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dlj04(at)josephson.com
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 Posted: Mon Oct 12, 2015 11:40 pm Post subject: detecting ground loops, was re: re: noisy 912 alternator |
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On 10/12/15 11:59 PM, AeroElectric-List Digest Server wrote:
| Quote: | > Hi Guy... just out of curiosity, can you explain how you test for
> ground loops? Using an ohm meter, apparently, but hoe exactly? Thanks!
A 'ground loop' is said to exist when a potential
antagonist (most often an alternator, motor or
sometimes a strobe) shares a ground path with a
potential victim (most often an audio system or
some instrument that responds to millivolt level
signals).
Hence, the victim's performance is compromised
by the DC/AC components of the antagonist's
functionality. Ground loops are avoided with
judicious attention to architecture where failure
to 'avoid by design' cannot be readily detected
except for the victim's mis-behaviors.
I'm not sure how any sort of instrument or
other measuring process would be useful in
detecting/analyzing a ground loop.
Some parts of the audio world are trying to get away from the name
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"ground loop" because the wording is confusing (it's seldom a loop and
can be in the power lead, not just ground.) We are using the term
"common impedance coupling" because a common path is used for two
currents, variations in one being superimposed on the other. A
differential scope is actually quite useful in analyzing this condition
-- you look for AC signals with one probe connected to each end of the
"ground" wire.
David Josephson
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jimkale(at)roadrunner.com
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| Posted: Tue Oct 13, 2015 5:14 am Post subject: detecting ground loops, was re: re: noisy 912 alternator |
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I have heard the term "Ground Loop" used a lot and as I understand the
problem, it mostly occurs when a unit that is sensitive to very small
fluctuations in voltage is used and the common ground circuit is not a
single common point, but a lengthy circuit distributed around the airplane
using wires or airframe components. This can lead to small resistances
distributed along the common ground circuit. As current increases in the
common ground circuit, these small resistances develop voltages along the
common ground circuit. Electrical devices, (radios, etc.) that are
sensitive to very small voltage differences and that are connected at
different points along the ground circuit, sense the very small voltage and
behave as if they are not connected to a common ground point, but to
different values of ground.
The solution seems to be to have a very current hefty single point (robust
common Bus Bar) to connect each of the using devices to so that minor
voltages cannot develop across distributed resistance in a long common
ground path. A hefty bus bar (preferably near the alternator/battery
current source): and all using devices have the common ground lead tied to
that point. Is that a layman's description of the solution?
This seems to be what is being discussed below. I am just trying to get it
all straight in my brain.
--
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