shielding/grounding question

Both. A metal container acts as a shield because no static electric field can exist inside of it. The electrons distribute themselfs on the surface to maximize the distance between them(since they repel).

But dynamically fields can exist and by grounding the circuit to it, any dynamic change on it will cause the circuit to follow it and hence the potential difference will be 0.

So even if you somehow gave the chasis a potential it wouldn't effect the circuit.... which it could otherwise.

e.g. suppose you ran a van de graff generator to put a large voltage on the chasis.... The circuit inside could be effected as the electrons move through the chasis to attempt to get to reduce the electric field.

Try using google next time? You'll get more out of it in the long run.

to

the

Thanks Jon

So it's better to have the chassis grounded from a shielding perspective. Now, does it matter where it's grounded to? For example, I'm trying to avoid ground loop problems, so I was going to keep my circuit ground separate from earth ground. Could the chassis be grounded to earth ground with my circuit ground (or, more accurately, reference) floating from it, or should the chassis be grounded to the circuit ground?

Thanks again

Well, if you ground in multiple places you reduce the ground impedence. Remember that the connection to ground isn't 0 resistance. Hence you end up increasing the potential difference between the two. I doubt it would be a problem though.

What I suggest is that you give yourself the ability to make several connections to the chasis then do tests. One central wire might be good enough. (if you have a ground plane for example) I've rarely seen more than

1 or 2 connections to the chasis in audio equipment though so it's probably not a big deal.

I imagine you'll have more issues with the circuit itself rather than the chasis and a well defined circuit doesn't need one except in rare occasions.

field

surface

any

the

perspective.

up

a

than

probably

occasions.

Thanks again Jon. Although I will naturally try to lay things out with no or few ground loops in the circuitry itself, I'm concerned about ground loops when using the circuit with other equipment. This particular piece of gear will have 5 guitar effects pedals mounted to it, and hopefully I'll be able to keep the ground that they will use floating from actual earth ground to avoid the problem with ground loops (and thus ground loop hum) when it gets connected to other equipment.

Thanks

"tempus fugit"

** The metal box and audio signal ground must be at the same potential to provide effective shielding.

Direct connection is not essential, it may be via a low value resistance.

Connection of metalwork to AC supply ground is purely a safety measure - it prevents the metal acquiring a voltage potential above that of ground.

..... Phil

it

Thanks Phil

Ugghhh... So if I use a metal enclosure, both the signal ground and the earth ground must be connected to the chassis - the 1st for shielding, and the other for safety, which introduces the possibility of ground loops with other equipment. When you say 'low vaue resistance' what type fo values are you thinking about? A few ohms or a few hundred ohms? Also, do you have any suggestions for combating the ground loop issue? Thanks again

"tempus fugit" "Phil Allison"

** A few hundred ohms is usually OK.

Maybe add a 47 nF cap across it for RFI suppression.

** Power the device with a wall wart.

No need for any safety ground then.

..... Phil