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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    • both happened, i bought two apms and they both failed, one to 0v the other to 5v

  • hey guys, thought i would just mention that rather than get the thing repaired or throw it out, just run the APM using a 3v UBEC and it will work just fine

    • How can we do that?

  • Okay... I tried to add a reply to my original post here: Comment # 1590992 to let folks know where to find the deleted material, but it wound up all the way at the end of this page, where clearly it doesn't do any good. I'm just going to formally ask this here then:

    I've started testing on a substitute regulator here: LT1117CTS-3.3 Regulator Mod and I've taken testing as far as I can with my resources. I am now asking for electronics-savvy volunteers who are equipped to abuse-test this mod and report back if the regulator provides adequately clean, stable power for the various sensors that are commonly used with the APM 2.5.2 and clone boards.

    I have several of the required regulators on hand; I'll gladly mail one to anybody interested in furthering this research.

    Thank you for your time,


    • Paul, you are one of the few around who are not using the tsp79147. I haven't read anywhere in your posts where you repeat the exact procedure that triggers the problem. Philip put the original reg through a lot of abusive tests including just about every short circuit you can think of and it passed all of them with flying colors. Specifically powering up a receiver(probably other loads too) while powered via USB is the Achilles heel. If you can do that a few times without a micro-nuclear meltdown then your mod passes the test.

      Also, with respect to the comparison of regulator technical details, the only thing that matters there is real performance. Sure one may be more stable than the other, but who cares if you can't tell the difference when flying? So you can also test that by flying and having some fun! Test out general stability, loiter, and alt hold performance. If performance is good enough for you then odds are it will be good enough for me and the rest of us too.

      No fancy scientific instruments needed.
      • Well, yeah... I kindof thought I made that clear; the reason I was asking for help is well... I don't really plan to push this thing very hard, and I don't know the product well enough to properly evaluate whether it's REALLY good enough. I don't have the budget right now to do big quads, and my Micro & Nano quads aren't really a good candidate for the APM controller. I really don't have a platform with which to abuse-test it.

        I've tested under the same conditions, as best I can duplicate, as Philip showed in his Video. On USB power only, with and without multiple loads, and with and without ceramic caps. I've demonstrated that I can cause enough of a disruption in the 5V rail to make the APM reset just by applying a 20uF ceramic capacitor at the rail. This I thought was valid evidence that what I suggested might be true; it needs a low-ESR ceramic cap (like those commonly used in modern, digital RX) to cause the voltage at the 5V rail to tank fast enough to cause this condition.

        My concern was that I could use this on one of my planes, which I intend to do, and have it function perfectly and declare it 100%. But if someone uses the mod on their big quad and it makes the gyros wig out under hard Gs because the power's too noisy or regulation isn't stable enough under light or heavy load, this would be something that could easily be seen and warned about, thereby prevented, with a better scope than I have at hand.

        Also, simply having better experience with the product would allow a user to pick up on things I wouldn't know are wrong... going by the example of this failure to begin with, I can guess that a lot of folks were flying this board with this failure and not realizing it for Ifni knows how long, just because they didn't know those occasional glitches weren't normal.

        Look... I've made my case, and there doesn't appear to be a lot of interest in taking up my little experiment, so I think I'm just going to let this die. Clearly I'm a lot more excited about this than anyone else is; I've already made a fool of myself, no sense in making a damned fool by pushing it.

        Thanks; I think it's time to just move forward with my build. ;)



        • Sorry, I didn't mean to embarrass you.  Quite the opposite actually.  I think your posts have been very helpful.  Your regulator replacement mod was a good idea.  You also backed up your choice of regulator with sound reasoning.  Your board is at the bleeding edge of the issue.  I'm eager to hear how your build goes and I'm sure others are too.    

        • Developer
          Hi Paul
          Great reply, and fair enough, no one wants to put a uas in the sky to end up with silly things bringing it down.

          I found the same, I could reset the APM by adding a cap while powered on. But that was not what the test was about. Two things....
          The USB cables I use are not long enough to fly with :)
          And the 3DR power module has a nice big cap on it.
          This is why we are not seeing this happen in flight.

          Many people have been flying with large caps added, as long as your power module can handle the inrush, that's all good
          I did some more testing with low esr caps, and though the APM may reset, the reg is fine.... It looks to be an inductance issue. And the Cap is a great solution to that.
          I appreciate your input, it has been useful :)

          • Phil -

            OK... I'm curious abut the inductance issue you refer to. I understand any cable has a certain amount of inductance; this can be exacerbated by internal resistance of small, long wires. Are you finding an oscillation (ringing) in the cable itself or as an interaction with inductors in the USB port's power supply?

            The former shouldn't be a big problem to resolve; just make sure to use a quality cable along with the buffer caps we've already done. But if it's interacting with inductors in the USB power supply... A better, lower internal resistance cable could actually make things worse, possibly even overwhelming the buffer cap.

            Oh, BTW... I've added some low-ESR 150uf Tantalum caps (because I didn't have any ceramic handy bigger than 47uf) on the I2C, INPUTS, OUTPUTS and ANALOG headers. I know the one on the INPUT header isn't really necessary as I already have one on the ANALOG header; but the board wouldn't sit level on my bench without it. ;)

            I'll probably need to move the caps once my case gets here anyways; I can put ceramics in place then.


            So far, so good; the LM1117 easily handles the inrush current and it appears to have cleared up some PWM noise I was seeing on the USB power in the 200mv range that was passing through the LM1117. Now all I'm seeing is the heartbeat down in the 10mv range. I've tried powering with 3 different PCs and 2 different USB wall-charger power packs; I can't make the APM reset or even lose comms from plugging in my big ceramic cap bank, or from any RX I have on hand. :thumbsup:


This reply was deleted.


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