sportav8r(at)gmail.com
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 Posted: Sun Feb 18, 2007 5:50 pm Post subject: OV module wiring update/erratum |
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In the process of changing my panel wiring to reflect the drawings I
recently posted here, I stumbled upon the need for one more change
(hopefully the last one) - by leaving in place the damaged OV module
that was fried during testing with a zero-to-18.3 volt power supply.
It became immediately apparent that there was a short somewhere (well,
duh!) and ended up blowing a 10 amp fuse in series with the avionics
standby battery. Ohmmeter probing revealed a short-circuit hard fault
existed from module pins 1 and 3 to pin 5 (ground). In my latest
iteration, I had connected pins 2,3 and 4 to one 5-amp fuse on the
avionics bus, and pin 1 plus one of the fast-on tabs to a second 5a
fuse on the same bus. For whatever reason, the fault current was
evenly-enough distributed between those two fuses that they both held,
while the 10 amp fuse in the battery + lead blew.
**The disturbing thing is that the entire 10 amp fault current was
carried to ground by the 22 AWG wire on pin 5. Sustained testing
could have gotten this wire really toasty**
To protect this ground wire indirectly at the 5 amp level, I have
moved all connections from the OV module to the avionics bus onto a
single connection protected by a 5 amp fuse (about time, eh?  This
has an additional benefit of preventing a blown OV module fuse from
taking out the trim system that formerly shared that fuse on the bus.
(Standard good-engineering practice, I know).
I realize that an _undamaged_ module won't be presenting any shorts to
ground when wired in this way, but in the event the replacement module
ever eats its lunch (I don't think it will, following the re-design of
my schematic), I don't want a fault current of potentially 15 amps
traveling down a 22 gauge ground wire.
Eric, see any problems with the way I've done it now?
-Bill
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