On Tuesday, April 7, 2020 at 6:19:36 PM UTC-7, 2G wrote:
On Tuesday, April 7, 2020 at 9:32:07 AM UTC-7, Dan Marotta wrote:
Well then, I must be really confused.Â* If the switch was a BREAK before
MAKE, the equipment would be momentarily without power.Â* Some devices
can survive that, perhaps with internal capacitance, but others will
lose power momentarily.Â* This may or may not be an issue.

Inrush current seems to be a big bugaboo to some people and, in some
cases it is, but what is the time constant associated with the inrush?Â*
How much current are we talking about and for how long? People keep
talking about the high battery "charging" the low battery during the
milliseconds that they are in parallel. Theoretically, yes, practically,
hogwash.Â* The heat required to blow a fuse or burn a wire does not rise
instantaneously; there's counter EMF to reduce the current...Â* So many
details that meant a bunch when you were taking a test back in school
but, practically speaking, don't mean squat in this case.

So, switch your batteries and protect your circuits however you wish.Â*
I'll stick with my make before break switches.Â* I'll even report back
when something fails due to their use, but don't hold your breath.

On 4/6/2020 10:43 PM, 2G wrote:
On Monday, April 6, 2020 at 11:23:21 AM UTC-7, Dan Marotta wrote:
Tom,

Isn't the rotary panel switch in a Schleicher glider a make before break?

OK, I'll stop posting to this thread now.Â* My solution, which I've
relied on for years without ever losing a connection or blowing a fuse
seems to be too simple to be acceptable.

On 4/5/2020 7:45 PM, 2G wrote:
The glider, an ASH 31 Mi, is already wired with a battery selector switch for the avionics, and is the way to go. You definitely don't want to parallel Pb and LFP battery's accidentally.

Tom
--
Dan, 5J
Dan,

Think about it: if it were MAKE before BREAK I wouldn't have had this problem and sure as hell wouldn't post a solution to a non-existent problem.

--
Dan, 5J

If my power switch was a break-before-make (also called a shorting switch) I would not have had a problem and certainly wouldn't have posted a fix to a non-existent problem. But it isn't. It was easier for me to put in a capacitor than replace the switch. It also protects against any switch bounce while switching.

People seem to be overly concerned with inrush current. High inrush current would only happen if there were a substantial amount of power supply capacitance in the LX9000 that needed charging. If that were the case it would handle a millisecond battery switch over without any problem, but it doesn't. Also, there is no inrush current during battery switching as the caps in the LX9000 are already charged.

Tom

The inrush current to the LX9000 is not the issue, nor is switching between batteries while the cap is charged. The issue (if there is one) is the inrush to the discharged cap when you first turn on the mains. It is a pretty complex and loosely speced system. Caps are very loosely spec'd, as are battery internal resistance, switch resistance, and of course the wiring in each glider is unique. That's why I suggested measuring it. When he switch is turned on, it will arc and bounce because that is what switches do. How much is the question. Quite easy to measure, hard to calculate accurately. It is quite possible that the step impedance in the system limits the inrush to an acceptable value - but you don't know what you don't know.

It would have been easy for LX to spec a switching power supply with enough residual energy to cover, say 20 ms switching interval. Apparently they did not. Or perhaps they are using linear supplies, in which case the garbage can is the appropriate resting place. I'll second Dave's suggestion of using one large battery bank instead of seperate small ones. It is better for battery life too. (In the 31 the second battery may be the starting battery so that isn't practical). Or better still, use a higher capacity LFP - my small instrument battery will power the panel for around 12 hours.