On Sunday, April 5, 2020 at 3:15:06 AM UTC-7, Tim Newport-Peace wrote:
At 01:23 05 April 2020, 2G wrote:
On one of my flights last year I had to switch between my avionics
battery
=
and engine battery when the avionics battery voltage dropped too low (I
had=
left the master on after the last flight and could only partially charge
t=
he avionics battery before launching). The switch over seemed to go okay,
b=
ut then I noticed that my LX9000 was giving me unbelievably short glide
dis=
tances. It turns out that the QNH altitude had been reset to the altitude
a=
t the time of switching. This was unacceptable, so I resolved to do
somethi=
ng about it before this season. The simplest solution was to add a
capacito=
r to the avionics power bus. The capacitor supplies power as the power
sele=
ctor switch is moving, and breaking, from the avionics battery, and
connect=
or, or making, to the engine battery (this is called a "break before
make"
=
switch. But how big of a capacitor to use? The basic equation involved
is:

V =3D I * t / C or C =3D I * t / V

where V is voltage, I is current and t is time.

Translation: the bigger the capacitor the smaller the voltage drop. If
the
=
requirement is to keep the voltage drop to 1 V, the current is 2 A (my
situ=
ation) and t is 0.1 s, then C =3D 0.2 F (200,000 =CE=BCF). The capacitor
wo=
uld also have to be rated for 16 V, min. That is a pretty big capacitor,
so=
I decided I could tolerate a larger voltage drop (4 V), which cuts the
siz=
e of the capacitor to 50,000 =CE=BCF. I ended up finding a suitably sized
3=
9,000 =CE=BCF capacitor rated for 25 V. A smaller capacitor could by used
i=
f the current drain is lower, which is likely for most gliders.
https://www.digikey.com/product-deta.../399-14301-ND=
/6928303

I installed the capacitor yesterday and monitored the bus voltage during
sw=
itch-over with an oscilloscope, which was anti-climatic: there was no
detec=
table drop in bus voltage. Apparently the bread-to-make time is very
short,=
perhaps a millisecond. Haven't had a chance to fly with it yet, but
should=
be able to soon. The scope waveforms and capacitor installation can be
see=
n at:
https://flic.kr/s/aHsmMo9rN7

Interesting. Many years ago Volkslogger had a similar issue and it is worth
examining their solution. Only the instrument in question (LX9000) needs
protecting, not the entire Avionics Bus.

They added an Electrolytic Capacitor, same as you did. But they also added
a Schottky Diode in series between the instrument/capacitor and the
supply.

The capacitor now only has to maintain the instrument in question and not
everything on the Supply Bus. In your calculation, the value of I is
greatly reduced (0.6A instead of 2A) and therefore the value of C is also
reduced, making for a much smaller capacitor.

As you can see from the installation photo, the physical size of the cap is not an issue.

Tom