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.

For 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 I can say is this 650 frame is much tougher than I thought!  After a few more reliability tests, I think I'll take my first, very sort, FPV flight in alt hold and maybe during a mission.  I don't when I'll try water :)

I've done testing frame toughness I hope!! Trees are not much more relaxing than water :)

Did you look at my video from yesterday ( Somewhere in the middle I go past the woods and cross the water and the entire time I was hoping don't crash now... with my luck it would have been hanging somewhere high in the tree or drop right over and into the little canal.

Do you know by any chance if you fly out of the FrSky telemetry range if the module start beeping? I've noticed that lately that if I go out about 1500 ft my TX starts beeping erratically.

Hi Detlef, thanks I'll order one,I didn't havn't any arming issues, just booting the board requires an already powered ubec, I suspect the startup voltage spike in the ubec is doing something to make the board not arm.

Opps did I write arm? I meant of course boot!! I ended up putting a switch over JP1 so I can turn the board on and off independent from the ESC and RX. I hope that will be gone when I get the new regulator on the board - if it doesn't escape my tweezers :)

:) No worries, I had advice over at the other forum that all the 3.3v sensors and chips would be damaged by the 5v, and to get a new board, that might be prudent advice...

This is my second board and before my first board died prematurely (pilot error) it was have the boot problems also so may have had a faulty reg also, would love to find out what is causing it.

On this 2nd board I was very careful not have usb and ubec powering at the same time, although the advice is it is fine to do so, but I forgot one time and it may be purely coincidental but it seen to develop the boot problem after that, has there been any discussion on that?

Although it does fly fine on pre RC3 firmware so....

Well that part is $1.30 at Digikey with shipping, what the heck before I trash it lets spend the $5. Like yours mine flew fine with 3.0.1 so the 3.3V got the burn-in but hopefully not the premature burn-up :)

Mine had that boot problem since new and I swapped it against another and that one did not develop any boot issues or damaged 3.3V regulator. So I assume my surrounding is fine and I often power my board from both sides.

If you power your board over JP1 with diode in fuse in place you have a voltage drop over both. So if USB and BEC have half way accurate 5V then your USB voltage will end up higher than the voltage from the BEC coming over fuse and diode on the board - so the BEC will not overpower the USB port. Backwards from the USB to the BEC the diode blocks it. So I would say its pretty safe that way and if then very minimal. Besides that the 3.3V regulator should not be affected by that at all - I don't see a reason.

Should you overpower your board with the BEC (high voltage) then there is a zener diode behind that fuse which in theories should blow the fuse at a certain voltage level. I doubt it will protect all chips (its probably too slow) but if your F2 was blown then you had a serious issue.

Thanks Detlef I'm powering from inputs with JP removed.

Am careful using a UBEC with sub 5.3 voltage.

UBEC and USB powering at the same time. Isn't this necessary, how else would we do the compassmot setup then, as this is supported over USB alone + motors need that extra power to go on their business.

Also, I was curious why is this 8 MHz change was made over existing 500KHz? The SPI bus speed increase would cause more power drain for the processor, but is their a visual performance improvement in this regard?

My brain cells kinda getting restless!

Well, Randy said that frees up some CPU time that he could use for other things. But the 5V on the 3.3V is wrong and even though you do not notice any problems with 500khz and 5V on the 3.3 it still has communication errors every once a while just not as badly noticeable as with 8MHz.

UBEC and USB power, I say that is perfectly fine! You have to ask the ones who say its dangerous :)

I just went through the reviews of the HKmega on HK's site and a few people have mentioned dead 3.3v regs, I'm wondering if it's feasible to rig a 3.3v ubec in there for durability?

Ha, I am a step further - my little jumpy capacitor showed up again - this time it had no chance to escape my soldering iron and its back on the board!!

This week hopefully comes the regulator and then I'll see if that will fix all issues...


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