> "On the avionics contactor relay, I thought the diode was for if it was switching
inductive loads to prevent back EMF. Is a diode still recommended for an
avionics bus with resistive loads? Do I misunderstand the diodes role here?"
A relay has two (or more) circuits. The coil is one circuit and the relay contacts
are in another circuit. The two circuits can be completely isolated from
each other. The purpose of a diode connected across a relay (or contactor) coil
is to protect the controlling switch from high voltage spark generated by
the relay coil when it is shut off by the switch. That high voltage has nothing
to do with the load on the relay contacts, whether that load is inductive or
resistive. In fact, a relay coil will produce a high voltage when it is shut
off, even if nothing at all is connected to the relay contacts. The bigger
the coil, the bigger the spark. A relay coil will not make as big of a spark
as a contactor coil. Connecting a diode across a relay coil will prolong the
life of the switch that controls it. The diode arrow should point towards positive.
> "Thanks for pointing out that the avionics relay is a single point failure. What
do you guys think about getting one fault tolerant by having two avionics
bus relays/switches in parallel - maybe one passing power from the Battery Bus
and one from the System Bus?"
Yes, that will work. What you are proposing is similar to the E-Bus. And it is
similar to what I proposed in my previous post, although I might not have explained
it clearly. Even if you do use two relays, the diode must still be used
to prevent high current from flowing from the avionics bus (AKA E-Bus) to the
system bus.
> "I'm not yet convinced that the Essential/Endurance Bus adds that much value."
The important feature of an E-Bus is that is has two independent current paths.
It would be easy to turn your avionics bus into an E-Bus by adding the alternate
feed path from the battery bus and a diode to prevent back-feeding the system
bus.
> "It seems, though, that it might improve reliability more for Z-12 to have dual
parallel battery contactors."
Yes, that will work, but will cost more and weigh more than using a relay and diode
along with an E-Bus that will accomplish the same thing.
It can be satisfying to design one's own electrical system. However, there could
be failure modes that you might not be aware of. The big advantage of using
one of Bob's drawings is that they have been proven over time and scrutinized
by many eyes. Any bugs have been worked out. Your schematic looks great in
colors. After a couple of minor changes, it will be similar to Z-11.
--------
Joe Gores
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