This is a discussion re the bad Accel, Gyro and Baro values that we're seeing with ArduCopter-3.1.  The increase in the SPI bus speed from 500khz to 8Mhz has exposed a hardware problem on some boards.  That hardware problem is that the 3.3V regulator has been blown so all sensors are running at 5V instead of the intended 3.3V.


How have these regulators been burnt out?

  • Attaching a radio receiver or MinimOSD to the APM while the APM is only powered through the USB (see video below)

  • Some clone boards seem to come from the factory with blown regulators.  3DR boards might also come with blown regulators although they do a specific check of the regulator as part of the regular QA process.
  • It is not (as far as we know) actually caused by the AC3.1 software itself, it just exposes the problem.  You could prove this to yourself by checking the 3.3V regulator (see video above) before and after the upgrade.


How can we fix the regulator?

Option #1:  If it's a new board (so that it's less likely you burned it out yourself) you could report the problem to the retailer that sold you the board and ask for an replacement.  If it's 3DR it's called an "RMA".

Option #2: if you're handy with a soldering iron you can replace the regulator yourself.  On the APM2.5.2 (and higher) boards it's not that difficult.  On the APM2.5 it's far more difficult.

3691073788?profile=originalFor APM2.5.2 : TPS79133DBVR

For APM 2.5: MIC5219-3.3YML TR

How can I stop it from happening again?

Do not connect any devices such as a radio receiver, MinimOSD, GPS, etc while the APM is powered especially while powered only through the USB cable.

Attaching a 100uF capacitor across any of the APM's radio input's 5V and GND pins will stop the regulator from being blown by plugging in a receiver.  video here!

There are very few reports of regulators being blown twice and no reports of it ever failing in flight.


Below are some graphs of the types of values that we are seeing on these boards.


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  • All good guys.   Randy and Paul hit the nail on the head.   Of course you we never have an issue disconnecting and reconnecting servo/signal wires... but when it's the FIRST servo wire... and the Rx is NOT yet powered... (too used to having Rx packs or the Rx powered by something other than the flybarless/flight controller).

    So in my case, yes I broke the new #1 rule without even catching it.  Connected lead one to Rx which powered up the Rx (I obviously thought it was already powered), in rush of current, bam. Blown 3.3v reg.

    Now I still feel that this is a hardware defect in the original design, it should be able to handle this (what if an Rx powered up and down in flight for some unknown reason... etc).  But since it's a pre-existing design, can't do much about that and now I'm just whining lol.  We'll see how the mods shake out.


    The good news is I just spent the last hour and a bit reconfiguring everything and before I touched anything I powered down both USB and Lipo power.  So far so good.  Even have the 3DR radio working happy again and sonar connected (Mission Planner still doesn't report Sonar Alt. but oh well, test screen in terminal does).


    Paul I'll have to read back to find the mod you are talking about and look at adding it to protect the board in future (tree-limbs and low flying wreak havoc on cables and boards lol).


    One thing I found while setting everything back up though is that the "Position" flight mode is gone from Mission Planner now, it's not in the drop down lists.   Did it get deleted in this new firmware?

    Back to configuring and soon a test "driveway" hover.



    P.S. yes I have three regs left lol.   Lots more chances to be dum or help nail this down to make it more robust.

    • Developer


          very few people really intend to use Position mode I think.  Normally they're looking for Loiter.  The flight modes are described on the wiki.

    • Glad you are not giving up. Also glad that the reason why you blew it up again is obvious: hot plugging/unplugging habbit:)))

      I ordered several 100uf ceramic caps from ebay vendor Paul posted a few days ago. They are on their way to me right now. I will have bunch of extra when they arrive and would be happy to sell anyone one or two at a cost. I will post it again once I receive them.  

      • Lol sounds good.   I'm also placing another order with Digikey for caps and more regs (just in case my testing gets a little "out of hand" or I revert back to "dumbass" mode (hmm, is that one of the flight modes? lol).



  • That standard way to protect a linear regulator is using the schottky diode from output to input, been that way for at least ~4 decades that I remember. Since I have not read all 74 pages, has then been tried and proven this method it IS NOT the fix?

    Any links to that testing? TIA

    • Yes, it has been proven that is not the solution.  Further, it has never been shown it has anything to do with the problem.  Modern regulators have an internal diode for just this reason.

      • They have a body diode (not a higher power protection diode) if they use a FET for the high current path, the block diagram does not show a FET, and even if it does, the body diode is weak compared to the surge current that can happen, and the body diode (if there is one) is not a low Vf schottky.

        Again, where is the link to this testing? I would like to see the setup and scope shots.

        • Why would they need a high power protection diode?  Have you found an undocumented large capacitor on the 3.3V output side?  What are you protecting against?  TI states that the internal diode is sufficient to drain the output under normal conditions, and an external diode would only be needed in unusual circumstances.

          If what you are saying is true, it should be easy to replicate this failure on the bench.  But that has never been done.  Plus, these things would be failing all the time, because everybody powers down their system at some time.

          Philip's hypothesis is the only one which has ever been able to produce a failure repeatedly.

          • Rob,

            EG; If you short a ceramic 100uF cap at say 10v, there can be 30+ amp spike (do to low esr of the cap) in to a short. If you short the input of the regulator, the output cap gets discharged though the linear regulator (a wimpy 0.5A regulator). Now plugging in a Rx is not shorting the input out, but depending on many factors like, how fast the input gets dragged down, to what voltage level, the esr of the conductors and a few others factors. this can make the reg fail or not. Electronics are fast, milli, and micro seconds are very slow (I measure down to the femto second at work, 10 to the minus 15 of a second), this type of problem w/o decent scope shots is like black magic, and you can guess or pray all day long.

            Your quote;

            "TI states that the internal diode is sufficient to drain the output under normal conditions, and an external diode would only be needed in unusual circumstances."

            Is shorting (or dropping input voltage) very fast considered normal conditions, heck no.

            If someone want to send me an APM 2.5.x and spare parts (to Newport Beach, CA, USA), I will test at work with 4ch 1GHz scope (DPO4104B), I can change SMT parts very quick, less than a minute for most parts up to 200 balls in bga or csp package, so changing this 5 pin part is child's play for me. 

            Rob, I don't need your basic electronic answers or questions they are not helpful (I understand electronics are not your background), so if you can't provide details, no need to answer my posts.

            We all wan't to get to the bottom of this, although my three APM's have never failed (sold my 3DR and have two RCtimer's), I just want the link to the post(s) that the (correct) protection diode emplemented and test method used does not work under valid test conditions. And I am willing to put in the time providing someone sends me a APM 2.5.x (working or not regulator with spares), I will test and share high resolution scope shots.

            EG, here is a scope shot I took today troubleshooting a customer issue (blowing up my companies part), maximum voltage spec's can't go over 5.5v, and it's going to 6.8v, it's over 5.5 for 24 nano seconds (laymen s terms, 24 billionths of a second, and my scope samples at 5GS/s(5 billion per/sec)), killing the IC driver. High resolution scope shots are very helpful in troubleshooting intermittent problems, been doing it for 4 decades (as application engineer).

            Good luck to the APM 2.5 solution.

            Red Cathode narrow HV spike.png

  • And it's dead again!!!

    This time I had everything up and running, calibrated happily and was starting to connect my Rx leads to get the radio setup.  Immeadiately the USB "disconnect" sound occurred and the board stopped communicatteing.

    Upon reconnection, I got the error message "Bad Gyro health".  Took it all apart and sure enough, the regulator has shorted now to 5v output.

    Something has changed guys, never had a problem connecting and disconnecting servo wires from an Rx to my APM before.

    Going to repair it once again and will give it one more go.   Should it blow again so easily, its getting sold.



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