I've studied quite a few schematics and was bothered by their
complexity and dependence on diodes. The 122NQ030
is an obsolete part and I don't want to be stuck trying to find one in the future. I redesigned using two batteries
and three solenoids - No Diodes.
These are my requirements:
1. I want my EFIS on an independent bus for start up. High side transients are not an issue but a low bus during
cranking is an issue and the EFIS could reboot.
2. E-bus should be isolated from main bus so it can be powered and not feed back to main bus.
3. Need a mechanism to charge both main and aux battery.
4. Want to be able to run ebus from main or aux battery.
Based on these requirements, which may be the same or different from your requirements, I came up with the topology in
the schematic below.
Operation of this circuit is as follows:
Prestart - Main Batt switch open, Aux Batt switch closed, X-feed switch open. E-bus is powered from Aux battery
and EFIS boots.
Start - Main Batt switch closed, Aux Batt switch closed, X-feed switch open. E-bus is powered from Aux, Main
cranks engine
Run/Flight - Main Batt switch closed, Aux Batt switch closed, X-feed switch closed. Batteries Charge.
Emergency Start - Main Batt switch closed, Aux Batt switch closed, X-feed switch closed. Engine cranked by
both batteries. EFIS may reboot during start.
Emergency Alt Failure - Main Batt switch open, Aux Batt switch closed, X-feed switch open. E-bus is powered
from Aux. Everything else powered down.
Features of this configuration are:
1. E-bus always sees full bus voltage and diode voltage drops are not an issue
2. Aux battery can provide cranking power
3. The extra solenoid is cheaper than diodes and heatsinks and mounting. They are also available in the field in
an emergency.
4. Altenator field can not be enabled unless at least one battery contactor switch is in the ON position.
