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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      • Thank you Paul. The problem now will be to find one at an acceptable price and logical shipping rate.

        • LOL... yeah, well...

          The reason small lots are expensive from places like DigiKey and Mouser is paying for "people time". When you buy parts by the reel, you get them for pennies because the reel only has to be handled once for a reasonably large sale.

          When you buy 1 or 10 pieces, you have to pay someone to cut those parts off the reel, package them for individual sale, inventory them, and stock them. Then when it sells you have to pay another person to pick the part and pack it and ship it.

          Now multiply that by 5,000 or 10,000 pieces per reel... that's where your cost comes in.

          That's why I recommended those eBay vendors; yeah, they only sell 10 or 20 pieces at a time, but they're still cheaper than DigiKey or Mouser for a single-piece buy.

          When you tinker with electronics, you really need to get used to buying in 10s and 20s and 100s; it's just part of the joy. ;)

          PK

    • That in the picture is a electrolytic capacitor. I assume the reason Philip picked a ceramic capacitor is because of the lower ESR - equivalent series resistance. That means the ceramic capacitor can give fast higher currents and in this case react faster to those dips.

      Philip can for sure answer that better if that would make a huge difference in this case - my guess would be not much. Kinda like the electrolytic is good, the ceramic is better in this application.

      • I will wait to hear from Philip. In the meanwhile if I use it on A2, it may not provide as much help as the ceramic one but it WON'T hurt right?

        I remembered the one the guy was trying to sell me yesterday. It was a Tantalum capacitor. He said that it was same as ceramic but I didn't much believe it and bought the above one which you call electrolytic.

        • If this problem is being influence by the inductive effect of a long USB cable, wouldn't a high ESR cap actually help?  I have read that you don't want very low ESR when trying to stop "ringing"?

  • Hi,

    Could someone please help me. I have a quad with a pixhawk on it with 3.2-rc2 (beta) loaded. I have been getting unexpected rtl's on it. well I am guessing it is RTL's, it happened 2 times on this log. the first time when I lost control I hit it to land (one of my mode's) but it never said RTL in the log. the second time (at about 77%) it says it changed to RTL then I hit land.

    I am not sure how to tell what caused this, I looked at the rc inputs which look ok to me and the batt voltage is good. any help would be appreciated.

    John

    2014-03-14 18-40-55.tlog

    • Developer

      John,

           This is very unlikely to be related to the 3.3V regulator issue.  Can you raise in the APM Forum?  Jonathan Challinger should be able to help you find the cause.

      -Randy

  • Philip, did any of the regulator actually go to 5V or let the 5V through? Or did all die and output 0V??

    I am just thinking 0V leaves you with a dead APM but 5V was still working at slower bus speeds which was the original issue that started this thread. Or is it just random how the reg gives up?

    • Developer

      most failed to 5v, then one to 0v. failure could be either way. so I think it may be random, not a big enough sample size to call it either way.

      • thx Philip, Jared,

        I was just thinking not that it would be a different problem we discovered. Because mine died this week going to 0V - the second one I only smoked and I gave up because my USB port kept shutting down when I also had the scope on it trying to record it.

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