Filtering supply lines

[Darrell Norquay]

At 02:14 PM 7/24/96 +1100, Stuart Woolford wrote:

>>  The only way to guarantee you are at a true 0v ground is to run 
>> a wire back to the battery terminal.
>
>and what defines the battery terminal as 0v ground?

The battery is the "power supply" for the entire electrical system.  I think
it was mentioned earlier that both the supply + and ground be run back to
the battery.  This helps in several key points:

*The battery is extremely low internal impedence, and tends to "short out"
high frequency noise.
*The battery acts as a large capacitor, absorbing both voltage spikes and
supplying heavy load currents for short periods of time, and tending to
average out the system voltage.
*The alternator is theoretically connected essentially directly to the
battery, therefore absorbing most alternator noise. 

>really - the battery terminal can be a particularly noise prone 
>ground point depending on the charging system design.. (from too much 
>car sterio install experience..)

If this is the case, then the battery ground lead should be repaired or
replaced.  Any voltage drop from the battery to the chassis will affect ALL
devices attached to the chassis.  In my car I have separate 2Ga ground leads
going to both the engine and the frame directly from the battery.  This
alleviated a lot of noise problems I was having with the stereo.  

Most grounding problems occur due to "ground loops", where there are several
different paths back to the power supply (battery) from different devices
within the car body.  Each of the paths has a different finite resistance,
and current flowing through them will generate a voltage across each of
these resistances.  This is what causes ground noise.  In an ideal world,
every device in the whole vehicle would have it's own separate ground wire
running directly back to the battery, then any noise or voltage drop
generated on any given ground lead would have no effect on any other lead.
Unfortunately, this is not the case, and auto manufacturers rely on the
assumption that the body itself is a single homogenous conductor, which
unfortunately it is not.  

The best you can hope to do is isolate your particular installation from the
rest of the system by running it's own power and ground leads directly to
the battery.  There is one caveat to this approach - you must ensure that
your ground lead is the ONLY path back to the battery from your system, if
there are any other "hidden" connections to chassis ground from the
installed equipment (ie a radio chassis or speaker - lead), all bets are
off, you've now created a ground loop instead of eliminating one...

>looking for a 'true' ground is usually not the solution, finding a 
>non-noisy one is quite a different job - I have usually found a good 
>solid frame ground to be good - you pick up so much noise on you 
>earth line to the battery that it gets real bad real fast. You just 
>need to find a frame ground with good conduction paths to the other 
>ground references you are linked to.

This is the standard method, unfortunately it is an approach that masks the
symptom rather than cures the disease.  Sometimes it is the only thing you
can do, to avoid, as you said, bad charging system design.  Grounding and
ground noise is a whole black art unto itself.




regards
dn
dnorquay at awinc.com