Developer

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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Replies

        • I am not sure exactly but maybe it is because USB cannot provide additional power demand immediately. I am pretty sure if you have a high-end motherboard with good power supply, you are in better shape but it would still be a gamble.

          You got lucky. I got lucky too because I did hot plug my Rx a few times when setting up.

          This doesn't burn the reg. 100% of the times but it does it. And statistics clearly show that it affects clone boards lot more than the 3DR boards. So if yours is original 3DR (mine is), odds are in your favor... 

          • Hi, Yeah, mine are all 3dr boards.

            thanks again,

            John

            • Hi,

              I was just thinking, what about the Pixhawk boards?   should they be batt powered before using USB? I am not sure if it is even possible, at least for loading FW. For FW it seems the board has to be disconnected from power then reconnected.

              I wonder if hitting the reset button would work instead of disconecting and reconnecting the USB?

              John

              • Developer

                John,

                     The Pixhawk have an improved power system including switching to the power provided through the output rail if the power module fails.  I'd be very surprised if you had to worry about this kind of thing with the Pixhawk.

                • Hi,

                  I kind of figured that.  Just wanted to be sure.  I just checked the last apm25 I have, all are good and I have plugged them all in to the USB with everything connected many many times for flashing FW and getting/clearing logs. they are all 3dr boards but I guess I just got lucky.

                  thanks,

                  John

  • 3701685174?profile=originalDetlef, Philip,

    I bought this 100uf cap yesterday. I am not sure if this is a ceramic or not; it does NOT seem so. The guy showed me some other small caps claiming that those were ceramic but there was no print on them. I couldn't read anything; didn't want to risk it.

    Would this 100uf 16v cap do the job for me if I plug it on A2 input?

    Thanks

    • Alex -

      The reason Philip recommended a 100uf ceramic capacitor across the 5V rail is twofold; 1, because they're more stable with changes in temp, humidity, age, etc, and 2, because they're very low ESR and serve much better at buffering the surge current than cheap radial electrolytics. Oh, and they're very small and light for their capacity, so a better fit for small flying things.

      This is what you want:

      4696308.jpg

      100uf Ceramic on eBay (US Vendor)

      100uf Ceramic on eBay (China Vendor)

      The tantalum electrolytics are a lot more stable, but not near as low ESR, so no, not as good at buffering the surge current.

      Rob - "Ringing" is caused by an unexpected LC bridge in the circuit; as we have no inductors at our end, not what we're addressing. It's more an issue of the source voltage (The USB cable) having varying amounts of current capacity because everybody's computer PC board has a different circuit for powering the USB. Add to that varying internal resistance of different USB cables, connection losses due to how clean the contacts are at each end, and we have a fairly unstable power source from any USB port.

      TL/DR version:

      We're just looking for a little brute-force filtration to keep the voltage at the 5V rail from tanking and causing the brownout/destructive latching issue we're seeing when plugged into USB. The 100uf ceramic capacitor is the least invasive means of providing that.

      PK

      • D'oh!

        Looks like I misspoke; I was out of the loop on the inductance thing. My own fault; I didn't see Philip's response to my own earlier post. Now I wanna know more about this inductance issue; I've had to chase down issues in USB-powered gear before, and it usually wound up being an especially cheaply made  Chinese cable, or a specific PC that had a compromised USB power circuit from being repeatedly overloaded.

        The end-result is still the same; we need some brute-force filtering at our end to make up for shortfalls in the USB power source.

        PK

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