AW: chassis grounding

Haible_Juergen#Tel2743 HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de
Wed Aug 28 20:07:00 CEST 1996


>   I have a question. I am building my ASM-1 project in a metal chassis. 
Should
> the chassis be connected to ground or not ? The holes for the mounting
> screws on the board are smack in the middle of the ground traces, but I
> could use plastic
> standoffs if this isn't a good idea. what do you think ?

My preferred method:

The enclosure is grounded (for obvious safety reasons), but this
enclosure ground is not identical with Signal Ground. (Insulated
Jacks!)
Now at one point I connect the two gronds by two strong diodes
(1N4007) (is "reverse parallel" the right word?), and a 47nF ceramic
in parallel to the diodes.
This method is perfect for me: Ground loops see a high impedance
(47nF at 50Hz), and a high (in terms of Hum) 0.7V threshold voltage.
Safety is good for this very unit (earth ground on enclosure), and even
other units that are connected to this one are somehow protected
(the dides can carry a lot of current if they have to!).
And the 47nF capacitor gives almost perfect shielding for the
electronics, because in terms of electromagnetic fields it is rather
low impedance, so for shielding signal ground and enclosure ground
are almost identical.

JH.

PS.: My largest project, the JH-3 Modular, is an exception of this:
I have lots of non-isolated jacks on aluminium front panels. But I won't
connect this to Earth Ground at all - just take care that the PSU is
perfectly insulated (epoxy, perhaps).



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