[TowerTalk] Station Ground

[Jim Lux]

At 03:45 PM 1/14/2005, Jim Brown wrote:
>On Fri, 14 Jan 2005 15:36:11 -0800, Jim Lux wrote:
>
> >If chassis and the 12VDC return are connected internally, you also don't
> >want the chassis ground being lower impedance to the power supply than the
> >intended 12VDC return wire, or your ground connection will carry more
> >current than the power supply wire, which is NOT a good thing.
>
>Huh?  Could you please explain exactly why?  A circuit diagram might help.
>
>Jim

The classic example is a mobile rig, with the -12V wire going to the 
battery, and the rig grounded to the car chassis at some point via the 
antenna coax. The chassis bonding  gets corroded, so now there's a 
potential difference between parts of the chassis due to, for instance, 
headlights or starter.  The radio becomes a low impedance connection 
between the two parts of the chassis.

There are also scenarios where the starter motor has an explicit ground 
wire that comes loose.  Likewise, the alternator.

But, for a more direct case, let's say you've got your radio hooked up with 
some sort of manky fairly small zipcord directly to the battery (or, say, 
the cigarette lighter aka 12VDC accessory outlet).  You've also got the 
radio chassis grounded to the chassis, and you've done a bangup job bonding 
the car's chassis, so there's a real low impedance.  Most of the 12V return 
current will flow through the chassis, rather than in the intended 
wire.  Now your power supply is over a huge loop, ripe for EMI/EMC problems.

If you have a fuse in both power leads (as is common practice), then having 
the chassis ground essentially makes the negative lead fuse useless, too.


Jim, W6RMK