Counter Electromagnetic Force (CEMF)
A potential problem inherent in any installation of electronic systems within a
vehicle is Counter Electro-magnetic Force (CEMF).
CEMF is caused when relays or solenoids, connected to the vehicle DC power
distribution, are de-energised. The voltage produced may exceed – 400 volts.
CEMF is produced by equipment such as the following:
• Electric fan brakes
• Air conditioners
• Starter relays
• Electric pump relays
CEMF is more than sufficient to damage or cause erratic operation of any
electronic system that is also connected to the same vehicle DC power supply.
CEMF can be eliminated by installing diodes at the relays and solenoids that
cause the problem, and more importantly at the power supply cable connections
on the receiver.
A 47V, 5W, Zener diode (1N5368 or equivalent) should be connected between the
receiver +ve (positive) power input terminal and ground, as illustrated in Figure 5.
Figure 5. Zener Diode Connected
Ground
Battery +ve (positive) supply
Zener Diode
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