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Backup system for BEC

Posted by Gareth Rens on January 10, 2012 at 8:00am

So after much reading and research into the whole "backup" BEC saga. I think I've found a solution!

My first idea was just to run a 5.5v backup lipo in parallel with the 5v UBEC output.
This idea has been vitoed by most. Too many unknowns apparently.
The other was to parallel all the BECs up, but this would cause harmonics due to all the square wave interaction.

So I figured we cant mix power supplies. Other than using a microproccessor controlled mosfet to switch
power supplies when and if the bec dies.

We use a relay :) Bear with me.

The main power supply powers the relay. With the relay closed (NO) power is routed via the bec to the apm.
If the bec fails, the power to the relay fails and is switched to the auxillary power supply (battery or another bec)

This completely seperates the two power sources.

Will the relay be able to switch fast enough to the auxillary power source without the APM losing power?
If not, im sure a power capacitor will keep the APM alive long enough for the bi-pass surgery to take effect?

Above is a recording of the ciruit just in case my explaining skills are dodgy :)

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Comments

  • jeff delvecchio May 8, 2013 at 3:32pm
    OK thanks Dean!

    the APM2.5 has a very odd power system. there are to date 20,134 different ways to run power to it. LOL
  • Dean Franks May 8, 2013 at 2:00pm

    Sorry, the filename is correct but I copied the link from the previous post.

    Version 2 UAVPWR file

    I don't know what the APM 2.5 power system looks like, we use a custom autopilot.

  • jeff delvecchio May 8, 2013 at 1:56pm
    hey dean... the package looks like its UAVPWR (not UAVPWR2) i think im looking at what you made before LOL

    thanks for the advice on the LC filtering. Does the APM2.5 have an lc filter and LDO when i apply 5V into one of the APM's input pins or when Im using the "PM" plug on the APM 2.5? I have "JP1" removed on the APM

    jeff
  • Dean Franks May 8, 2013 at 11:53am

    Design complete and files uploaded to http://71.6.151.114/pub/UAVPWR2.zip

    Includes LDO for 5V regulated output, LC filter in input to LDO, mounting holes and BOM file with exact recommended part numbers.

    Notes:

    I don't think it will do much good to put LC filtering on the output to the servos to clean up the servo power.  Putting LC filters at the servos themselves (or at the end of their leads) would be more effective, but I suspect the real benefit comes from filtering the ESC and servo switching noise out of the 5V power supply that ultimately supplies the various sensors.  The Exar LDO has a fairly good power supply noise rejection ratio, and the LC filter on the inputs helps further reduce the noise.  On the APM board, there will be another LC filter and LDO (pretty much all sensors are 3.3V) further reducing the power supply noise.

    Be careful substituting the 10uf ceramic caps on the LDO.  Ceramic capacitors have significantly reduced actual capacitance with operated with a bias voltage (6.5V ish on the input and 5V on the output in this case).  The X7R caps are better, but you will want a 16V cap (or more) to get most of the 10uf we are hoping for.

    The mounting holes are sized for #3 screws.

    The board should run fairly cool in free air, don't thermally insulate it in any way.  If you are paranoid, eliminate the solder mask on most of the top and bottom land areas (keep some around the surface mount pads and both sides of the through hole pads to ease assembly).

  • jeff delvecchio May 8, 2013 at 9:32am

    hi dean... that sounds great.

    I just realized that my LC filter wont handle the load of servos (1A max). Can you advise me on what kind of filter i can build that can handle the load... or do i even need one since you are building one into the new design for the APM?

    I am preferring to have clean power throughout the entire electrical system of the tri.

    I am now going to use the LC filter that i have to feed my FPV power.

    thanks

    jeff

  • Dean Franks May 7, 2013 at 9:57pm

    Jeff,  sorry, I forgot about that.  I will post another board in the morning with an additional regulated 5v output using the OKI switching regulator and an additional LC filter on the output to limit noise into the digital/sensor section.

    Dean

  • jeff delvecchio May 7, 2013 at 9:54pm

    thanks dean! im gonna look this over. looks like were not using the 2 output idea then, right? i wish i had one idea of what your design means... electronics theory mostly eludes me.

    is it ok if i share your design  on other forums ( crediting you )?

    hi gustav... the drop is the problem.... otherwise i would use one.

  • Gustav Kuhn May 7, 2013 at 2:26pm

    I humbly suggest again to look at just using diodes to OR your supplies/batteries ?

    Get practical, take a diode like 1N5819, run 500mA through it, measure the drop.

    And it can easily handle peak currents up to 10 A ! Looking at Dean's circuit, with those output capacitors, the diodes will present an almost flat supply voltage to the load.

    No complicated electronic devices to go belly-up, and you can build the thing on Vero Board......

  • Dean Franks May 7, 2013 at 12:31pm

    Design complete and files uploaded to http://71.6.151.114/pub/UAVPWR.zip

    Contains PCB Artist (free download) CAD files and PDFs of schematic and board layers.

    Dean

  • jeff delvecchio May 5, 2013 at 10:28am
    hi dean....

    just found this... based on a LTC 4416.

    http://www.rcgroups.com/forums/showpost.php?p=24509775&postcoun...

    not sure if this helps.

    jeff
    RC Groups - View Single Post - Simple Bus-Tie Circuit using Schottky Diodes
    RC Groups - the most active Radio Control model community: electric and fuel rc airplanes,rc helis,rc boats and rc cars. Features discussion forums,…
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