Author: "Brian" <bsowell@...> Time: Mon Dec 6, 2004 10:59 am PST Link
In order to use the Keep Alive and Clearance Delivery features of the
EXP BUS 2V, AND use a firewall mounted Master Contactor to be able to
swtich off the #8 AWG Main Battery lead running into the cockpit, I'm
considering doing the following. Please tell me what you think.
Desired State:
I want to use the Keep Alive terminal to power my Dynon EFIS so that
internal battery stays charged and the clock stays current, and
I want to use the Keep Alive terminal to power the Clearance Delivery
feature so I can use my comm intercom & music system with the master
off.
I want the large #8 wire running into my cockpit to the EXP BUS to be
switched off for safety reasons.
Everything is installed, so going back and making big changes is not
desireable. I want the solution to be simple to install.
I'm currently using the 40-amp-rated master relay on the EXP BUS with
the battery lead coming directly from the battery (no master
contactor, fuse, or any other protection). I have a firewall mounted
Master Contactor installed that is currently only supplying power to
the starter through the starter contactor. I installed the master
contactor 'just in case I need it someday', and to allow me to swtich
off the starter if the starter relay got stuck closed. When I close
the master switch, both the 40 amp EXP BUS relay, and the Master
Contactor close.
Proposed Solution:
Move the EXP BUS battery lead to the switched side of the firewall
mounted Master Contactor. Normally this would switch the EXP BUS off
completely when the master is off, disabling the Keep Alive and the
Clearance Delivery features.
But, if I also install a fuse, fuseable link, or other (say, 10 amp
protection) between the two terminals on the master contactor, then
the EXP BUS could draw up to 10 amps through the main battery lead
through this fuse. If a short occurred in the main battery lead with
the master off, the fuse would protect it.
Discussion:
Since the EXP BUS Master relay opens when the firewall mounted master
contactor opens (they're on the same switch), then the only way the
the EXP BUS could continue to draw through this 10 amp 'short
circuit' is through the keep alive feature which is limited by a 3
amp polyfuse, and the #6 avionics bus connector (for comm and
intercomm) which is limited by a 7 amp polyfuse.
The only down side I can see with this is that the EXP BUS now has
both the master contactor and the 40 amp master relay through which
to draw power. It would work fine, but not a good idea from a
reliability standpoint. However, I do not have any critical systems
on the EXP BUS. If one of these fail, my dynon has a battery backup,
as does my GPS. My comm and intercom could be turned on using the
Clearance Delivery feature. I would lose my lights, my engine
instruments, and my transponder.
Any downside I'm not seeing?
Brian
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