Edit : from the posts below I deduce the issue seems specific to my config. Threfore I remove temporarily my request to add a line a caution in the wiki as it might not be a general problem for everyone, until further investigation and identification of the root cause.

This post is a follow up of this one :

I purchased this capacitor to serve as an APM power protection:


I thought this would protect my APM from very short voltage drops and ultimately avoid erventual APM resets in mid air. WRONG.

I connected this capacitor on the A1 to serve thus as a short power supply on APM.

What happens then when connected ?:

multicopter is on the ground (after landing for example), motors are armed, throttle is at zero. Then if you turn your radio off -> RTL is triggered ! -> motors start full throttle to reach RTL altitude. (of course at the same moment your radio is off so you can only observe a flyaway)

This behaviour was tested and repeated multiple times with same results (on my config).

Another scare we can avoid...very very bad idea this stupid capacitor.

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  • MR60

    Yes I agree it is a mystery still. The last tests I have done yesterday trying to reproduce the problem did not reproduce the problem, but there were a few differences:

    -The APM was moved to my newly built frame, thus APM has been replugged with all wires, including the capacitor.

    -I have now a camera gimabl with servos connected on APM (was not the case initially)

    -the "home position" is less than 2 meters, ie at the same place of this last test

    -> impossible to reproduce.

    What I want still to try out :

    Do the same test, but this time : do it with the home position set more than a few meters away from the test location. This is the only explanaible difference.

  • Developer
    @hugues: can you connect the capacitor to the Outputs power rail and test if you have the same response of RTL?

    We are thinking that the protective fuse/diode maybe playing a part in the issue. The APM2.0 does not have this part so would explain the difference related to the issue

  • Hughes; try changing the size and/or type of capacitor that you are using, it's possible that the cap has provided a fantastic receiver for some kind of electrical noise, changing it's size, type, or adding a small ceramic cap across it may fix that.  also if you run telemetry, disable that for a test as well and see if that has an effect.   

  • I ran a cap bank for several months so the GPS retained its lock when swapping batteries. I had no issues. It was connected to the PWM input power rail along with the BEC.


  • R_L : I agree with your statement regarding the voltage and current sense pins. They probably do not want anyone mistaking those for the inputs. I think remember checking all the +5 / Vcc pins there and they agree with the schematic wiring.

    Hardware like the APM can't stay static for long and rare is the user that can dig into the device and actually fix/replace anything on it.

    I bought a Feiyu FY-20A 3axis stabilizer off eBay a week ago just to get under the hood. The documentation clearly shows to NOT connect RX and servo inputs in a certain way. I never powered it up but cleaned off the servo tape adhesive and started noticing evidence of an 'event'. One of the large surface mount caps was black in the center half cracked open.

    Could it have been a mis-wire by a user? Maybe. Could the cap have let go? Maybe also. Regardless I removed it and applied power to see what would happen. It looked like it booted fine with LED indications as the documentation described. I performed the initialization procedure and the LEDs did accordingly.

    I had to pull out my inspection microscope to start picking through the components. Feiyu coated the board probably for mechanical stability and moisture resistance. This made seeing part numbers difficult. Most were obsolete now and the device has been replaced by the FY-30A

    $8 (shipping included) gets me a lot of entertainment...  :O


  • HERE is the reference to not connecting to the 5V pins. Scroll down the page a bit to see it.


  • I have been wanting to make this graphics for a the perfect discussion has arrived...

    3692714687?profile=originalThe JP7 header has a warning about connecting to the 5V pin (highlighted in red boxes). The implied reason is for the current and voltage sensor input but there is no reason on the schematic that this should not be used. Looking at the pcb layout in Eagle, the entire 5V section of the header does not have a clear connection trace as seen in the Output header (clipped but with a red arrow).

    There is some variation between APM2.0 and 2.5 regarding the JP1/power end of the board but the IO pins are the same.

    It would take one of the board designers to explain why the 5V trace is not clearly indicated on the board layer. I turned on all layers in Eagle and have never been able to make sense of how it works even thought the schematic clearly shows they are all connected together.

    They would also have to clarify why the prohibition of connecting to 5V on A1 and A2. Is this just a sensor warning or is something else up that is not clear between the schematic and the board layout?


  • When stuff gets stranger than fiction, rest assured Quantum mechanics are at work, LOL

  • ... just for my understanding, isn't i possible to detect, if a vehicle is grounded, so in this case, the Failsafe could/should be different than RTL?

  • I have observed this behaviour as well when doing bench testing with capacitors.  I believe the issue is this:

    1) The 32u2 browns out before the AVR chip.  We know this, it was revealed during the investigation of my last Octo crash.

    2) When the 32u2 browns out, it stops sending a signal, so the AVR responds with a failsafe event.

    3) Normally, this is not a problem, as the AVR browns out a few milliseconds later.

    4) Large capacitance on the board will hold up the AVR just long enough that it can throttle up the ESC's before it also shuts down.

    However, I don't think it's necessarily a reason to not use a capacitor, as they can serve a helpful purpose.  This is mostly a problem when bench testing as you have observed.  I almost got bit a few times myself!  

    If you are operating safely, it should not be a problem.

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