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Hi Jim,
I think you are thinking of the diodes reverse voltage breakdown rating when
you say "30V forward bias". The alternator would have to somehow go negative
in order for this to happen, and that can not happen because the main
battery would prevent it. The 1N5829 I recommended is rated at 20V, this
should be more than enough for any condition that I can think of.
If an alternator does go over voltage, and by the way, this is unfortunately
a common failure mode like you said. What will happen is you will get a huge
surge current through the diode trying to charge the 13.8 volt battery to 50
volts. This could destroy a normal diode and perhaps leave it shorted, or
open depending on how long it burned. Quick surges tend to leave the diode
shorted like you said. However, these 25 amp continuous duty diodes also
have a very high surge capacity. These diodes will handle a surge of 800
amps! This is a single surge of 800 amps for a single half sine wave of
60Hz. That is roughly 800 amps for 8.3 milliseconds. You have a very big
alternator if it even gets close to 50 amps, let alone 800, so this will
give plenty of time for the breakers to kick off, or the crowbar to kick in
and the diode will be fine.
Regards,
Ed Armstrong
Jim Wrote:
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 th is
transient without shorting out? If so the backup battery will continue t o
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.
LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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