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Chapter 3
Power and Signal Return
3.1
INTRODUCTION
Many automotive EMC problems are attributed to “bad ground”
connections. Bad ground seems to be the cause of many problems in all
types of electrical circuits. The reason that there are bad ground connections
is simple. There is not a “ground” anywhere on a vehicle! The reason there
is no ground connection is also simple. The vehicle is intended to travel on
the ground, not attached to it. Actually, the one time when there can be a
ground connection on a vehicle this is shown in Figure 3.1:
In this case, if the ground connection is maintained, it can be seen that
the vehicle is of little use as a transportation method if it can only travel as
far as the ground cable allows it.
The use of the term ground unfortunately has become used to describe
the path where the return currents are assumed to be flowing. As a matter of
fact, there is a circular definition of the term electrical ground. Many
writings on electrical circuits refer to the ground as the “sink of the power or
signal currents”. That definition MAY be satisfactory if the return currents
read this definition and then consult with the circuit designer to find out
where they should be flowing! There are interesting definitions that have
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been developed. One heard recently was the concept of “dirty” and “clean”
grounds. This emphasizes the fact that a “ground” is not what it is supposed
to be, since we seem to keep making more definitions when realize when our
existing ones do not work!
It is correct to look at the path of the return currents as the “return”.
Doing so will eliminate the underlying assumptions about ground
connections that are not always true. For example, it is sometimes assumed
that the ground is a zero impedance path and can sink infinite amounts of
current. The problem is that in the real world, there is no such thing as zero
impedance and there is also a limitation on the current carrying capability of
the return path. The other problem with referring to “ground” connections is
that there are at least three uses of the term “ground”. We will discuss these
later. It is the authors’ intention to never use the term “ground” in this text
when power or signal return path is actually meant. (However, some old
habits are difficult to eliminate!)
Let's look at some basic facts in order to develop our concept of return
rather than ground.
The first concept is that every current must return to its source. This
is a fact of nature. If this were not true, there would be pools of
charge created by the accumulation of current flow, which does not
happen.
The next item is that the majority of the current takes the path of
least…IMPEDANCE. We're sure many of you learned current takes
the path of least “resistance”. That is a true statement – when we are
dealing with low frequencies or D.C. Once we have a frequency
greater than D.C., which is nearly all the time (including “pulsed”
DC, which has a period near zero), then we need to understand that
impedance is important.
The last key point is that in order to understand the circuit, the
current source and return must be known for each current. If they are
assumed to be on the same line, and it is not understood where the
currents flow, this can lead to difficulty in creating a model of the
actual conditions.
In summary:
1. There is no ground connection on a vehicle (or for any electrical
circuit that does not have a wire or cable going from the circuit to the
earth).
2. Current takes the path of least impedance.
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Power and Signal Integrity / 19
3. Frequencies greater than DC means that currents will flow different
from what was assumed to be the path in the DC current flow.
3.2
CURRENT PATH
Let us now look at the impact of these statements. What they imply is
that, if the current has a frequency greater than D.C., then the concept of
impedance must be considered. This means that the current paths may be
defined by either the inductance or capacitance of the circuit, NOT ONLY
THE RESISTANCE!
If the current path is defined by the inductance of the circuit, then a major
contributor is the size of the current loop. This will be discussed in more
detail later. Note: implicit in this definition is the actual current loop – not
the assumptions about the wiring harness, since the harness may not always
be conducting the current it is assumed to be conducting. If the impedance
is defined by the capacitance of the circuit, this is due to the relative location
and spacing of the conductors, which are clearly not the DC circuit paths.
Least resistance may not be equal to least impedance! Many times in
solving EMC problems at the circuit board level there is the incorporation of
a “ground plane”. This again is an example of using confusing terminology.
The purpose of the plane, which normally consists of a conductive surface
over the majority of the area, is to allow the current to define its own “least
impedance” path back to the source. What is significant is that this path may
even be the path of higher resistance, yet lower impedance! This would
seem to contradict “common sense”! See Figure 3.2.
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There are some conditions where it is appropriate to refer to the “ground”
connections. These are generally related to safety considerations and
primary power in residential and/or commercial installations. In this case,
there are connections that routed back to a rod that is driven into the ground
(earth). The purpose of this is to provide an alternate path for the current to
flow in the event of a circuit fault. This ground connection is the third pin
on the three-prong electrical connectors that are in use today. Along with
the ground connection, today's electrical codes require that there be a
“polarity” to the connection. This is also intended to protect the operator
from a safety issue or concern. Photos of the three-prong connector and the
polarized connector are shown below, with the connections labeled.
In the electrical codes, there are also reference to the “hot” and “neutral”
connections. Figure 3.3 also shows which lines connect to which terminals..
There may be a second meaning of the term grounding – this is typically
used in electronic circuits. This may actually be a voltage reference, where
the current in the voltage reference line is very near zero. This is shown in
Figure 3.4.
This type of connection should not be called grounding – it should be
called voltage reference, because that is the function it is performing.
Let's now look at another concept that is frequently used, and see if we
can better define the actual conditions that are taking place. These are
shown in Figure 3.5., and should be called single and multi-point return
connections.
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