Offline
Brattain Member
Karma: 279
Posts: 15314
Measurement changes behavior
 « Reply #3 on: December 12, 2009, 05:11:34 pm » Bigger Smaller Reset

Is this a solution looking for a problem.

Lefty
 Logged

SF Bay Area (USA)
Online
Karma: 78
Posts: 5454
Strongly opinionated, but not official!
 « Reply #4 on: December 12, 2009, 05:25:59 pm » Bigger Smaller Reset

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...
 Logged

Offline
Full Member
Karma: 1
Posts: 242
Engineer
 « Reply #5 on: December 12, 2009, 05:48:34 pm » Bigger Smaller Reset

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...
 Logged

Manchester (England England)
Offline
Brattain Member
Karma: 277
Posts: 25517
Solder is electric glue
 « Reply #6 on: December 14, 2009, 04:14:36 am » Bigger Smaller Reset

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.
 « Last Edit: December 14, 2009, 04:15:03 am by Grumpy_Mike » Logged

Offline
Full Member
Karma: 1
Posts: 242
Engineer
 « Reply #7 on: December 14, 2009, 10:24:36 am » Bigger Smaller Reset

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

Quote

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

Manchester (England England)
Offline
Brattain Member
Karma: 277
Posts: 25517
Solder is electric glue
 « Reply #8 on: December 14, 2009, 11:04:23 am » Bigger Smaller Reset

Quote
I was hoping you'd spot this thread.

In general I don't do weekends.
 Logged

 Pages: [1]   Go Up