17 April 2016

Avionics/Wiring: Bus topology resolved.

I offered much thought towards how I would setup the bus architecture in the aircraft.  Here is what the system will look like at a high level.

As noted above, I have decided on three buses.  Left-to-right, Main, Avionics 1 and Avionics 2.

Here is a list of the items on each bus.  An asterisk, "*", indicates the device is additionally switchable.

1 Start AHRS 1 and GPS* AHRS 2 1
2 Alt Field* AE Fuel Guardian USB Console Power 2
3 Fuel Pump* Unassigned Capacitive Fuel 3
4 Dimmer and Defrost* EFIS Left* EFIS Right 4
5 Trim* GTR-200* GMA-240* 5
6 Flaps* GTX-330ES* GTN-650 Nav/GPS 6
7 Land Light Left* TruTrak ADI-2 GTN-650 Comm 7
8 Land Light Right* SkyRadar DX Roll and Pitch Servos* 8
9 Tail Nav/Strobe*
LEMO Power 9
10 Wing Nav/Strobe*

11 EIS and MAP Sensor

12 Pitot Heat*

13 Hobbs and ELT


My thinking was to keep the Main bus for devices that typically need to always be on, including for engine start.  Avionics 1 would be for devices I would want available should I lose my alternator and need to conserve energy as I look for a place to put down (the SkyRadar DX has an integrated AHRS that is accessible via Wi-Fi on my phone/table, the ADI-2 provides multiple pieces of flight info, as does the EFIS provides, including synthetic vision).  Avionics 2 would be for all remaining devices that are not necessary for VFR flight.

At this point, I do not feel it necessary to place a backup battery in the system to power the Avionics 1 bus.  Batteries very rarely fail catastrophically and I'm more concerned about an alternator failure.  In a worst case scenario (no radio, no lights), I can reduce constant current consumption to under 5 Amps (no attitude indicator or backup AHRS) with both Avionics 1 and 2 busses off, or under 8 Amps with the Avionics 1 bus on.

I will use the ES 24115 Master Relay (a Cole Hershey 24115, 85 A continuous duty SPST) supplied by Van's to provide power to all busses.  

However, both Avionics 1 and 2 busses will be switchable by independent SSRs (solid state relays).  I will be using the Crydom DC100D40 with the KS-100 protective cover.  This 100 V, 40 Amp DC relay has the following characteristics of note:
  • 30 Amps continuous load at 50 °C without a heatsink 
    • Though the airframe will act as a heatsink, this gives a great safety margin.
  • 0.28 V maximum output voltage drop.
  • 2.5 mA minimum load current.
  • 0.1 mA maximum off-state leakage current of (irrelevant when Master is off).
  • 4-32 V control voltage range.
  • -40 to 100 °C operating temperature.
  • 2.53 ounce weight.

The SSRs (left image) will each have a diode across their loads.  This is to ensure that any inductive reactance in the load (i.e., my avionics, of which the autopilot servos are the greatest such contributors to inductive reactance) will be properly diode suppressed (right image) when the SSRs are turned off and will thus not fry the latter's trigger circuit (parts ES Diode Master and ES Diode Starter on page 43-04 serve the same purpose for the ES 24115 Master Relay and ES 24021 Starter Solenoid).  Here is where you can learn about a "flyback diode" and Crydom also provides guidance.

I will be using automotive ATM style blade fuses (available in values from 1 to 40 Amps).  I have decided against circuit breakers because if a circuit exceeds its designed current, I wouldn't want to reset it in flight before investigating the issue on the ground.  Blade fuses can be found with an integrated LED that illuminates should the fuse blow (ATO style here).  The LED is self-limiting as, should itself fail, it fails into an open circuit.

Thus, I will need a fuse block.  I wanted every device on its own fuse, so I will need at least 40 positions available, preferably more to accommodate future expansion.  I decided on the Cooper-Bussmann 15310-1-2.  It has 60 positions and is non-bussed.  So a daisy chain of wires mates adjacent fuses to create busses (being careful to avoid exceeding current capacity).  I will dispense with its oversized cover (the 15310-1-1 has a smaller cover but I am unable to find a supplier that sells it in single unit quantities) or trim it to a shallower size.  With a footprint of approximately 2.42"x4.17", it will easily fit into the ~3.9"x5.3" space in the F-14106 Wiring Channel after cutting a mounting panel for the purpose.  The block uses TE Connectivity 1-968855-1 (16-20 AWG) and 1-968857-1 (12-14 AWG) terminals as well as Tyco AMP MCP2.8 Series #828922-1 cavity plugs.

A ground block is important to reduce the possibility of pesky ground loops, which tend to introduce noise into the audio system.  Whilst the "forest of tabs" approach is well proven, I wanted the additional stability of screw terminals (which can back out over time, so their torques will have to be verified at each annual inspection - nothing's perfect).  I chose this block from Fastronix.  It has a single binding post with 12 positions for ground terminals.