pitts_pilot(at)bellsouth.
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 Posted: Tue Nov 02, 2010 3:22 pm Post subject: Battery breaker vs contactor |
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On 11/2/2010 6:53 PM, mmayfield wrote:
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OK, so the aircraft we're talking about has a very simple electrical system. Day VFR only, a comm, a transponder, a few miscellaneous widgets like boost pump, smoke pump, engine monitor, a couple of solenoid valves, and the engine is the venerable M14P which is an air start motor. So the battery is also correspondingly small, and recharged by the B&C SK10 alternator.
The traditional basic electrical schematic has an avionics master but I'm discarding this because, well, I just can't see why it is needed at all for this setup. Any issues there?
Not really. The avionics master came to life as a way to turn off all
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the radios etc. at once instead of using each front panel switches.
There was a fear that starting and shutdown would cause spikes on the
buss that killed the avionics. May have been true, I don't know.
However, most all newer electronics have internal switching power
supplies and are relatively immune to that scenario. One downside to
having the avionics master was that the on/off switches were usually
volume controls, which stayed in one spot ..... and wore a hole in the
variable resistor part, creating a noisy spot or requiring replacement.
| Quote: | This diagram is also drawn with no battery contactor, but a main battery circuit breaker instead, and the master switch is 14AWG wire in one side and out the other to the bus, I believe.
How does this rate compared to using a battery contactor instead? I do recall Bob talking of this being a bit of an unconventional layout (regarding a B&C wiring diagram a while back). Would it be better to specify a battery contactor in a conventional arrangement, or for this system would it not matter too much?
Typically, the battery master contactor was placed close to the battery
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and had two jobs. #1 was to supply battery power to the aircraft, and
the starter, and #2 was to remove power to all things that might run
down the battery if left on. It also allowed the user to cut power if
there was a healthy short to ground and prevented ensuing fires. To
answer your question, it would be better to have the master contactor
available just for the safety issue.
Linn
BTW, you're not putting the M14P in the -10 are you??? 
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