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AC3.1 & blown 3.3V regulator (was AC3.1-rc5 SPI speed problem)

Posted by Randy on October 30, 2013 at 12:37am

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.

3691073724?profile=original

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Tags: accelerometer, batometer, spi, noise

Replies

    • Rob_Lefebvre > Bill Isenberger February 19, 2014 at 1:42pm

      Bill, that is an interesting thread.

      Has Joe suggested what the actual mechanism is, whereby a low 5Vcc from your USB could cause the 3.3V regulator to fail?  I can't figure that out at all.  Since the 3.3V reg is a super low dropout, it should have no problem at all down to a 5Vcc of about 3.4V.  Your USB was putting out more than that.  And even should it go below that, an LDO usually fails "well", it simply puts out less voltage.  At 3.0V input, it would put out 2.9V.  

      Joe tends to guess at things, and provides no backup information or data, so you have to take what he says with a grain of salt.

      I see that your board already has the "protection diode" installed, which is something else that he came up with.  Previously he has stated that this is the cause of the failures.  But now you've had a failure, despite that diode being installed.  So now we're guessing again.

      I'd like to suggest another cause, that is at least plausible.  

      You are using a UBEC to power the board of unknown construction or quality.  Have you or anybody else ever checked the output of it on an oscilloscope to see if it is spiking over 6V on power up?   Using this external BEC instead of the dedicated voltage input on the APM board, bypasses the the overvoltage protection circuitry, leaving you vulnerable to this sort of thing.

      Another idea, and this is admittedly grasping at straws a bit, but something to think about...  Some switching regulators (UBEC's) will fail very badly when they go undervoltage (you had that bad battery).  Some of them will actually pass through full unregulated voltage when that happens.  Not a good thing.  Again, you've bypassed the protection circuitry.  

      • Mario bernard > Rob_Lefebvre February 22, 2014 at 9:03am

        Hi Robert

        I think that your UBEC theory fits perfectly with what i experienced yesterday.

        I had a perfectly good arduflyer board that i was using to test my (testbench components) to make sure they where all adequate. (usb cable, power source, etc)

        No bad gyro, good compass calibration, good gps lock (even in my basement), etc.. 

        Recycled the board a few times and reconnected (without changing any connections)

        At some point i disconnected the power from my 12v power supply to the bec and reconnected it...

        I reconnected via 3d radio.... as i did before...., good connection, BUT   oups...... bad gyro health....and the horizon started going awire....

        I instantly checked the 3.3v regulator......   and got a 5v..

        I decided to check the ubec output  peak at power on.....   and  yep,   i get a 5.9 peak for a split second...

        I wish i would have seen your post before......

        The ubec used is a Hobbyking low noise Ubec...  http://www.hobbyking.com/hobbyking/store/__4319__TURNIGY_3A_UBEC_w_...

        The one that Jabramm is showing on arduflyer build thread..

        Hope this info will help in the troubleshooting, but for me, the only thing that i did was recycling the power.

        So that's it for me..... now i need replace those regulators....

        ....

         

        • Developer
          Randy > Mario bernard February 24, 2014 at 5:01pm

          Mario,

               That's great feedback.  So we now know that it doesn't require you to do anything silly like plug an ESC cable in backwards to cause the failure.

          • Mario bernard > Randy February 24, 2014 at 7:52pm

            Tried 2 identical HK becs

            i get a steady 5.98 on the first when connecting and then it goes down  to 5.23

            The second one gives me 5.88 and goes to 5.22.

            Not shure if that 5.98 peek is long enough to blow the Vreg, but if it is, no wonder i now have 3 blown boards.... Especially since this Ubec is a permanent component of my testbench.

            • Detlef > Mario bernard February 25, 2014 at 4:06am

              Hi Mario, did you see my last post? I hooked 3 different BECs to my scope with no-load power-up. None of those showed any overshot near 6V. The only time I got 5.9xV was because the jumper on the lowe rf noise BEC was original set to 6V when I took it out of the bag.

              But neither my Fluke with MAX hold mode or the scope show any high voltage like you report it. I think I have 2 more of the low rf noise BECs laying around if you want me to test additional. But I think your multimeter is maybe reading wrong or you must have real bad luck having two units that overshoot so badly?

              • Mario bernard > Detlef February 25, 2014 at 4:52am

                HI,

                Unfortunatly, i dont have a scope and my voltmeters are not top quality either. Nevertheless, i tried with two different multimeters and they both give me about the same high peaks.

                I tried another HK bec ( a bigger one)

                http://www.hobbyking.com/hobbyking/store/__6320__TURNIGY_5_7_5A_8_4...

                i get 5.5 peak on this one.

                All my measures (even the previous) where taken with the ubecs set on 5volts.

                • Detlef > Mario bernard February 25, 2014 at 5:11am

                  I just got an idea, have you tried to measure just plain your battery voltage with your multimeter to see if it would show a high reading at the time you connect it?? 

                  Then you could be sure its your multimeter just showing a high reading that is not there.

                • Detlef > Mario bernard February 25, 2014 at 5:07am

                  Weird!! If I get a chance tonight I can try all three HK low rf noise BECs that I have laying around here. My scope is not the most accurate in voltage readings but fast enough that it would show any overshot in the curve. That's why I put the Fluke parallel to have an accurate voltage reading.

                  The only one that did actually overshoot was the 26V SBEC and that by aprx 0.5V before it was swinging in and get stable. That went so quick that the Fluke didn't even catch that overshot that you can clearly see on the scope screenshot.

        • Detlef > Mario bernard February 22, 2014 at 10:22am

          Hello Mario,

          I've got a few of those BECs and I checked the one I have on my APM under load condition with my scope. The one I tested never overshot to 5.9V but it does not go exact to 5V. It ends somewhere at 5.15-5.2V

          How did you measure the 5.9V? BEC open - no load? Multimeter?

          I am not saying that that BEC did not blow your 3V regulator just I think its not that general to blame that specific BEC because there are probably plenty of us who do not have an issue with it.

          I don't know why jabram recommends it, he never posted any performance data/measurements that put it above any other BEC. Until someone actually does a comparison test with other BECs I would also not believe that LOW NOISE statement from HK - but it sure helps their sales! I bought it because its cheap and hey they say "low noise" why not...

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