Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

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Magnetism 67
Figure 4-33. Radiation interference: EMI waves
travel through the air and are picked up by wiring that
acts as a receiving antenna.
EMI Suppression Devices
Just as there are four methods of EMI transmission,
there are four general ways in which EMI is
reduced, as follows:
• By the addition of resistance to conductors,
which suppresses conductive transmission
and radiation
• By the use of capacitors and radio choke coil
combinations to reduce capacitive and inductive
coupling
• By the use of metal or metalized plastic
shielding, which reduces EMI radiation in
addition to capacitive and inductive coupling
• By an increased use of ground straps to
reduce conductive transmission and radiation
by passing the unwanted signals to ground
Figure 4-34. The capacitor attached to a GM HEI
ignition module protects the module from EMI.
Resistance Suppression
Adding resistance to a circuit to suppress RFI
works only for high-voltage systems (for example,
changing the conductive core of ignition
cables). The use of resistance to suppress interference
in low-voltage circuits creates too much
voltage drop and power loss to be efficient.
The only high-voltage system on most vehicles
is the ignition secondary circuit. Although this is
the greatest single source of EMI, it is also the
easiest to control by the use of resistance spark
plug cables, resistor spark plugs, and the silicone
grease used on the distributor cap and rotor of
some electronic ignitions.
Suppression Capacitors and Coils
Capacitors are installed across many circuits and
switching points to absorb voltage fluctuations.
Among other applications, they are used as follows:
Figure 4-35. Interference-suppression capacitors
and choke coils are attached to electric motors, like
the Bosch wiper motor shown. (Reprinted by permission
of Robert Bosch GmbH)
• Across the primary circuit of some electronic
ignition modules (Figure 4-34)
• Across the output terminal of most alternators
• Across the armature circuit of electric motors
Radio choke coils reduce current fluctuations
resulting from self-induction. They are often
combined with capacitors to act as EMI filter circuits
for windshield wiper and electric fuel pump
motors (Figure 4-35). Filters may also be incorporated
in wiring connectors.