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In analyzing the reliability of a diode being used for battery isolation
consider the following happening:

The alternator fails resulting in a temporary 50 Volts over voltage.  The
overvoltage causes the over voltage protector "crowbar" to trip the
alternator field deactivating the alternator.  Will your diode survive this
transient without shorting out?  If so the backup battery will continue to
power your essential bus for the expected duration while you are getting
the aircraft saftely back on the ground.  If not the backup battery is
powering  the primary bus and the backup battery may not last as long as
expected.  If your isolation diode is only rated for 30V forward bias
perhaps you should look for another.  Unfortunately, diodes usually fail in
the shorted mode which is not good when they are being used to isolate
critical systems. We must consider failure mode possibilities else the
system may not work as expected.

Jim Frantz  

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Please send your photos and drawings to marvkaye@olsusa.com.