>I am using a Radio Shack Cat# 276-1185 rectifying diode array for
>automatic isolation of my essential buss. My problem is that while smoke
>testing the electrical system I noticed a 0.7 volt drop in voltage
>across the diode array. This was a no load, engine off check and my
>input voltage from the battery is 12.43 volts with 11.73 reaching the
>essential buss. Is this OK? My gut says no.
What you have observed is built into the physics of how
diodes work. They are not perfect "switches" . . . while
their reverse conduction is very tiny (this is what makes
them good isolators to prevent backflow of energy) their
forward conduction has a small but relatively fixed voltage
drop.
The very common silicon power rectifier may be expected to
drop between .6 and .8 volts under normal operating
conditions. It is possible to buy germanium diodes and
special silicon diodes (schotky) with lower drops but for
the application we're discussing, it's not necesary. Here's
why.
Consider that when the alternator is running, bus voltage
is 13.8 volts or greater. If you feed your essential bus
from the main bus through the recommended isolation diode,
this puts your essential bus at 13.1 volts or greater. Is
this a "bad" thing?
Consider further that when the alternator is not running,
battery only operations will be at 12.5 volts or LOWER.
Under these conditions, everything on the essential bus
is supplied with a voltage LOWER than when the alternator
is running.
If the goodies on the essential bus are EXPECTED to operate
battery-only until the battery's useful energy is depleted
(down to 10.5 volts) then I suggest that the 0.7 volt drop
you've observed is nothing to be concerned about. The design
task you need to complete is make sure that all goodies on
the essential bus will indeed function in the battery-only
mode. All products for TC aircraft must be functional at
some performance level down to 9 volts! Some suppliers of
hardware to the amateur built industry have not done their
homework. I get several messages a year from folk who find
that some shiny new gizmo on their panel won't work unless
the bus is at normal, alternator working levels.
This survey of goodies in your airplane has nothing to do
with whether or not a diode is installed . . . even if
you replaced the diode with a switch (bad idea), the
electro-goody's shortcomings will still be there.
Bob . . .
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