metal project box grounding

[Jamie Morken]

Hi,

I am putting my circuit board (low power mixed analog and digital) in a
metal project box and am wondering if I should attach any grounding wires to
the metal box, and what types of metal boxes work best for shielding. The
interface cord to the box will have 5 lines, 2 for power and 3 digital
interface lines. I am deciding between an aluminum box and a stainless
steel box. Thanks for any tips on grounding and the required wiring!

cheers,
Jamie Morken

 

[Tim Shoppa]

I am putting my circuit board (low power mixed analog and digital) in a
metal project box and am wondering if I should attach any grounding wires to
the metal box, and what types of metal boxes work best for shielding.

That depends entirely on what you are shielding against. Electric
fields? Magnetic fields? Alpha particles? Gamma rays?

The
interface cord to the box will have 5 lines, 2 for power and 3 digital
interface lines.

Those lines will almost certainly be the conduit for the vast majority
of noise into and out of the box. Any switchers on the power lines
will put out nasty stuff in the tens to hundreds of kHz region, and the
digital lines, if carrying signals with high slew rates, will add to
it on every transition.

Again, it's unclear on whether you want to reduce susceptibility of your
circuit to noise or if you want to reduce the radiation of noise from
your circuit. Slew-rate limited digital lines, and EMI filters on both
ends of the power lines, will help with the latter.

I am deciding between an aluminum box and a stainless
steel box.

No difference. Look at ventilation holes and the holes where your
conductors go in/out, though. Feedthrough capacitors are remarkably
more effective than holes with wires sticking through

Thanks for any tips on grounding and the required wiring!

Minimal grounding: at least one ground for every power and signal line.

Maximal grounding: 4-inch-wide copper strap tied to the box, going to
a radial ground system made of #2 or thicker wire, 8-foot long copper-clad
ground rods, and all connected by Cadwelds.

Your needs will probably fall on the "minimal" side, but not knowing
the details of your needs it's hard to tell.

Tim.

 

[Walter Harley]

I am putting my circuit board (low power mixed analog and digital) in a
metal project box and am wondering if I should attach any grounding wires to
the metal box, and what types of metal boxes work best for shielding. The
interface cord to the box will have 5 lines, 2 for power and 3 digital
interface lines. I am deciding between an aluminum box and a stainless
steel box. Thanks for any tips on grounding and the required wiring!

What Tim S. said is quite true. It is really hard to give general
guidelines for grounding; to get good results you need to understand how
grounding works and how interference signals are generated, transmitted, and
received. And the best box in the world is not going to do you much good
when there are wires (also known as antennas) poking through it.

But ***IF*** you're just doing something simple (low frequency, not
particularly worried about achieving very low noise, etc.) and you just want
to do avoid egregious mistakes:

Yes, you should ground the box. Ground it directly to a ground point that
can handle some junk current on it, like the power supply ground. If you
have any AC wiring going to your box (that is, if you're not either battery
powered or wall-wart powered), ground it to the AC mains ground, and make
sure the connection is secure enough that it won't come loose over time with
vibration, heating and cooling cycles, and wiggling of connectors.

Aluminum or steel doesn't matter, in this scenario (it does make a
difference, but not at this level). Use whatever's easiest for you to
machine, but still strong enough for your application.

 

[Roy J. Tellason]

I am deciding between an aluminum box and a stainless steel box.

Dunno why you'd want stainless, unless you _need_ it for some reason. I do
know that it's a real BITCH to work with! Especially compared to aluminum...

 

[Spehro Pefhany]

Dunno why you'd want stainless, unless you _need_ it for some reason. I do
know that it's a real BITCH to work with! Especially compared to aluminum...

Shipping cost, both of the parts and the final product, can be
affected as well.

Best regards,
Spehro Pefhany