Re: Re: Ground loop boogieman ... can someone explain this

> Ground loop boogieman ... can someone explain this in some
> detail or point me to a link that does?
>
> I think I know basically - a "ground loop" is where you have two
> different return paths for your current but they ain't both
> at the exactly the same potential therefore .... problems,
> current flowing in a direction it should not be ... but
> I'd like to understand this (.... better ... or maybe
> "at all" if the above sounds offbase).

"Ground loop" is probably the most incorrectly used phrase in all of
electronics.

Ground loops are almost never BAD things, and frequently are GOOD
things.

The "ground problem" is basically this. Wire is not a pure thing, but
always has resistance ( and inductance and capacitance, but for this
discussion, I'll lump them all together and talk about resistance
alone ).

Current flowing through a resistor causes a voltage drop, hence current
flowing through a wire causes a voltage drop.

--- ASCII ART BELOW. ---

Consider this circuit

0v -------A--------B
* *
+5v -------C--------D

where

--- is "wire "
A*C is a device drawing some current
B*D is another device also drawing some current.

Let us assume the power supply represented by 0v and +5v is an ideal,
nothing wrong with it in any way, designed to be perfect, power supply.

It would appear that both A and B are at 0 volts, ground potential, but
in fact they are not.

Since current is flowing and wires have resistance, A is some finite and
measurable voltage above ground, and B is some finite and measurable
voltage above A.

That is B > A > 0

Really, the circuit should be redrawn as

0v ---r1--A---r2--B
* * Figure 1
+5v ---r3--C---r4--D

where r1 - r4 are resistors.

Now, often, this resistance effect doesn't matter. If A*C and B*D were
light bulbs, B*D would be simply be dimmer and we wouldn't worry.

Say A*C was a BIG heater, drawing lots of continuous current in a nice
steady manner. B*D might then have only 4.8 volts across it, with .1
volt being eaten up in the ground leg to A and another .1 volt being
eaten up in the +5 leg to C.

B*D would probably still work fine. It has a wee bit less voltage to
work with than expected, but not so much as to cause a problem. And the
voltage would be quite steady.

An important point to note here is that the + side of things
frequently needs as much attention as the ground side.

The problem comes when A*C is NOT a steady draw ( like in a chopper
motor controller, a stepper motor controller, an audio amplifier, etc )
and B*D REQUIRES an unfluctuating supply voltage. In this case, the
actual voltage at B ( and D ) is wavering and unpredictable and bad
things happen.

If you were to add another wire like so

|----------r5--|
0v ---r1--A---r2--B
* *
+5v ---r3--C---r4--D

so that a "ground loop" is formed, things might actually improve ! The
problem would probably not be fixed, though. A*C would still try to
return current to ground through r2 as well as r1, with the new r5 now
providing a drop. But things would probably improve as a result of
this "loop".

The "solution" , on the ground side, is

|----------r2--|
0v ---r1--A B
* *
+5v ---r3--C---r4--D

with the best wiring practice being

|----------r2--|
0v ---r1--A B
* *
+5v ---r3--C D
|----------r4--|


The drawings above are a representation of "single point grounding" and
solves the problem.

It is worth noting that the optically isolated inputs of the Gecko
devices are there to solve this exact problem of "how to ground
things".

Since invariably the power supply for the step and direction signals
will not be the same power supply as used for the Gecko motors/logic,
and worse, the two power supplies are a distance apart, the best thing
to do vis-a-vis grounding is to not ground them at all !

There need be no electrical connection between the computer and the
Geckos.

We are left with the additional problem of supplying 5 volts for these
devices, but opto-isolators are by far the easiest of the possible
solutions.

Hope this helps.

Alan

P.S. All this refers to the DC component of our projects. The AC
component is a different matter entirely. Our AC house wiring is also
laid out in a bus, as in Figure 1 above. This is fine for safety, but
unacceptable for true "grounding". When building my electronics shop,
I ran a separate wire from the ground receptacle of every AC outlet box
back to a big ground rod pounded into the ground, and a FAT wire from
that rod to the cold water inlet.

Measurement of microvolt signals with multiple pieces of test equipment
connected is now possible

I continued this practice when wiring the shop.


P.P.S. At a block party a number of years ago to fete the opening of
a new subdivision, our band was called upon to play.

We set up at the end of the cul-de-sac, running an extension cord from
one of the houses to power all the instrument amps and another one from
a house across the street to power the mics and PA gear. Quick tests
showed all was well.

Lead singer got on stage to make some opening remarks, grabbed his
guitar strings so as to keep guitar noise down, "kissed" the mic, as
was his style, and got a REAL shock.

Turned out that there was enough of a difference in ground potential
between the two houses to give him a zap.


--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta