You guys must pass the salt like this:
https://www.youtube.com/watch?v=b3PgxWaQMaQ



On 4/5/2020 11:03 AM, kinsell wrote:
On 4/5/20 9:45 AM, jfitch wrote:
On Saturday, April 4, 2020 at 6:23:44 PM UTC-7, 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 the avionics battery before launching). The
switch over seemed to go okay, but then I noticed that my LX9000 was
giving me unbelievably short glide distances. It turns out that the
QNH altitude had been reset to the altitude at the time of
switching. This was unacceptable, so I resolved to do something
about it before this season. The simplest solution was to add a
capacitor to the avionics power bus. The capacitor supplies power as
the power selector switch is moving, and breaking, from the avionics
battery, and connector, 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 = I * t / CÂ*Â* orÂ*Â* C = 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 situation) and t is 0.1 s, then C = 0.2 F (200,000 μF).
The capacitor would 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 size of the capacitor to 50,000 μF. I
ended up finding a suitably sized 39,000 μF capacitor rated for 25
V. A smaller capacitor could by used if the current drain is lower,
which is likely for most gliders.
https://www.digikey.com/product-deta...301-ND/6928303


I installed the capacitor yesterday and monitored the bus voltage
during switch-over with an oscilloscope, which was anti-climatic:
there was no detectable 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 seen at:
https://flic.kr/s/aHsmMo9rN7

What is the inrush current when you first switch the power on? Must
not be enough to blow the fuse, but that'd be something I'd want to
O'scope with a current probe.


Yep.Â* High enough current might eventually erode the switch contacts,
or even damage the capacitor.

--
Dan, 5J