Sunday, February 18, 2018

Small EFIS fixes and thoughts about power supply

Added time variables to the EFIS. UTC, local time, flight time and stop watch. This also includes a count down timer that pops up, like on the default screen 1,2 etc.

But it's getting a bit cluttered. Perhaps bar graphs (big fat ones) and numbers would be better than these round dials and numbers. I have to try, I still have a couple of pages left in the EFIS. Programming it, once understanding the principles, is really easy though, rather fun actually.

The optically isolated DC-DC solution (Aviogard) may take some time to mature it seems. The Aviogard from MGL is not a viable solution as of today. There is one other guy doing something similar (I think), but not very much of details other than a battery charger as of yet. Its a really interesting site though. Experimental Avionics. If I have learned anything from this, it is that the backup battery MUST be on the isolated side, so it will supply power to the avionics also if the DC-DC converter should fail (a very real scenario as I have experienced). This also means a real battery charger solution must be in place. And while doing that, why not install a modern type of battery, NiMH or LiFePO4. (having said that, I have already ordered a Mascot DC-DC charger, and will rebuild the Aviogard with a new UQQ, configured in accordance to the specs, and try it on the kitchen table at least).

Anyway, a plan B is obviously needed, or building will stop. The obvious plan B is a traditional system, but with a backup battery for avionics. Plan B basically consist of removing the Aviogard, replacing it with a simple switch, essentially what is called an "avionics (master)/switch" in most certified aircraft. I will lose the galvanic protection completely, but:

  • I will be able to isolate the avionics when starting the engine AND have power to everything through the backup battery.
  • The backup battery is charged by the main system, and can be turned on and off.
  • I can switch on and off the backup battery switch and avionics switch to see the voltage from each directly on the EFIS, monitoring each system without an extra gadget (IEFIS Extender)
  • I can isolate the avionics in flight running only from the backup battery if needed.

The plan B will look something like this:

The reversed biased schottky diode is mainly to prevent accidentally starting the engine with the backup battery, which will completely overload the backup battery and wires. No power should ever run from the secondary system to the primary system. This will also simplify the backup mode.

  • Starting:
    • PM ON
    • SM OFF
    • SB ON
  • Normal operation
    • Everything ON
    • If everything in the primary system goes down, the diode will assure all power from the backup is going to the avionics. The engine runs fine on mags alone.
  • Testing
    • SM ON/OFF - reading V from backup battery and main 

No comments: