If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
Battery switching without tears
Wouldn't a make-before-break switch solve the problem in a much simpler
fashion?Â* That's what my Stemme uses and switching between main and tail batteries is a non event. On 4/5/2020 4:02 AM, 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. -- Dan, 5J |
#12
|
|||
|
|||
Battery switching without tears
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 / C or 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. |
#13
|
|||
|
|||
Battery switching without tears
On Sunday, April 5, 2020 at 10:18:00 AM UTC-5, Dan Marotta wrote:
Wouldn't a make-before-break switch solve the problem in a much simpler fashion?Â* That's what my Stemme uses and switching between main and tail batteries is a non event. Depends on what happens between make and break. There will be some amount of surge current from the full battery to the empty one. If the current times the time exceeds the fuse IT, you can blow a battery fuse. Long wires out to the tail and quick switching could make this less likely. |
#14
|
|||
|
|||
Battery switching without tears
On Sun, 05 Apr 2020 09:10:53 -0700, stu857xx wrote:
On Sunday, April 5, 2020 at 10:18:00 AM UTC-5, Dan Marotta wrote: Wouldn't a make-before-break switch solve the problem in a much simpler fashion?Â* That's what my Stemme uses and switching between main and tail batteries is a non event. Depends on what happens between make and break. There will be some amount of surge current from the full battery to the empty one. If the current times the time exceeds the fuse IT, you can blow a battery fuse. Long wires out to the tail and quick switching could make this less likely. So would a schottky diode inline with each battery. It will cost you a whole 0.25v voltage drop and even make the switch unnecessary if you don't mind drawing from both batteries at once. I power my logger that way, but split the panel feed so one battery runs flight instruments and the other does radio and T&B. Backups? My nav system is a PNA thats good for at least a couple of hours on internal battery and my backup vario is a Borgelt B.40 thats good for 8 hours plus off the fresh PP3 dry battery it uses for an alternate power source. -- Martin | martin at Gregorie | gregorie dot org |
#15
|
|||
|
|||
Battery switching without tears
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 / C or 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. High enough current might eventually erode the switch contacts, or even damage the capacitor. |
#16
|
|||
|
|||
Battery switching without tears
Well, when I had my LAK-17a, I had a make-before-break and I never blew
a fuse.Â* The idea is to flip the switch, not see how slowly you can keep it in the "make" position.Â* I would imagine the "make" time is of the order of no more than a couple of thousandths of a second.Â* Some body with a scope could measure that time and let us know. On 4/5/2020 10:10 AM, wrote: On Sunday, April 5, 2020 at 10:18:00 AM UTC-5, Dan Marotta wrote: Wouldn't a make-before-break switch solve the problem in a much simpler fashion?Â* That's what my Stemme uses and switching between main and tail batteries is a non event. Depends on what happens between make and break. There will be some amount of surge current from the full battery to the empty one. If the current times the time exceeds the fuse IT, you can blow a battery fuse. Long wires out to the tail and quick switching could make this less likely. -- Dan, 5J |
#17
|
|||
|
|||
Battery switching without tears
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 |
#18
|
|||
|
|||
Battery switching without tears
On Sunday, April 5, 2020 at 10:03:31 AM UTC-7, 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 / C or 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. High enough current might eventually erode the switch contacts, or even damage the capacitor. The commonly used rotary switches are already being run over spec in modern panels. Schleicher and most others are typically using something like the NKK MRY106 or equivalent, these are called a 2A switch but that is the AC rating, DC rating is 1A. A modern panel uses something like 1.5A, and that is before you key the PTT switch. When I redid my panel I used an MRT23 which is 3A, and paralleled the contacts for a 6A total. Rotary switches with a higher rating are normally much larger physically. |
#19
|
|||
|
|||
Battery switching without tears
On Sunday, April 5, 2020 at 1:01:09 PM UTC-5, Dan Marotta wrote:
You guys must pass the salt like this: https://www.youtube.com/watch?v=b3PgxWaQMaQ delightful, but not quite the same A 27 at a recent contest had two separate switches plus an old guy to implement make and eventually break. The result took a Walmart run to get a replacement fuse. The diode seems a more certain alternative. |
#20
|
|||
|
|||
Battery switching without tears
On Sun, 05 Apr 2020 12:15:53 +0000, Martin Gregorie wrote:
I added small capacitors (18mm diameter x 35mm long, 35-50v, so 1000-2000 uF capacity or thereabouts for each capacitor - thats a guestimate since I can't read their markings without major surgery), one for each battery, and with a 0.1 ohm resistor in series with it. Correction: still don't how big the capacitor is but the current limiting resistor in series with it is 10 ohms (just measured it) so the capacitor won't charge or discharge more at than 1.2 amps. The resistor is a big wire-wound resistor in a rectangular ceramic case with the capacitor taped onto it, so is both mechanically and electrically strong. But, as I said, since this fixed the problem thats enough to prevent vario and radio resets if I smack the XLR connector powering my panel. Nav is fine - got its own internal battery. The same setup may well sort out a break-before-make switch too, with the advantage that fitting it across the +12v and ground lines on the panel side of the switch can most likely be done without disturbing anything that's already in your panel and is both inexpensive and easy to make and install. -- Martin | martin at Gregorie | gregorie dot org |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Airplane tears off winglet on jet bridge | a[_3_] | Piloting | 0 | July 8th 10 08:06 PM |
Tears in the eyes, - 1 attachment | RobG | Aviation Photos | 4 | June 17th 08 10:51 AM |
The Tears Of Finding The Truth | algaga | Piloting | 9 | January 3rd 08 04:33 PM |
“Particularly on May 19th”— with the tears of his father | X98 | Military Aviation | 0 | May 18th 04 10:34 AM |